PRESENTED BYPREM BABOOM.Sc.,B.Tech(Chemical Engineering),M.Phil, M.B.A
Fellow of Institution of Engineers IndiaAn Expert for www.ureaknowhow.com
WHAT IS TEMPERATURE?
• Qualitatively, the temperature of an object determines the sensation of warmth or coldness felt by touching it. More specifically, temperature is a measure of the average kinetic energy of the particles in a sample of matter, expressed in units of degrees on a standard scale.
• Temperature is the degree of "hotness" (or "coldness"), a measure the of the heat intensity. When two objects of different temperature are in contact, the warmer object becomes colder while the colder object becomes warmer. It means that heat flows from the warmer object to the colder one.
• Thermometer helps us determine how cold or how hot a substance is. Temperatures in science (and in most of the world) are measured and reported in degrees Celsius (oC). In the US, temperatures are commonly reported in degrees Fahrenheit (oF). On both Celsius and Fahrenheit scales, the temperature at which ice melts (water freezes) and the temperature at which water boils are used as reference points.
• On the Celsius scale, freezing point of water is defined as 0 oC, and the boiling point is defined as 100 oC.
• On the Fahrenheit scale, water freezes at 32 oF and boils at 212 oF.
• On the Celsius scale there are 100 degrees between freezing and boiling of water, compared to 180 degrees on the Fahrenheit scale. This means that 1 oC= 1.8 oF.
• Thus the following formulas can be used to convert temperature between the two scales:
• (1) F = 1.8 C + 32 = 9/5 C + 32 (2) C = 0.56( F -32) =5/9( F-32)
DEG F DEG C DEG RDEG K
ABSOLUTE ZERO
ICE POINT
STEAM POINT
00-273-459
491273032
671373100212
Types Of Measurement
• There are four basic types of temperature measuring devices, each of which uses a different principle: – Mechanical (liquid-in-glass thermometers,
bimetallic strips, etc.). – Thermojunctive (thermocouples). – Thermoresistive (RTDs and thermistors). – Radiative (infrared and optical pyrometers).
Mechanical temperature measuring devices
• Principle of operation: – A change in temperature causes some kind of
mechanical motion, typically due to the fact that most materials expand with a rise in temperature. Mechanical thermometers can be constructed which use liquids, solids, or even gases as the temperature-sensitive material.
– The mechanical motion is read on a physical scale to infer the temperature.
Liquid-in-glass thermometer
– The most common and well-known thermometer is the liquid-in-glass thermometer.
– As the temperature rises, the liquid expands, moving up the tube. The scale is calibrated to read temperature directly.
• Usually, mercury or some kind of alcohol is used for the liquid
Bimetallic strip thermometer
– Two dissimilar metals are bonded together into what is called a bimetallic strip, as sketched.
– Suppose metal A has a smaller coefficient of thermal expansion than does metal B.
– As temperature increases, metal B expands more than does metal A, causing the bimetallic strip to curl upwards as sketched.
Working of bimetallic strip
Pressure thermometer • considered mechanical, operates by the
expansion of a gas instead of a liquid or solid. (Note: There are also pressure thermometers which use a liquid instead of a gas.)
• Suppose the gas inside the bulb and tube can be considered an ideal gas. The ideal gas law isPV = mRT
oR is a constant. The bulb and tube are of constant volume, so V is a constant. Also, the mass, m, of gas in the sealed bulb and tube must be constant. Hence, the above equation reduces to P = constant times T.
– A pressure thermometer therefore measures temperature indirectly by measuring pressure. • The gage is a pressure gage, but is
typically calibrated in units of temperature instead.
Pressure thermometer
MERCURY IN STEEL THERMOMETER
• MERCURY - IN - STEEL thermometer works on the principle of expansion of mercury due to rise in temperature. The whole system is filled with mercury under pressure. The definite volume of mercury contained in the bulb expands under effect of temperature to be measured. The increaase in volume of confined mercury is transmitted through the capillary to the coiled burdon tube which uncoils proportionally to volume increase resulting an indication of temperature by the pointer.
GAS FILLED DIAL THERMOMETERS
• Gas Filled Thermometers deploy Nitrogen Gas at high pressure as an expansion gas filled into a closed system comprising of a bulb (of Steel or Chrome – Moly Steel), a microbore capillary (of Steel or Stainless Steel) and a spiral or ‘C’ shaped Bourdon Tube (of MS or SS). This system when heated at bulb end the Gas in the bulb expands and a pressure is generated within which moves the spiral / ‘C’ Shaped bourden as it is the only the elastic element. This movement is transmitted to a rack and pinion movement which drives a pointer thus showing temperature on a calibrated dial.
THERMOCOUPLE
• The Thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, e.g., one chromel and one constantan. These two wires are connected at two different junctions, one for temperature measurement and the other for reference. The temperature difference between the two junctions is detected by measuring the change in voltage (electromotive force, EMF) across the dissimilar metals at the temperature measurement junction.
CONSTRUCTION OF THERMOCOUPLE
Typical Thermocouple Circuit
CONSTRUCTION OF THERMOCOUPLE
CONSTRUCTION OF THERMOCOUPLE
GROUNDED JUNCTION
UNGROUNDED JUNCTION
EXPOSED JUNCTION
LIFE OF THERMOCOUPLE
THERMOCOUPLE
THERMOCOUPLE
TYPICAL RTD CIRCUIT
RTD ELEMENTS
LEAD EFFECTS
3 WIRE RTD
Thermistors
– A thermistor is similar to an RTD, but a semiconductor material is used instead of a metal. A thermistor is a solid state device.
– A thermistor has larger sensitivity than does an RTD, but the resistance change with temperature is nonlinear.
– Furthermore, unlike RTDs, the resistance of a thermistor decreases with increasing temperature.
– Thermistors cannot be used to measure high temperatures either, compared to RTDs. In fact, the maximum temperature of operation is sometimes only 100 or 200 oC.
From the circuit diagram, it is clear that this is a simple voltage divider. Rs is some fixed (supply) resistor. Rs and the supply voltage, Vs, can be adjusted to obtain the desired range of output voltage Vo for a given range of temperature.
Radiative temp.measuring devices (radiative pyrometry)
• Principle of operation: – Radioactive properties of an object change with
temperature. – So, radioactive properties are measured to infer the
temperature of the object. – The advantages of radioactive pyrometry are:
• There is no physical contact with the object whose temperature is being measured.
• Very high temperatures can be measured.
• The fundamental equation for radiation from a body is the Stefan-Boltzmann equation,
– where • E is the emissive power radiated per unit area (units
of W/m2). • is the emissivity, defined as the fraction of blackbody
radiation emitted by an actual surface. The emissivity must lie between 0 and 1, and is dimensionless. Its value depends greatly on the type of surface. A blackbody has an emissivity of exactly 1.
• is the Stefan-Boltzmann constant,
• T is the absolute temperature of the surface of the object (units of K).
Infrared Pyrometer
• An infrared pyrometer infers the temperature of a hot surface by measuring the temperature of a detector inside a detector chamber as shown below:
IDENTIFICATION BY COLOUR
• DARK RED :540 DEG C
• MEDIUM CHERRY RED :680 DEG C
• ORANGE :900 DEG C
• YELLOW :1010 DEG C
• WHITE :1250 DEG C
Features of Pyrometer– An optical pyrometer is useful for measuring very
high temperatures (even flames). – An optical pyrometer works by comparing a
glowing wire of known temperature to the glow (optical radiation) from a hot object.
– When the internal wire and the glow of the object are the same color, the temperatures are assumed to be equal.
• The temperature of the internal wire is controlled and known, and thus the temperature of the object can be inferred
Attribute Thermocouple RTD Thermistor
Cost Low High Low
Temperature Range
Very wide-450ºF
+4200ºF
Wide-400ºF
+1200ºF
Short to medium-100ºF+500ºF
Interchange ability
Good Excellent Poor to fair
Long-term Stability
Poor to fair Good Poor
Accuracy Medium High Medium
Repeatability Poor to fair Excellent Fair to good
Sensitivity (output)
Low Medium Very high
Response Medium to fast Medium Medium to fast
Linearity Fair Good Poor
Self Heating No Very low to low High
Point (end) Sensitive
Excellent Fair Good
Lead Effect High Medium Low
Size/Packaging Small to largeMedium to
smallSmall to medium