Temperature Measurements:Temperature are one of the most important physical properties that needs to be determined (or measured) in any process. For example in heat exchangers in order to calculate the heat transfer coefficient you need to determine the temperatures of the hot and cold fluid (in and out) which requires using temperature devices measurements. Temperature is the measure of energy motion of the particles that composes the material1 and it can be related to kinetic energy through Boltzmann Constant (k=1.3806 10-23 m2 kg s-2 K-1). Over the years various temperature devices have been developed some are electronic and others are not. Some are contact measurements and other measures the radiation power emitted by a source.Thermocouples:Thermocouples are temperature measuring devices that measures the potential difference between two wires made of dissimilar metals joined together at a junction2. When two pieces of dissimilar metals are joined together they produce a voltage difference depending on the temperature of the junction, this effect is known as the Seebeck effect3. In 1821 Thomas Seebeck discovered that when thermoelectric semiconductor has a temperature difference between its ends a voltage will be produced and this voltage will depend on the type of material and the temperature difference4. In a thermoelectric semiconductor materials there are free electrons that behaves like a charged gas. When placed under temperature gradient the colder end will have higher electron density because the hotter end will have higher energy electrons that will diffuse faster than those in the cold end. As a result of the high electron density at the colder end a voltage will be produced4. This voltage can be calculated by: (1)V: Voltage (mV)

The voltage will depend on both the temperature difference between both ends and the Seebeck coefficient which will depend on the materials of the thermoelectric. Thermocouples are contact temperature measurement devices that can be used to measure temperatures of both solids and liquids. They are made of two dissimilar metal wires joined together at one end, called measurement junction, and the other ends are connected to signal conditioning circuitry, called reference junction2. One of the wires act like a anode and its usually made of electron rich metal and the other acts like a cathode and is made from electron deficient metal. Figure 1 shows a schematic of thermocouple. Thermocouples measure the temperature by placing the measurement junction at a desired spot and connecting the reference junction into a reader. When the measurement junction is hotter than the reference junction electrons will move toward the reference junction which produces a voltage. This voltage could then be sent to a reading device where it will be changed into a temperature value.

Figure 1: Scheme of the thermocouple (image was obtained from analog.com)2.In industry, thermocouples are usually mounted to the walls of the equipment which temperature needed to be specified. Figure 2 shows a typical industrial arrangement of a thermocouple. Between the hot junction and the junction box there is a wall that isolates the cold junction so that there will be a temperature gradient. Additionally the thermocouple is covered in metal or ceramic tube to protect the wires from corrosion and other effects that could damage the wires; however, this protecting materials will cause a delay in the temperature measurement because you will have to wait until the inside wires (hot junction) temperature reaches the outside temperature which is why the protecting tube is usually made from a high thermal conductive materials. Figure 2: Typical arrangement of an industrial thermocouple used to measure fluid temperature. (Image obtained from Chemical and Biochemical Reactors and Process Control, Knovel.com)5There are various types of thermocouples that are made of different metal wires. For example, J-type thermocouple is made from Iron and Constantan6; but the most common thermocouple is K-type thermocouple made from Chromel & Alumel2 (Chromel is an alloy made mostly from Nickel with a small portion of Chromium, Alumel is also a nickel alloy with a small portion of Aluminum). Table 1 shows a list of standard thermocouples with the materials that the two wires are made from, operating temperature range, accuracy, and some applications.Table 1: Standard thermocouple types with wires materials, temperature range, accuracy, and some typical applications.(Data obtained from Chemical and Biochemical Reactors and Process Control, Knovel.com)5Thermocouple TypeWires MaterialsTemperature Range (K)Accuracy (K)Typical Applications

BPlatinum- 30% Rhodium2732000 4In high stability and long life application such as a heat exchanger

EChromel-Constantan100 1100 1.5Furnaces, acid production

JIron-Conatantan100 1100 1.5Polymer manufacturing, paper production, chemical reactors

KChromel-Alumel100 1300 1.5Boilers, acid production, reactors, superheater tubes

NNickel-Chromium-Silicon100 1300 1.5Semiconductor manufacturing, Power station boilers, heat treatmen

RPlatinum-13% Rhodium273 1850 1Heat exchangers, furnaces

TCopper-Constantan30 700 0.5Food processing, lubricating oils, Sulfuric acid manufacturing

Thermocouples can measure a wide range of temperatures depending on the type of wires used it could go from 273-2000 K 5. However not all metal wires can measure such a range; for example, type-K thermocouple ranges from 100-1300 K 5. This wide temperature range makes thermocouple a common temperature measurement device in industry for various applications. Additionally the accuracy of a thermocouple ranges from 1-4 K5 which is considered somewhat accurate. However since thermocouples are differential measurement devices, they measure temperature based on the temperature difference between measurement junction and reference junction, changing the temperature of the reference junction could affect the accuracy of the measurements. The precision of a thermocouple is 0.1 K. Thermocouples have various advantages which make them desirable, at some cases, over other temperature measuring devices. One advantage is the wide temperature range that thermocouples can be used to measure, for example it can measure temperatures of cryogenics and can also measure the temperature of a jet-engine exhaust2. Another advantage is the robust as thermocouples are not affected by vibrations or shocks which make them suitable to measuring temperatures near pumps or any other vibration generating device2. Moreover thermocouples does not self-heat which can cause a high imprecision. One more advantage is that thermocouples produce rapid responses which make them ideal for measuring high temperatures that could cool off quickly3. One last advantage is that thermocouples are self-powering devices and they dont need an energy source for them to work3. Thermocouples also have couple of disadvantages that limits there usage and requires another temperature measuring device. One disadvantage is that thermocouples requires substantial signal conditioning in order to change the voltage into a temperature value and might require the use of reference tables or thermocouple readers2. Since thermocouples are made from dissimilar metals corrosion could occur on the metals which might affect the precision of the temperature measurement and requires the thermocouple wires to be changed. The accuracy of thermocouples depends greatly on the reference temperature of the cold junction and if the cold junction is not well-insulated this might produce inaccurate measurements. Since thermocouples already produces electrical signal in the form of a voltage, there is no need to come up with a way to change the measurement into an electrical one. However, thermocouples need a reader that could change the voltage into a readable temperature measurement or reference table can be used to do that. In order to use thermocouple reader or reference tables you need to know the thermocouple type because different types will produce different voltages which can result in inaccurate temperature measurements. 1-Physical chemistry book2-http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf3-http://www.omega.com/temperature/Z/pdf/z019-020.pdf4-http://www.thermoelectrics.caltech.edu/thermoelectrics/index.html5- Coulson and Richardson (Knovel)6-http://ocw.metu.edu.tr/pluginfile.php/1870/mod_resource/content/0/AE547/AE547_8_temperature.pdfConstant Volume Gas-Thermometer:Another contact temperature measuring device is Gas thermometer. Gas thermometer is temperature measuring device that measure temperature by the variation of pressure or volume. There are two kinds of gas thermometer one with constant volume and the other with constant pressure. Constant pressure gas-thermometer are not used in practice because of the high error due to the non-uniformity of tube bore7, so only constant volume gas thermometer will be discussed. Constant volume gas thermometer consists of a bulb, filled with a gas, connected to a pressure measuring device such as manometer8. Figure 2 shows an apparatus of the constant volume gas thermometer. To measure the temperature of a fluid the gas bulb is submerged in fluid and as the gas expands a pressure difference occurs that could be related to the temperature of the fluid. The basic principle behind the constant volume gas thermometer is that temperature and pressure of a gas with a constant volume is directly proportional. As the temperature of the gas increases the pressure also increases and when taking the pressure difference, using a manometer, that pressure difference could be used to calculate the temperature of the fluid. The equations used to calculate the temperature in a constant volume gas- thermometer are:If we are assuming ideal gas behavior such as for Helium: (2)P: Pressure (Pa)V: Volume (m3)n: number of moles (mol)T: Temperature (K)R: gas constant (which equal 8.314 )If we are not assuming ideal gas behavior such as for hydrogen Van Der Waals equation can be used: (3)a: quantity related to the intermolecular forces between atoms ()b: quantity related to the molecular volume of the gas ()When taking both volume and number of moles as constant the equation relating pressure to temperature is7: (4)

By expanding equation 4: (5)

(6)By substituting equation 6 in equation 7 and rearranging: (7)To measure the pressure at a reference point, the gas bulb is placed at the reference temperature usually 0 C. Then the Pressure is measured again at the desired temperature and given the parameters for and the temperature could be calculated using equation (7). The pressure gauge in figure 2 is a closed-tube manometer. This manometer has one end connected to the gas bulb and the other connected to an evacuated tube. The purpose of the mercury reservoir is that during operation the reservoir must be lowered or raised to keep the level of mercury constant at the manometer to ensure that the thermometer operates at a constant volume8. The height h could be related to the pressure at the temperature we want to measure and used to calculate the temperature of the fluid.

Figure 2: Apparatus of the constant volume gas thermometer. (Image obtained from Massachusetts Institute of Technology) 8The temperature range of the constant volume gas-thermometer will be largely affected by the gas used in the bulb. In general the range of the gas thermometer is (300 to 1900 K)7, which is a wide temperature range. The accuracy of the constant volume gas-thermometer ranges from (0.003 to 0.02 K)9 depending on the type of gas and the pressure gauge used. The precision for Helium and hydrogen is (0.2 K); However for Nitrogen it is a little bit higher (around 2 K)9.The advantages of constant volume gas-thermometer are that it is more temperature sensitive than other thermometers. For example compared to a fluid thermometer, the constant volume gas-thermometer is much more sensitive to small temperature. Another advantage is that it has a wide temperature range which makes them ideal for measuring low temperature fluids. Gas-thermometer works preciously and uniformly over a wide range of temperature which makes them preferable for sensitive temperature measurements7. Additionally gas-thermometer are cheaper than most type of thermoelectric thermometer and can be used to almost the same range as some of these devices; which gives them an advantage over thermoelectric devices especially for fluids that might corrode or damage the thermoelectric devices. In the other hand, there are couple disadvantages of constant volume gas-thermometers that limits there usage and make them invaluable as temperature measuring devices for some applications. One disadvantage of gas-thermometers is that they take a long time to measure the temperature7. Since gas-thermometers operates based on pressure increase, it takes the gas some time to expand and pushes the manometer fluid so that a pressure value could be determined. This disadvantage limits the use of gas-thermometer for the cases where the fluid is heating or cooling rapidly because of the time needed for the gas to adjust. Another disadvantage is that these devices are not easily portable so if there a temperature measurement needed somewhere else, it will be hard to move the gas-thermometer around. One more disadvantage is that the gas-thermometer needs to be submerged on the fluid in order to get an accurate temperature measurement, so if there is not enough fluid to submerge the gas bulb these devices cannot be used to determine the temperature. Finally these devices are large contact devices and it can impact the temperature of the fluid significantly especially if the gas bulb was at a different temperature from that of the fluid. The constant volume gas-thermometer doesnt produce an electric signal which could be related to pressure; instead it gives pressure values that can be used to calculate the temperature. The advantage of producing electric signals is that electric signals are more precise and they eliminate the operators bias errors. To produce electric signal, electric sensors (such as those in cars gas tank) could be placed at the sides of the manometer and as the mercury rises it will conduct electricity between the two sensors in the side which will determine the height. After that the signal could be sent to a device that can uses the manometer fluid height to determine the temperature by calculating the pressure difference given the gas used in the bulb. 7-SRM university (ppt).8- http://curricula2.mit.edu/pivot/book/ph1902.html?acode=0x02009-Reduction of gas thermometer (Knovel.com)

Reversible Liquid-crystal thermometer:Liquid crystal thermometer is temperature measuring device that changes color depending on the temperature. There are two kinds of liquid crystal thermometers one that is reversible and can be used multiple times; the other is an irreversible thermometer which can only be used once. For this assignment only reversible liquid crystal thermometers will be discussed. The basic idea behind these thermometer is that some physical and chemical properties of a material changes as the temperature, it is at, changes. One of these physical properties is the substance color. For example when Iron is heated it starts as black and as you increase the temperature, it starts glowing red then turns into white at very high temperatures. Although Iron changes color at different temperatures, the temperature range for the color change is high which makes it impractical to use as a temperature indicator; however, there are materials that change temperature over a small range of temperatures and they are called cholesteric liquid-crystals. Cholesteric liquid-crystals are grease-like substances that produce compounds having color transition ranges at temperatures from (-30 to 120 C) 10. Additionally there are some liquid crystals that have a different temperature response ranging from (1-50 C) 10. The fundamental physics behind these labels is the idea that at various temperatures, the molecules of the substance emits radiations with different wavelength according to Plancks Law. In 1900 Max Planck proposed a blackbody radiation law that describes the behavior of the radiations emitted from a blackbody at different temperatures1. Although Plancks law predicted accurate results, the physics behind the blackbody radiation wasnt understood until the development of Quantum Mechanics. From quantum mechanics, the electrons in the material have quantized energy levels that they can occupy. The spaces between these energy levels becomes closer as you go to the high energy levels; electron transition can only occur between these levels and electrons will only absorb the light that correspond to the difference between these energy levels1. Figure 3, shows energy diagrams and the allowed electron transitions between them. When the electron absorbs light, it goes to a higher energy level and then when it comes back to its ground state, it emits back radiation. As you increase the temperature of the element, you move electrons to higher energy levels and they absorb and emit different wavelengths of radiation1; which correspond to different colors of visible light. For liquid crystal thermometer, the same physics are used as you increase the temperature the crystals absorb and emit different wavelengths that correspond to different colors. Figure 3: Generic energy level diagram that shows the allowed electron transitions. (Image obtained from Lawrence Berkeley National Laboratory)11The fundamental equation which governs the behavior of the liquid-crystal thermometers is Plancks radiation law1 which is:

T: Temperature (K)The frequency is related to the wavelength as:

By changing frequency into wavelength and differentiating, maximum wavelength emitted is1:

Figure 4 shows the different components that construct liquid-crystal thermometers. At the top a polyester film is used to cover and protect the liquid-crystal ink. After that there is a white and black graphic print that shows the temperatures and there range. Each hole in the graphic black print is filled with liquid crystal ink which is then covered by a black backing. The purpose of the black backing is to hold the liquid crystal ink in place and to provide a black back ground that makes the temperature readings easier. Finally an adhesive and carrier and a release linear films are attached to the thermometer. When the liquid-crystal thermometer is used to register the temperature, the liquid-crystal ink turns into different color depending on the type of ink used. The temperature value where the ink changes its color is the registered temperature value. There are various types of liquid-crystal inks that can be used for different temperature ranges and they have different colors that correspond to the temperature reading13. For example some use liquid-crystal inks that change into green at the measured temperature, while other change into red for the measured temperature.

Figure 4: Multiple layers that construct Liquid-Crystal thermometer. (Image obtained from LCR Hallcrest)12 The range of the liquid-crystal thermometers varies greatly depending on type of the liquid crystal ink used for the thermometer. In general liquid-crystal thermometers ranges from (-30 to 120 C)13; however there are a wide variety of liquid-crystal thermometers, commercially available, that have different temperature range. Additionally the accuracy of these thermometers will vary greatly but in general these liquid crystal thermometer have an accuracy of ( 2 C)13. Similar to the accuracy, the precision of the liquid-crystal thermometers will also vary depending on the type of crystals used; as some crystal will be more sensitive than others. But the most common liquid-crystal thermometers have a precision of (2 C)13 and (5 C)14. Table 2 shows different liquid-crystal thermometers models, their temperature range, accuracy, and precision.

Table 2: Liquid-Crystal thermometer models that are commercially available with temperature range, accuracy, and precision. (All Data were obtained from Omega.com)14Model NumberTemperature Range ( C) Accuracy ( C)Precision ( C)

RLC-50-30/0 -30 0 25

RLC-50-0/30 0 30 25

RLC-50-30/60 30 60 25

RLC-50-60/90 60 90 25

RLC-50-30/90 30 90 210

RLC-50-90/120 90 120 25

The liquid-crystal thermometers have various advantages that make them desirable for some applications. One advantage is that liquid-crystal thermometers are inexpensive and easy to buy; which make them appropriate for simple temperature measurements such as room or refrigerator temperature measurements. Another advantage is that the liquid-crystal ink is non-toxic which makes them harmless and they can be used in food refrigerators or fridges. One additional advantage is that they are easy to move and carry around compared to other devices, such as the gas-thermometers. Additionally liquid-crystal thermometers are fast measurement devices that dont require a long time to produce a temperature measurement. Some of the disadvantages, that the liquid-crystal thermometer has, are these thermometers have a short life time and they cant be used for a long time; which make them undesirable for many industrial applications. Another disadvantage is that all the materials that these thermometers are made from corrode easily and they require a special care to prevent that. For example if they were prolonged exposed to ultraviolet or sunlight, they will become malfunction. Moreover liquid-crystal thermometers cannot be used for sensitive temperature measurements because of their high inaccuracy and imprecision. Also there limited temperature range make them undesirable for application that involve measuring a wide ranges of temperatures such as in heat exchangers or furnaces. Since liquid-crystal thermometers dont produce an electric signal, they cannot be used for applications that involve recording temperature electronically. However these thermometers could be designed to produce an electric signal that could be sent to a computer or an automatic recording machine. To achieve such a conversion, to an electric signal, a color sensitive sensor could be attached to each temperature increment. This sensor gets activated only if it absorbs a specific color, for example green. When the liquid crystal thermometer registers a temperature the ink will change into green at the measured temperature, then the sensor at that specific temperature value gets activated, because it absorbs the green color, and send an electric signal to a reader which register that temperature.

10-ASM handbook (Knovel.com other account)11-Berkely lab (in Bookmark google chrome) 12- LCR Hallcrest13-telatemp (Pdf in the desktop)14-Omega (pdf in my computer)

Task 2:Pressure:Pressure is considered one of the most important parameters that need to be specified for any process. The importance of pressure arises from the fact that at different pressures the physical properties of a material changes which could affect the outcome of a process. Through the years, there have been a lot of pressure measurement devices that was developed to measure pressure such as barometers, Bourdon gauges, and McLeod gauges. All these devices had different ranges and different accuracies and were used for different applications.

Piezoelectric Pressure sensors:Piezoelectric is derived from the Greek word Piezo which means to squeeze15. Piezoelectric effect is the production of a voltage when a piezoelectric material strained15. The Piezoelectric pressure sensors are devices that use the piezoelectric effect to measure the pressure through an electric signal16. There are some materials that exhibit piezoelectric effect and they are used in the production of piezoelectric pressure sensors. In order for the material to be piezoelectric, it has to be anisotropy which means that its not symmetric in all directions such as quartz crystals. There are naturally occurring piezoelectric materials such as quartz and tourmaline and there are synthetic piezoelectric materials which are ceramics15. Piezoelectric materials when subjected to pressure, the crystals making the piezoelectric materials will realign themselves and produce a charge across the crystal15; the realignment of the molecules inside the crystal causes a change in polarity, because the density of charge at one end relative to the other will change, which produces an electric charge. However the electric charge gets produced only when there is a dynamic pressure change; for a static pressure change, the piezoelectric material will produce a charge and then it will dissipate slowly until there is no electric signal. Since there is a change in charge over time in a piezoelectric material, this produces current: (11)

I: Current (A)

Then a voltage can be produced from the current of the piezoelectric material, which can be calculated by15: (12)V: Piezoelectric generated voltage (V)

P: Pressure (Pa)D: Thickness of the material (m)Figure 5 shows a scheme of a piezoelectric pressure sensor. When a pressure is applied on the piezoelectric material, a change in polarity occurs which causes an electric charge to be produced. This electric charge passes through an operational amplifier (the triangle) which amplifies the signal to make it detectable. The current produced also passes through a resistor (I/V converter) that changes it from a current into a voltage. The output of this circuit is a DC Voltage that gets sent to a reader which can then convert these voltage values into pressure. The type of piezoelectric material must be specified in order for the reader to produce the right pressure value.

Figure 5: Scheme of a piezoelectric pressure sensor. (Image obtained from Auburn University)16

The ranges for piezoelectric pressure sensors are different depending on the type of the piezoelectric and the material it is made from. For example a piezoelectric silicon pressure sensor packed in a surface mount configuration has a pressure range of (0-500 psig)17. While a piezoelectric silicon pressure sensor packed in dual-in-line configuration has a range of (0-1 psig)17. Additionally the precision of these piezoelectric pressure sensors also vary depending on the piezoelectric material used to make the pressure sensor but in general these pressure sensors have an accuracy of (0.25% psig)17; which for the surface mount configuration corresponds to (1.25 psig) and for dual-in-line configuration (0.0025 psig). Similarly the precision of the piezoelectric pressure sensors will vary; for example for a surface mount configuration the precision is (0.05 psig)17. However there are more accurate piezoelectric pressure sensors with an accuracy of (0.05% FS)18. Piezoelectric pressure sensors have numerous advantages that make them desirable pressure measurement devices and make them common in industrial applications. One advantage is that they are sensitive pressure measurement devices and you could get a highly accurate pressure measurement18. Another advantage is that they are small devices that could be attached to a pipe to measure the pressure without affecting the flow rate. Additionally the piezoelectric pressure sensors are fast response pressure sensors that dont take a long time to get a pressure value17. Also these pressure sensors produce an electric signal which makes them ideal for automatic recording of pressure values. In the other hand there are couples of disadvantages that limit the use of the piezoelectric pressure sensors and prevent them from being used in certain applications. One disadvantage is that these sensors only measure dynamic pressure and not static. Dynamic pressure occurs when there is a change of pressure over time; however, if the pressure is constant over time (static) then these devices cant record that pressure value because there is no rate of change of charge that produces current16. Moreover the piezoelectric pressure sensors are temperature sensitive and with the change of temperature there accuracy gets affected greatly16. Due to their pressure sensitivity they cant be used in application where there is a heating involved such as in furnaces or heat exchangers. Additionally the piezoelectric pressure sensors are somewhat expensive especially the one made from quartz crystals because of the high price of quartz17. Since piezoelectric pressure sensors already produces electric signal, they could be used to automatically record data. Additionally the electric signal measurements prevent error due to operators bias. Also the recording machine or computer could be placed far away from the process that you are trying to measure its pressure, to prevent any damage that could occur to your machines. 15- Piezoelectric and Piezoresistive Sensors (udayton in desktop)16-piezoelectric (auburn desktop)17-Berkeley (bookmark)18- Institute of physics publishing (Smart desktop)

Pirani gauge:Pirani gauge is a pressure measurement device that is used to measure the pressure of gases at a really small range (10-4 to 100 Torr)19. Pirani gauges, similar to thermocouple gauges, uses the idea of heat transfer in a vacuum to measure the pressure of a gas. The basic idea behind a pirani gauge is that an electrically heated wire made from Platinum or Tungsten is mounted on a glass tube and then connected to a vacuum apparatus20. Then any change in pressure causes a change in the wires temperature which causes changes in the wires resistance20. For instance at a high pressure there are more gas molecules that collides with the wire and makes it loses its heat; which reduces the temperature of the wire and causes a change in resistance21.The change in the wires resistance could then be transformed into an electrical signal that can be sent to a reader where it can be changed into a pressure value. The pirani gauge uses the idea that thermal conductivity of a gas at low pressure is linearly related to the pressure of that gas22. The relationship between thermal conductivity of a gas and the pressure of that gas are linearly dependent and can be calculated as20:

K: thermal Conductivity (W/m K)

P: Pressure (Pa)Figure 6 shows a scheme of a pirani gauge. When taking a pressure measurement, the gas enters from the unknown pressure inlet to the inside of the pirani gauge. Inside the gauge is evacuated and there are four Tungsten wires that are connected to an outside circuit23. The outer circuits heats the Tungsten wires to a specific temperature and when the gas enters heat transfer between the gas molecules and the Tungsten wires occur. This heat transferred from the Tungsten wires to the gas molecules reduces the temperature of the wires which causes a change in the resistance of the wires23. The change in resistance can then be measured and sent as an electric signal to another circuit where it can be changed from an electric signal into a pressure value.

Figure 6: A scheme of a pirani gauge that shows Tungsten wires and the inlet for pressure. (Image obtained from Measurements and Instrumentation knovel.com)23The range of the pressure values that the pirani gauge can detect varies greatly depending on the type of the materials used for the wires inside the vacuum chamber. In general the pirani gauges can detect very low pressure values ranging from (3*10-3 20 Torr)24; however there are other kinds of pirani gauges that can detect up to (100 Torr). Similarly both the accuracy and the precision of these gauges will also vary. Most of the commercially available pirani gauges have an accuracy of ( 2*10-4 Torr)24 and they have a precision up to ( 1.2 *10-5 Torr)24.There are numerous advantages for using pirani gauges and the most important one is that they can measure low pressure of gases. In industry it is sometimes required to measure the pressure of a specific gas and because gases have low pressures; sometimes it is hard to do these types of measurements. Using a pirani gauge can provide these measurements to a high level of accuracy. Additionally pirani gauges produces an electric signal that could be sent to an automatic recorder to automatically register the data. Also the digital measurements usually considered more accurate because they eliminate the operators bias error. Another advantage is that these gauges are small and they could be easily transported from one site to another. Moreover they are usually made from materials that dont corrode easily such as Platinum which gives them a long life time. In the contrary there are disadvantages to using pirani gauges; such as they need an external power source to heat the wires which limits there applications in places where there is no power source. Another disadvantage is that they are temperature sensitive and temperature could have a major effect in their accuracy24. Additionally they are sensitive to shocks and vibrations which make them invaluable for pressure measurement near pumps22. One extra disadvantage is that they need calibration before being used and usually McLeod gauges are used to calibrate them. Pirani gauges produce pressure measurement as electric signals which can be converted into pressure readings using a reader. The benefits of the electric signal measurements are that it prevents error due to operators bias. Also the recording machine or computer could be placed far away from the process that you are trying to measure its pressure, to prevent any damage that could occur to your machines. 19- Pirani gauge (mksinst) (desktop)20-Instrumentation reference book (Knovel)21-Vacuum deposition (knovel)22- Engineering PhysicsBy Rajendran (look it up online)

Bourdon Tube:Bourdon tubes are simply tubes that are bent into an oval cross-section which utilizes the concept of materials elasticity to achieve pressure measurements. When subjected to pressure the coil inside a bourdon tube uncoils and causes a needle to move through a graduate scale, which provides the pressure readings. One end of the bourdon tube is sealed and attached to a pivoted quadrant via connecting link. The upper part of the quadrant is a toothed segment that moves and causes the needle to move with respect to a fixed scale25. The other end of the tube is open so that the measured pressure can be applied via a block which is fixed at; it also carries the pressure connection and provides the measurements25. The materials that the tube is made from have to have an elastic property across the pressure range that the tube can measure; it also has to be selective to the type of fluids used in the measurements. Since the fluid enters the bourdon tube the materials has to be corrosion resistance, if the fluid to be measured is corrosive, and it should retain its elasticity. Most common materials used for bourdon tube that are corrosion resistance are Phosphor bronze, beryllium cooper, and stainless steel25. Both the type of the material and the thickness of the tube are selected based on the pressure range and the application that the bourdon tube will be used in. Bourdon tube gauges measure gauge pressure relative to ambient atmospheric pressure. Since the basic idea behind a bourdon tube is elastic deflection, pressure could be measured as a function of elastic deflection. From Hookes law:

Where tensile stress is:

And tensile strain is:

Figure 7 shows a bourdon tube and the inside components of it. When pressure is applied, as shown in figure 7, the C-shaped tube start elastically uncoils which causes the tube to expand. This tube expansion causes a rotation on the quadrant attached to it. The quadrant has a toothed gear mounted to it and its motion causes the gears to rotate; which also causes the needle pointer to move on a fixed scale. The motion of the needle depends on the elastic expansion of the tube which will depend on the pressure applied. The pressure range of the bourdon gauge depend on the elasticity range of the materials used to make the tube, if this elasticity level was exceeded then the tube enters the region of inelastic expansion where it is permanently deformed.

Figure 7: Mechanism of bourdon tube gauge and its components. (Image obtained from KOBOLD koboldunirota.hu)26

Bourdon gauges have different pressure ranges that they could be used over depending on the materials used to construct the tube. The pressure range for any bourdon tube is correlated to the elasticity of the materials because beyond the elasticity range the tube will deform permanently. Since bourdon tubes are used to measure the pressure of fluids at high pressure they could range up to (10000 psig)27. In general, bourdon tube pressure range is around (0-1000 psig)26 . Additionally these bourdon gauges have an accuracy of (1% FS) 27; which ranges between ( 1 psig) to (0.01 psig) depending on the type of bourdon gauge used. The precision of bourdon gauges also vary depending on the type of gauge used but in general these gauges have a precision range that goes from (0.05 psig)28 to (100 psig)28.

There are many advantages that make bourdon gauges a favorable pressure measurement device, one advantage is that these gauges give accurate pressure measurements. Most of industrial application depends greatly on pressure measurements and having an accurate pressure measuring device can help with the design and analysis of a process. Another advantage is that these gauges are considered inexpensive pressure gauges compared to other types of gauges such as piezoelectric pressure sensors and viscometer gauges. Also bourdon gauges are safe in taking pressure measurements even at high pressures; which make them desirable pressure measurement devices especially for industrial applications. Moreover bourdon tubes are simple in construction and easy to read since they dont require any sophisticated reading machines. The simplicity of the bourdon gauges give them an advantage over many pressure gauges because they are easy to fix and troubleshoot. One additional advantage is that bourdon tubes could be made from corrosion resistive materials which gives them the ability to be used for pressure measurements for corrosive fluids such as acids25. On the other hand there are couple of disadvantages that limit the use of bourdon tubes and need to be considered. One disadvantage is that bourdon tubes are sensitive to shocks and vibrations which make them unusable for pressure measurements near a vibration source, such as a pump, because of the high inaccuracy. Another disadvantage is that they respond slowly to pressure changes, so that they cannot be used if quick pressure measurements are needed because they take time to adjust. Additional disadvantage is that the needle on a bourdon tube could be twisted easily, especially if subjected to a high pressure, which can cause it to deform permanently. One last disadvantage is that bourdon tubes are sensitive to temperature and high temperature changes could change the elasticity of the tubes material causing it to produce inaccurate pressure measurement. Since bourdon tubes doesnt provide an electric signal, and electric signals are sometimes desirable, some modifications could be added to the bourdon tube to produce the needed electric signal. At the tip of the bourdon tubes needle, a small capacitor could be placed and additional capacitors could be placed on the measurement scale. Figure 8 shows a rough scheme of a bourdon tube needle with a capacitor. As the needle moves through the fixed measurement scale and stabilizes on a value; there will be two capacitors, in front of each other, which will produce a voltage that could be sent to a reader. The reader then converts that voltage into a readable pressure value.

ReaderCapacitorsPointer

Figure 8: Rough scheme of an electric pressure measurement using a bourdon.25-Instrumentation reference book (knovel)26- Kobold pdf (desktop)27-Wika (desktop)28-baileymackey desktop

Flow Rate:Although flow rate measurements are not as common, as pressure and temperature, they are one of the most important types of measurements especially for chemical engineers. For any process that involves components and mixtures the flow rate of these components have to measure and specify to correctly analyze and design the process. Flow rate is the amount of materials that flows through an area at a given time. There are three types of flow rates: Volumetric flow rate (L /s), Mass flow rate (kg /s), and mole flow rate (mole /s). Although all of these types are important in designing a process, measurement devices are mostly used to measure volumetric flow rates. Flow rate measuring devices were developed through the years and they became more precise and sophisticated as the technology advanced. A simple flow rate measurement device will be a graduated cylinder with a stop watch, but due to the high error involved it is not a practical device to use especially in industry. Venturi Meter:Venturi meter is simply a pipe that converges to form a cone and then expands back. Since there is a convergence in a venturi meter the fluid is accelerated through the smaller area and causes a pressure drop. The pressure drop can be measured using any pressure measurement device and then the flow rate can be calculated from that pressure drop. The basic principle behind venturi meters is Bernoullis equation which is an energy balance between the inlet of the venturi tube and the outlet of the tube. Assuming that the friction losses through the tube is minimal and can be neglected is a valid assumption that simplifies the flow rate calculations through a venturi meter. Venturi meters could be used to measure the flow rate of liquids as well as gases.The main equation for calculating flow rate through a venturi meter is Bernoullis equation which is:

(17)

Assuming that there is no height difference:

Given that the volumetric flow rate is constant:

Substituting equation 19 into equation 18 and rearranging29:

Figure 9 shows a cross sectional area of a venturi tube. The fluid flows through the inlet area and the inlet cone starts getting narrower which causes a drop in pressure. As the fluid passes through the throat the cone start expanding again and the fluid retains its flow pattern. At the high and low pressure tap a differential pressure measuring device such as a manometer could be placed which will give the pressure drop between the inlet and the venturi throat. By measuring the pressure difference and knowing the inlet and throat area, the volumetric flow rate can be calculated easily from equation 20. Additionally a digital pressure measuring device could be attached to the high and low pressure tap which will provide a digital pressure reading that could be translated into a flow rate.

Figure 9: Short-form venturi tube with its components. (Image obtained from Industrial Instrumentation)29The flow rate range that a venturi meter can measure depends greatly on the size of the venturi tube used. Some venturi tubes goes up to (1.38 m)30. The range of a venturi tubes gets specified by the differential pressure that it can handle; some venturi tube can handle a nominal pressure of (6.3 MPa) 31 which translates for water into around (10,000 L/min). The accuracy of venturi tubes ranges from (0.5%)30 for (Reynold >75000) and (1.5%)30 for (Reynold>200000). However, the precision of venturi meters will depend on the pressure device that is used to take the measurements, some venturi tubes will have digital pressures with a precision of (10 Pa)31 which will translate into (10L/min) for water in a (6.3MPa) venturi tube.

There are many advantages to use a venturi meter for flow rate measurements; one of them is the head loss in venturi meters is low compared to other flow rate measuring devices such as an orifice plate. The benefit of having a small head loss is that you can have accurate measurements and you also dont lose your fluids pressure, which will force mean that a pump has to be attached to increase the flow rate of your fluid. Another advantage is that venturi meters can handle large flow rates along with small flow rates. Depending on the size of a venturi meter used you can measure both high and low flow rates. Moreover venturi meters accuracy doesnt get highly affected by the wear and the instillation conditions such as orifice plates30. Additionally venturi meters have less pipes relaxation than orifice plates which ensures that the pipes are not under stress and increase the safety of the process.

Although venturi meters have a lot of advantages, they also have some disadvantages that limits there applications. One disadvantage is that venturi meters are expensive. Taking into effect the cost of venturi meter limits there uses across a pipe, for example if you have a long pipe and you want to measure the flow rate across multiple places, it will be hard to use three or four venturi meters because of their cost. Additionally venturi meters are hard to replace, which makes it a problem when cleaning is required. Another disadvantage is that venturi meter only measures the pressure drop and an additional reader is needed in order to convert the pressure measurements into a flow rate values.

Since the venturi meters dont provide an electric signal, further modification is needed to convert it into an electric signal. In venturi meter what you measure is the pressure drop and not the flow rate directly. In order to convert the venturi measurements into electric signal, digital pressure gauges can be used to record the pressure values. Then these pressure values could be sent to a reader or software that could change the pressure readings into flow rate measurements. For the reader to be able to convert pressure values into flow rates the density, inlet area, and throat area of the venturi meter have to be specified. This means that if the fluid changes or another venturi meter, with different areas, was installed these configurations has to be reset to provide a correct pressure measurements. 29-Industrial_Instrumentation Flow (desktop)30-Omega (bookmark)31-alibab.com (bookmark)

Electromagnetic flow meters:Electromagnetic flow meters utilize faradays law to provide a volumetric flow rate measurement. The electromagnetic flow meters are simply two magnets that surround a pipe and as the fluid moves between these two magnets, it induces a magnetic field which can be measured and converted into velocity measurements. Given the diameter of the tube the velocity could be converted into a volumetric flow rate. Faradays law states that when a conductor moves through a magnetic field, it induces a voltage32. The induced voltage will depend on the velocity of the conductor, which is the fluid, and this voltage signal can be converted into a velocity value. This type of flow meters works for liquids only because gases dont have high conductivity that can induce a voltage when passed through a magnetic field.

The basic law behind electromagnetic flow meters is Faradays law, given that the velocity and the magnetic field are at right angles to one another and that the conductive fluid is along a line between the two electrodes Faradays law can be reduced to33:

E: electromotive force or the induced voltage (V)K: proportionality constant that can be electrically determined B: magnetic field (Tesla)D: the distance between the electrodes (m)v: average velocity of the fluid (m/s)Using the definition of flow rate:

A: cross sectional area (m2)Substituting equation 22 in equation 21:

Figure 10 shows a scheme of an electromagnetic flow meter. The pipe has a magnetic field going through it and two electrodes are mounted to the side of the pipe. As the fluid flows through the pipe, it acts like a conductor moving through an electric field. When a conductor moves through an electric field, it induces a voltage that is produced. The electrodes conduct the voltage produced to a Volta meter that measures the voltage. Given the area of the pipe, electromagnetic meter constant, and the magnetic field strength the volumetric flow rate can be calculated. The electromagnetic meter constant depends on the type of materials the electromagnetic flow meter is made from and its usually specified in the electromagnetic flow meter; however, it can be easily calculated given a known volumetric flow rate. Since the voltage induced across the electrodes is directly proportional to the volumetric flow rate, as you increase the flow rate the voltage induced increases.

Figure 10: Scheme of an electromagnetic flow meter. (Image obtained from Sensors magazine sensorsmag.com)34

The range of the electromagnetic flow meter varies greatly as the magnet, pipe, and electrode distance vary. But in general electromagnetic flow meters have a velocities range of (0.1- 10 m/s)35 which can measure up to (0.707 m3/s) given a pipe diameter of (300 mm)35. Additionally pipes inner diameters range from (9.40-300 mm)35. The accuracy of the electromagnetic flow meters also vary with the type used but they range about (0.5% to 1%) from the volumetric flow rate which is considered pretty accurate, for example a (0.7 m3/s) flow rate will have an inaccuracy of (7*10-3 m3/s). Additionally the precision of electromagnetic flow meters is (1*10-6 m3/s) which is highly precision.

There are many advantages that make electromagnetic flow meter a desirable flow rate measuring device. One advantage is that they produce an electric signal that could be transmitted into an automatic logging device to record data. Another advantage is that these flow meters have a high precision and accuracy. In industrial applications, especially the ones that require an accurate flow measurement such as in reactors, electromagnetic flow meter are favorable because of their high accuracy. Additionally electromagnetic flow meters can measure wide ranges of velocities going from (0.1 m/s to 10 m/s). One more advantage is that electromagnetic flow meter is easy to install and operate unlike orifices or venturi tubes. Easy instillation also means that they are easy to uninstall and move them from one pipe to another if it was needed. Also electromagnetic flow meters dont operate on head losses which means that they keep their original pressure and there is no need for additional pumping.

In the other hand there is couple of disadvantages that limit the usage of the electromagnetic flow meters and prevent them from being used in certain applications. One disadvantage is that electromagnetic flow meters get affected by electromagnetic fields; so if there was a magnetic field close to the measuring area, it will affect the measurements accuracy to great extent. Another disadvantage is that electromagnetic flow meters are limited to measuring a specific range of flow rates depending on the pipe size. If there was a higher flow rate, another electromagnetic flow meter has to be used. Additionally electromagnetic flow meters are expensive devices which mean that they can be used for only precise flow rate measurements. Since the electromagnetic flow meter operates using a magnetic field they can cause some disturbance to other machines and affect the precision of their measurements. One extra disadvantage is that electromagnetic flow meters are temperature sensitive and the environment temperature has to be in the range between (-5 to 55C)35 for them to work precisely.

Electromagnetic flow meter produces an electric signal which makes them preferable for automatic data logging. Also these electrical signals could be transmitted to a device further away from the pipes if there was a reason preventing the reader from being near the pipes, such as heat transfer or splashes. Additionally having an electric signal prevent error due to the operators bias which decreases the in imprecision in the measurement and provide a more reliable results.

32-Sensor technology handbook (Knovel)33-automaatika (desktop)34-Sensorsmag (bookmark)35- electromagnetic flow meter omega

Ultrasonic Flow Meter:Ultrasonic flow meters are devices that measure the flow rate of a fluid by utilizing the Doppler Effect. Doppler effect (Doppler shift) is the change on the waves frequency as the sources is moving away or toward the observer1. In ultrasonic flow meter, the device emits an ultrasonic signal which gets reflected by gas bubbles or solid suspended particles (discontinuities) in motion36. This device then utilizes the shift in frequency between the wave going out, of the device, and the wave coming in; which can then be related to the speed of the fluid. When ultrasonic waves are transmitted through the pipe, they get reflected by the discontinuities in the fluids motion with a slight shift in frequency that could be detected and related to the flow rate of the fluid36. Ultrasonic flow meters can be used to measure the flow rate for both liquids and gases; however, there are different devices for each one of them. Since the Doppler effect ultrasonic flow meters only works if there are suspensions in the liquid, this method doesnt work for clear liquids because they dont have any discontinuities in their motion that could reflect the ultrasonic waves. Another type of ultrasonic flow meters was developed which is called the time of flight ultrasonic flow meter37. This time of flight ultrasonic flow meter measure the time it takes for the ultrasonic wave to travel through a fluid, then the time difference between these two times can be related to the velocity of the fluid. In time of flight ultrasonic flow meter, usually the ultrasonic signal travels opposite to the fluid motion which causes a delay in the receiving time of the signal and this delay time could be related to the velocity of the fluid37. This type of flow meter doesnt need require any discontinuities in the flow so it can work perfectly for clear fluids.

The main equation for the Doppler effect ultrasonic flow meter is derived from the Doppler effect. The velocity of the fluid in a Doppler effect flow meter is37:

For this equation to be used the speed of sound through the fluid has to be specified. In addition, the main equation for the time of flight ultrasonic flow meter is37:

Figure 11 shows a Doppler effect ultrasonic flow meter. The flow meter device measures the velocity of the fluid by transmitting an ultrasonic wave across the pipe. When this ultrasonic wave gets hit by a discontinuity in the fluids motion it reflects it back to the receiver device. Then the frequency shift is calculated

Figure 11: Scheme of the Doppler effect ultrasonic flow meter working principle (Image was obtained from Engineeringtoolbox.com)37Figure 12 shows a time of flight flow meter. As the fluid passes through the pipe, the transmitting device sends an ultrasonic signal across the pipe. Without the fluid flowing the ultrasonic wave would reach at a specific time (t) but as the fluid flows through the pipe, it will delay the receiving time of the signal. This delay in the receiving time can be measured and used to calculate the velocity of the fluid.

Figure 12: Scheme of time of flight ultrasonic flow meter. (Image obtained from Engineeringtoolbox.com)37

The range of the Doppler effect ultrasonic flow meter differs greatly based on the type of fluid flowing, pipe size, and the quality of the ultrasonic transmitter. But generally the velocity ranges from (o.1 to 10 m/s)38 and sometimes it can go higher to around (50 m/s)39. Additionally the pipes sizes vary and can go from (76mm to 3m)39 in diameter. Which will correspond to flow rates of (4.5*10-4 to 0.71 m3/s)? The accuracy of Doppler effect ultrasonic flow meter is (1%)38 and can go as high as ( 5%)38 for high flow rates. The precision of Doppler effect flow meters is about (1*10-4 m3/s)38.

Although time of flight ultrasonic flow meter has the same range as the Doppler effect flow meter for the low flow (0.1 to 10 m/s)38, at the high flow measurements the time of flight ultrasonic flow meter can go from (0.03 to 100 m/s)39. Depending on the type of flow measured, time of flight flow meters have an accuracy of (1%)38 for the low flow rates and (5%)38 for the high flow rates measurements such as the Doppler effect ultrasonic flow meters. However for the precision time of flight flow meters have higher precision about (2*10-5 m3/s)39.

The advantages that make Doppler effect ultrasonic flow meter desirable in industrial applications are these flow meters dont obstruct flow such as the case for orifices or venturi tubes. Another advantage is that Doppler effect ultrasonic flow meter doesnt causes any pressure losses which can effect the motion of the fluids and require additional pumping. Additionally they dont require a lot of power to operate and taking measurements with them. Reducing power costs is required in industrial applications so that they can reduce the price of operation which can allow for more profits or less expensive products. One extra advantage is that Doppler effect ultrasonic flow meter are corrosion resistance which makes them ideal for measuring the flow rate of corrosive fluids such as concentrated acids or bases. On the other hand there is couple of disadvantages that limit the uses of Doppler effect ultrasonic flow meter. One disadvantage is that Doppler effect ultrasonic flow meter can only measure the flow rate if there was discontinuity in fluids motion, so if the fluid is clear these devices cant be used to measure the flow rate. Having some solid depositions in a fluid can sometimes damage your machines especially if the fluid was sent through a heat exchanger or a reactor. Another disadvantage is that Doppler effect ultrasonic flow meter require a power source to operate so they cant be easily moved from one spot to another. One extra disadvantage is that Doppler effect ultrasonic flow meter are sensitive to the fluids density and temperature. Because Doppler effect ultrasonic flow meter uses the sonic speed through the fluid to make velocity measurements, density and temperature can affect the speed of sound through the fluid which will also affect the fluid velocity measurements.

For time of flight ultrasonic flow meter, there are lots of advantages that favor them, in some applications, over other flow rate measurement devices. One advantage is that time of flight ultrasonic flow meter dont obscure the flow of fluid. Such as the Doppler effect flow meter, obstructing the flow path can have undesirable effects and sometimes generate turbulence where its not needed. Another advantage is that time of flight ultrasonic flow meter dont get effected by temperature or fluids density. Unlike Doppler effect flow meter, time of flight ultrasonic flow meter can measure the fluids velocity pretty accurately even if there was temperature changes and varying density. Additionally time of flight ultrasonic flow meter can measure the flow rate of clear fluids which make them desirable in area that are close to a reactors opening or a heat exchanger. One extra advantage is that time of flight ultrasonic flow meter are bi-directional flow measurement devices37. Whether the fluid is going from left to right or opposite these devices can still get an accurate measurement of the fluids flow rate.

As with any measurement device, time of flight ultrasonic flow meters have couple disadvantages that needs to be considered when using them to measure the flow rate of fluids. One disadvantage is that time of flight ultrasonic flow meters are not self-powered devices and they actually require a power source for them to operate. Another disadvantage is that the operating principle for the time of flight ultrasonic flow meter requires a high frequency sound transmitted through the pipe37. If there were some slurries in the fluid or gas bubbles, this could affect the accuracy of the measurements. One extra disadvantage is that time of flight ultrasonic flow meter cannot reliably measure the velocity of Liquids with entrained gases. This means that the fluids have to be purified before entering the measurement pipe.

Both Doppler effect ultrasonic flow meter and time of flight ultrasonic flow meter produces an electric signal which can be transmitted to an automatic logging device. Electric signals are also helpful because they eliminate errors due to operators bias. Additionally having an electronic signal means that you could place your reader, which is often expensive, far away from the piping and the measuring area to avoid heat or splashes .36-Omega (bookmark)37-Engineering toolbox (bookmark)38-The essential guide to flow measurements (desktop)39-Ge measurments and control

Task fourConcentration:

Another important property that needs to be specified in any process is concentration. Concentration is simply how much solute is dissolved within a solution1. Probably not as important as temperature or pressure but concentration is an important parameter to be measured for any given processes. As a chemical engineer, for any reaction or separation process both the concentration of your inlet and outlet has to be specified in order to correctly calculate and predict the outcome of a process. Through the years many devices have been developed to measure the concentration of solutions and probably the most common one is spectrometers.

Refractometers:The basic idea behind a refractrometer is the measure of index of refraction through a solution. Since the index of refraction is dependent on concentration, as well as temperature and path length, measuring the index of refraction of a solution can be used to determine its concentration40. As the concentration of the solution increases, the index of refraction also increases in a linear relationship. By measuring the index of refraction, the concentration of the solution can be determined given the type of solution because for every substance there will be a different relation between the index of refraction and the concentration40. One way to determine concentration from index of refraction is to measure the index of refraction for couple of standard solutions, known concentration, and then determine the concentration of your sample from the fit of the curve.

The basic law behind Refractometers is snells law25:

By determining () the index of refraction of the solution can be calculated as:

With and determined in the experiments apparatus.

Figure 13 shows one type of commonly used refractrometers called Abbe refractrometer. In abbe refractrometer, light is shined through two surfaces A and B. While the transmitted light goes from B to A, it gets refracted at the interface between A and B. Then a receiving device receives the transmitted light and calculates the angle it was received at compared to a normal line. By knowing the angle of the light going in, angle of the light out, and the index of refraction of B; index of refraction of A can be calculated from equation 27 (snells law). In general B will be air with an index of refraction of 1 or glass and A will be the solution we want to determine its concentration.

Figure 13: A scheme of an Abbe refractrometer. (Image was obtained from Instrumentation reference book knovel.com)25

Refractrometers are commonly used devices in research and industry to measure the concentration of solutions. The range of refractrometers vary greatly depending on the type of solution used, temperature, and the model of refractrometer used; but in general they have a range of (0 to 120 g/L)41 which is considered a wide range for most application. The accuracy of these devices is about (0. 1 g/L)41. Additionally refractrometers have a high precision of (0.001 g/L)41 which is more precise than most of the concentration measuring devices. There are many advantages that make refractrometers a desirable concentration measuring device, one of these advantages is that refractrometers produces a digital measurement that could be translated into an electric signal for automatic data logging. Another advantage is that refractrometer have a high accuracy and precision which makes them reliable in taking sensitive concentration measurements especially in research. Additionally refractrometers are inexpensive devices and provide a wide range of concentration measurements for a good price. One extra advantage is that refractrometers are easy to use and to get readings from compared to other complicated devices such as spectrometers.

On the other hand there is couple of disadvantages that limit the use of refractrometers such as temperature sensitivity. Since refractrometer uses index of refraction to obtain concentration measurements, temperature changes can produce an inaccuracy in concentration measurements. Another disadvantage is that refractrometers are not self-powered and they need a power source in order for them to operate. One extra disadvantage is that refractrometers need to be cleaned regularly because any dust or solid particles can change the index of refraction and causes an inaccuracy in the concentration measurements.

Since refractrometers already produces an electric signal, there is no need for further modification in order for them to do so. Using an electronic signal eliminates the operators bias error from the measurement which produces more accurate measurements and can be used for automatic data logging. 40-Kpatent (desktop)41-Omega (desktop)

Conductivity Probes:The basic idea behind a conductivity probe is measuring the concentration of a solution by measuring its conductance42. As the concentration of a solution increases the number of ions available to conduct electricity increases, so as the concentration increases the conductance also increases. Additionally all solutions are made from anions and cations and once an electric current is passed through the solution, they get separated as the anions travel toward the positive electrode and the cations travel toward the negative electrode; this movement of the anions and cations in a solution conduct electricity through solution and can be used to determine the concentration of the solution. There are couple of conductivity probes that can be used to determine the concentration of a solution and they all use the same principle stated above; one type of conductivity probes is toroidal conductivity probe and another common probe is the conductivity cell.

The basic law for conductivity probes is42:

Molar conductivity of an electrolyte at infinite dilution ( ) is given by Kohlrausch's law42:

Both and can be obtained from tables based on the ions present in the solution. From equation 28 and equation 29:

Figure 14 shows the various components of a conductivity cell. As the solution flows through the conductive cell probe, an electric current is passed through the electrodes in the side. The metal or graphite electrodes supply a current that ionizes the solution into cations and anions and then the conductivity of the solution is measured. Electrodes are usually made from graphite to prevent corrosion as these probes are used to measure concentration of highly corrosive solutions. The insulating materials in the pipe is used to prevent fluid or electricity leakage. As the solution flows through the conductivity cell and the conductance is measured, concentration can be determined from these parameters given the type of solution flowing to determine the ionic conductivities of the anion and cation.

Figure 14: Cross section of a conductivity cell with major components (Image obtained from Coulson and Richardsons Chemical Engineering book knovel.com)42

The range of conductivity probes vary depending on the type of electrodes used and the model of conductivity probe used. In general conductivity probes can measure a conductance range of (10 mS/cm to 1 S/cm)43; which will measure a various concentration depending on the solution used. Table 3 shows the concentration range for some solutions that conductivity probe can measure; however, there are more elements that can be measure using the conductivity probe. Additionally a known standards of a solution can be prepared and then measure their conductivities so that a linear equation could be obtained and then used to determine the concentration of the unknown solution.

Table 3: Some elements with concentration range that can be measured using conductivity cell. (All data was obtained from omega.com)43

ElementConcentration range (g/L)

HNO0 280

HF0 300

NaOH0 300

HSO0 800

NaCl0 260

NaSO0 220

HCl200 350

The accuracy of conductivity probes ranges between (0.1 to 0.01 mS/cm)43 depending on the conductivity range used and the cell constant. The cell constants offered for a conductivity probe are (0.01, 0.1, and 1 cm-1 )43. Conductivity probes can measure the conductivity to a high precision of about (0.001 mS/cm)43.

There are several advantages that make conductivity probes a desirable concentration measuring devices one of which is that conductivity probes can measure a wide range of concentrations. Given that they have a high conductance measuring range, they can measure concentrations up to (1000 M). . Another advantage is that conductivity probes can measure the concentration of corrosive solutions without being affected. Since the electrodes are made from graphite or highly corrosion resistance metals, the life time of conductivity probes are higher than many other concentration measuring devices44. Additionally conductivity probes are insensitive to contaminations in solutions. Unlike many concentration measuring devices, such as spectrometers, conductivity probes dont get affected by contaminations in solutions44. One extra advantage is that conductivity probes produce an electric signal which help with automatic data logging and removing the operators bias error.

Conductivity probes also have couple of disadvantages than needs to be considered before using them for concentration measurements. One disadvantage is that conductivity probes dont produce a direct concentration measurement but measure the conductance through a solution and then concentration could be measured. Another disadvantage is that conductivity probes are somewhat expensive compared to other concentration measuring devices. Moreover conductivity probes are temperature sensitive; to get an accurate measurements temperature has to be within the range of the device or it will not produce an accurate measurements. 42- Coulson and Richardson (knovel)43- Omega (Flash drive)44-knick (flash drive)

Colorimeter:Colorimeters use the same principle as spectrometers but with the exception of using only visible light. Colorimeters operate based on measuring the intensity of color emitted from a sample then relating it to concentration45. The absorbance and transmission through a solution could be related to concentration through Beers law. Colorimeter shines light with different wavelength that can be determined manually, and then measure the intensity of light going out from the sample.

Transmittance through a solution could be calculated as45:(31)

T: Transmittance

Absorbance could be related to transmittance as45:(32)

A: absorbance

Using Beer Lamberts Law, absorbance is related to concentration as:(33)

Figure 15 shows a calorimeter with the essential components of the device. To measure the concentration of a sample, the sample is placed between the light source and the light detector. Then the light source shines a light with a specific wavelength through the sample. The light passes through the sample and some of it gets absorbed. The intensity of light coming out is then measured and related to the light intensity of the initial light. By measuring the intensity of the light in and out of the sample transmission can be calculated. Absorbance can then be calculated from transmittance and concentration could be related to absorbance through Beer lambert Law.

Figure 15: A scheme of colorimeter that shows the different components of the device. (Image was obtained from California State University Stanislaus)46 The range of colorimeter will vary depending on the available wavelengths. In general there are four different wavelengths corresponding to different colors: 470 nm for blue, 525nm for green, 591nm for yellow, and 625nm for red. The absorbance range that a colorimeter can measure (manufactured by Vernier), is (0.05 to 1.00)45 and the transmission ranges between (10% to 90%)45. The accuracy of the colorimeter is ( 0.14% transmittance)45. Additionally the colorimeter have a high precision of (0.02% transmittance)45.

There are many advantages that make colorimeter a desirable device for measuring concentration, one of them is that colorimeter provides a direct electrical signal. The direct electrical signal is useful for automatic data logging and it is easier to record. Another advantage is that colorimeters have a high accuracy and considered a reliable measuring devices especially for educational and research applications. One additional advantage is that colorimeters only uses visible light, which makes them safer than other devices that uses ultraviolet rays to measure concentration. Moreover colorimeters are easy to handle and take measurements with, unlike spectrometers which requires an experienced operator.

On the other hand there are couple of disadvantages for colorimeters. One disadvantage is that colorimeters need a power source to operate. Having to supply power to the device to operate, limits the mobility of the device. Another disadvantage is that colorimeters needs special tube to use for measuring absorbance which requires constant cleaning and purchasing of new tubes if needed. Additionally colorimeters cant be used to measure the concentration of flows since they can only measure a specific amount of the sample at a time; which makes them undesirable for industrial applications that requires the constant measurement of concentration.

Since colorimeters already produces an electric signal there is no further modifications need to be done. Having a direct electric signal can be used for automatic data logging. Additionally being able to transmit the electrical signal makes it possible to place the reader far away from the measuring site which ensure the safety of the computer.

45- Colorimeer (vernier)46- science.csustan.edu