Electronic Calibrator

ELECTRONIC CALIBRATOR FOR ELECTRONIC CALIBRATOR FOR CONDUCTIVITY TRANSMITTER CONDUCTIVITY TRANSMITTER

AND AND DO TRANSMITTERDO TRANSMITTER

A PROJECT BY:

SHUCHI PARASHAR B.TECH(EC)VII

SEMESTER ROLL NO.: 6578

ID No. BTBTE07059

Overview of Forbes Marshall For over half a century, Forbes Marshall has been building steam

engineering and control instrumentation solutions that work for process industry. Forbes Marshall has grown from a modest; Mumbai based trading company to a multi-divisional, ISO 9001 certified global company manufacturing advanced engineering products for process and power industries across the World.

Forbes Marshall’s goal is to provide solutions in Energy, Efficiency and Process Automation, using the best technology the world has to offer.

Some of the major products of Forbes Marshall are Conductivity, DO, pH 2-wire and 4- wire transmitters, dust monitors/opacity monitors; gas analysers, which can analyse up to 7 gases, flow controllers, electromagnetic flow meters, and ultrasonic flow meters, vortex flow meters, control valves steam, pressure regulators, back

pressure regulator (reverse of pressure regulator) steam and water analyzer; Vibtrans, it is a digital vibration monitor and transmitter etc.

INTRODUCTION Electronic Calibrator for Conductivity transmitter and DO transmitter A gadget for Testing and Calibration of Conductivity Transmitter

and Dissolved Oxygen (DO) Transmitter.

The calibrator is specially designed for the SMART Pro Transmitters.

This calibrator is an integrated form of the present method used for the testing and calibration.

It calibrator consists of power supply, conductivity simulator, pH simulator, PT100 simulator and NTC simulator.

Also it has HART connectivity and open collector relay outputs.

More efficient and accurate.

STUDY OF SMART Pro TRANSMITTERS

pH Transmitter SMART Pro 8966

Conductivity Transmitter SMART Pro 8967

Dissolved Oxygen Transmitter SMART Pro 8968

General description Microcontroller based user

friendly menu based two wire transmitters.

The SMARTPro 8966/67/68 are HART compatible.

It takes the input and converts It into 4-20 mA of current output.

Enclosure meets IP67 protection and protection against electromagnetic interference.

Front view Side view

General description The transmitters have inbuilt

non-volatile memory, Galvanically isolated current

output separately adjustable high and

low set-point hysteresis and delay for relay output.

Built with HALL EFFECT electromagnetic sensors.

Measurement results are displayed on UV protected LCD display.


Specifications:

SUPPLY Type 2 wire Supply voltage 12-36VDC Load Resistance 600V @24VDC ACCURACY Display Conductivity ±0.5% of Full Scale Display Temperature ±0.5% of Full Scale Output Current ±0.1mA OUTPUTS Current 4-20mA 1.linear 2.Bilinear 3.Log Logic outputs 3 User configurable “Open collector” output Display 2 Line 10 Character per line LCD

INPUTS Primary Sensors Conductivity Secondary sensors PT100/PT1000 Data Input 3 Button Keypad and Magnetic pin Temperature -20 to 70 oC


Theory of Conductivity

What is conductivity?

How well a solution conducts electricity depends on a number of factors:

• Concentration

• Mobility of ions

• Valence of ions

• Temperature


How is conductivity measured? Conductivity may be measured by applying an alternating electrical

current (I) to two electrodes immersed in a solution and measuring the resulting voltage (V).

Migration of ions


The Importance of Conductivity

The measurement of conductivity is a rapid and inexpensive way of determining the ionic strength of a solution. However, it is a nonspecific technique.

Quality control purposes. Surveillance of feed water purity. Control of drinking water. Process water quality. Estimation of the total number of ions in a solution.


What is a conductive solution?

Conductive solution or Electrolytes are substances containing ions. The ions formed in solution are responsible for carrying the electric current. Electrolytes include acids, bases and salts and can be either strong or weak. Most conductive solutions measured are aqueous solutions, as water has the capability of stabilizing the ions formed by a process called solvation.


Definition of some terms:

Resistance The resistance of the solution (R) can be calculated using Ohm’s law V = R x I R = V/I

Conductance Conductance (C) is defined as the reciprocal of the electrical

resistance (R) of a solution between two electrodes. C = 1/R (S) The conductivity meter in fact measures the conductance, and displays

the reading converted into conductivity.


Cell constant This is the ratio of the distance (d) between the electrodes to the area

(a) of the electrodes. K = d/a Conductivity Conductivity is the ability of a solution to pass current. κ = C • K κ = conductivity (S/cm) C = conductance (S), where C = 1/R Resistivity This is the reciprocal of the conductivity value and is measured in

ohm•cm. It is generally limited to the measurement of ultrapure water, the conductivity of which is very low.


Calibration Determination of the cell constant required to convert conductance

readings into conductivity results.

Conductivity Temperature Compensation Conductivity measurements are temperature dependent. The degree to

which temperature affects conductivity varies from solution to solution and can be calculated using the following formula:

Ct = Ctcal {1 + α (T-Tcal)}

Where: Ct = conductivity at any temperature T in °C Ctcal = conductivity at calibration temperature Tcal in °C, α = temperature coefficient of solution at Tcal in °C.


Total Dissolved Solids (TDS) This is the measure of the total concentration of ionic species of a

sample. Its magnitude is relative to the standard solution used to calibrate the meter.

TDS factor Conductivity readings are converted to TDS readings by multiplication

with a known mathematical factor. The factor depends on the reference material used to prepare the standard.

Salinity Salinity is a measurement without unit corresponding to the weight of

dissolved salts in seawater.


Operating Principle

A typical conductivity transmitter applies an alternating current (I) at an optimal frequency to two active electrodes and measures the potential (V).

Both the current and the potential are used to calculate the conductance (I/V). The conductivity meter then uses the conductance and cell constant to display the conductivity.

Conductivity = cell constant x conductance

A sinusoidal voltage of known amplitude powers the measuring cell. A jumper setting on CPU card can select the frequency of this excitation signal as either 70Hz or 1100Hz. An oscillator of low output impedance to minimize cable capacitance effect delivers this signal.


Operating Principle

The amplifier converts the current going through the probe in voltage by means of detector circuit, which eliminates the effect of external interferences and capacities.

A special ADC ADG452 digitizes the analog voltage, suppresses the noise on the signal and gives accurate conductivity and temperature measurements. The digital data is further processed by a dedicated microcontroller PIC18LF672D.

The Microcontroller further processes the data, displays the results on LCD display and generates retransmission current output with HART.

The Microcontroller accepts the inputs from keyboard during programming and calibration. The application specific parameters such as measuring range, temperature compensation are user programmable through keyboard.


Operating Principle

Block diagram


Conductivity Electrode/Sensor/Cell

Conductivity electrodes are cylindrical and the two electrodes (cathode and anode) are arranged concentrically, the outer electrode is cathode and the inner is anode. The electrodes are usually made of platinum metal.

An AC signal is given to one of the electrodes and the sensor is

dipped in the material whose conductivity has to be found. Due to the resistance of the material there is some change or attenuation in the signal provided (in volts) this change is calculated and since conductivity (C) is related to the resistance (R)

R =K/C; K is cell constant

Then the change is calibrated and the value displayed in terms of conductivity (S/cm) on the LCD display.


Conductivity Electrode/Sensor/Cell

Testing of conductivity electrodes

Continuity test

HV (high voltage)/megger test

Resistance measurement


Applications

Water treatment/process industries

Chemical/petrochemical industries

Steel industries

Pharmaceutical industries etc.


Specification

SUPPLY

ACCURACY Display pH ±0.01 pH Display mV ±1 mV Display Temperature ±0.5% of Full Scale Output Current ±0.1mA

OUTPUTS INPUTS Primary Sensors pH Secondary sensors PT100/PT1000 Data Input 3 Button Keypad and Magnetic pin Temperature -20 to 70 oC RANGES pH 0-14pH ORP -2000mV to 2000mV


Theory of pH

pH is defined as follows:

The lower case letter “p” stands for the negative common (base ten) logarithm, while the upper case letter “H” stands for the element Hydrogen. Thus, pH is the logarithmic measurement of the number of moles of hydrogen ions (H+) per liter of solution.

pH = -log [H+]

The pH term translates the value of hydrogen ion concentration, which ordinarily ranges between about 1 and 10× -14 gram equivalents per liter into numbers between 0 and 14.


Theory of pH

The pH loop is made up of three components, the pH sensor, a preamplifier and a transmitter.

The measuring electrode is sensitive to the hydrogen ion concentration, develops a potential (voltage) directly proportional to the hydrogen ion concentration of the solution. The reference electrode provides a stable potential. The preamplifier is a signal-conditioning device; it takes the high impedance pH electrode signal and changes it into the impedance, which the analyser can accept. The comparison and measurement is done in the transmitter and the pH value corresponding to that measured voltage is displayed.


Operating Principal

The mV signal from the pH electrode is given to the high input

impedance preamplifier. This amplifier is internal to the transmitter when the sensor is very close to the transmitter.

A special A/D convertor digitizes the analog voltage, suppresses the noise on the signal and gives accurate pH and temperature inputs to microcontroller.

The microcontroller further processes the data, displays the results on LCD display and generates retransmission current output. The microcontroller accepts the inputs from keyboard during programming and calibration.


Operating Principal

Block diagram


pH Sensor/Electrode

The pH electrode consists of two electrodes: Measurement electrode: which must be constructed of a special

glass, must be fragile to create the ion selective barrier to screen out all the hydrogen ions from other floating ions in the solution and is chemically doped with lithium ions, which make it, react electrochemically to hydrogen ions. Its purpose is to measure the solution’s pH.

Reference electrode. It is made from a neutral solution of pH. There is a porous separator to exchange ions. Its purpose is to provide the stable, zero-voltage connection to the liquid solution.


pH Sensor/Electrode


pH Sensor/Electrode

When the sensor is immersed in the solution according to the hydrogen ion concentration the voltage of the measuring electrode changes while there is no change in the voltage of reference electrode, which is a neutral solution. A solution in the reference electrode also makes contact with the sample solution and the measuring electrode through a junction, completing the circuit.

Testing of pH electrode Using 4.01 pH Using 7 pH (neutral) Using 9.21 pH


Applications

pH measurement is a very important in many liquid chemical processes.

Paper/ textile/biotechnology

Pharmaceutical industry

Manufacturing industry

Food production application

Water treatment plant.

Dissolved Oxygen (DO) Transmitter SMART Pro 8968Specifications

SUPPLY ACCURACY Display DO concentration ±0.5% of Full scale Output Current ±0.1mA OUTPUTS INPUTS Primary Sensors Dissolved Oxygen Secondary sensors NTC 22K Data Input 3 Button Keypad and Magnetic pin Temperature -20 to 70 oC Ranges DO Saturation 0.0 to 500% Concentration 0.05 to 50.0 ppm or mg/l Pressure 0.000 to 4.999 Bar (user defined) Relative Humidity 0 to 100% (user defined) Salinity 0.00 to 50.00 (user defined)

Dissolved Oxygen (DO) Transmitter SMART Pro 8968Theory of Dissolved Oxygen

Every liquid takes up oxygen until the partial pressure of oxygen in the liquid is in equilibrium with air or the gas phase in contact with it.

This physical distribution of gaseous oxygen in liquid is called DISSOLVED OXYGEN. The main sources of dissolved oxygen in water are the atmosphere and photosynthesis.

Quantity of oxygen dissolved in water depends on the following factors:

Atmospheric pressure Atmospheric Temperature Salinity of water: Dissolved impurities in water reduce the solubility

of oxygen. Oxygen concentration depends on temperature and pressure:

O2 (conc.) Pressure 1

Temp.

Dissolved Oxygen (DO) Transmitter SMART Pro 8968Theory of Dissolved Oxygen

Dissolved oxygen is measured in terms of the following parameters: % Saturation Mg/liter or PPM

The various types of Dissolved oxygen transmitters available are: Polarographic (Clark Cell Method) Sensor DO transmitter: Use an

external voltage keeping the potential difference between the anode and cathode to less than 0.5 volts. Power is supplied by DO transmitter. This sensor has a silver anode surrounded by a gold cathode.

Galvanic Sensor Oxygen Meter: It does not use an external voltage the DO sensor produce its own current and the difference between the anode and cathode is greater than 0.5 volts. These are more stable and accurate when compared to the Polarographic oxygen meters. This electrolyte is usually silver and lead with a potassium hydroxide electrolyte.

Dissolved Oxygen (DO) Transmitter SMART Pro 8968DO Sensor

The sensor used for DO measurement is an electrochemical sensor. This sensor consists of two metallic electrodes dipped in electrolyte. They use gas permeable membrane to allow only O2 to pass to the sensor. And effectively blocks all the other interfering molecules that may cause undesirable current and faulty readings.

The galvanic oxygen probe consists of two metals. These two metals produce some voltage in mVolts in presence of electrolyte.

There are 2 types of electrodes used: Dissolved Oxygen electrodes using a Galvanic cell


oxygen electrodes using a Polarograph:


The current flow is established between anode and cathode, O2

molecule dissolved in electrolyte travels to cathode and gains electron and produces hydroxyl ion combination with water. Hydroxyl ions give electron at anode and forms oxide. The diffusion rate of oxygen through this membrane is proportional to partial pressure of oxygen in sample.

The current generated by the sensor is in turn proportional to the oxygen concentration in sample. This current is measured by the transmitter and displayed.

Dissolved Oxygen (DO) Transmitter SMART Pro 8968Applications

The need of DO transmitter is, it indicates if the liquid is useful for a specific application. The following are some applications of DO transmitter:

For fish and microorganisms in water. DO measurement is important in the degradation process in waste

water treatment Corrosion process in pipelines, water /sewage treatment plants Shelf life of beverages. Environmental analysis of rivers and lakes. And also for the study of microorganisms in biotechnology

Photographs

HART FIELD COMMUNICATION PROTOCOLHART (Highway Addressable Remote Transducer)

HART (Highway Addressable Remote Transducer)

Communication between a control system and intelligent field instruments is easily implemented using HART.

The HART protocol is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system. It is a bi-directional communication protocol that provides data access

HART technology is easy to use and very reliable when used for commissioning and calibration of smart devices continuous diagnosis.

The HART protocol makes use of the Frequency Shift Keying (FSK) standard to superimpose digital communication signal at a low level on the top of 4-20. It communicates at 1200 bps. Can be used in various modes.

The HART protocol uses two different frequencies (1200 and 2200 Hz) to represent binary 1 and 0.


HART Data Link Frame Format


It is a master/slave protocol. Slave caries current signal, and master caries voltage signal. Current signal is converted into a corresponding voltage by the loop

load resistor. All devices should use voltage sensitive receiver circuits. A 250-ohm resistor is connected in series when the HART device is to

be connected while testing the transmitters for HART read and write communication.

Compatibility with standard 4-20 mA. It can help to gain early warnings to variances in devices,product or

process performance. Reduce maintenance cost. Enable automated record keeping of compliance data.

Calibration and testing

Calibration is the process of determining the performance parameters of an instrument, by comparing it with measurement standards

Calibration is the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy.

Two important measurement concepts related to calibration are precision and accuracy.

Successful calibration has to be consistent and systematic. Testing of that system or instrument is done after the calibration

process is completed. Testing is done for checking that all the performed calibrations are accurate.


Need of calibration

Calibration is done to minimize the error to the maximum possible extent so that the instrument or system shows the true and exact output value.

Calibration can be called for: With a new instrument. When a specified time period is elapsed. When a specified usage (operating hours) has elapsed. When an instrument has had a shock or vibration, which potentially

may have sudden changes in weather. Whenever observations appear questionable.


How testing and calibration is done?

A calibration is performed using a calibration procedure, which is a documented, validated and controlled method for making the

comparisons. It should define the parameters to be measured, the measurement

standards to be used and the data to be collected. The person performing the calibration is trained and qualified and

understands the scientific and physical principles of the measurements.

The results (data) of a calibration procedure may indicate a need for adjustment or other repair

Example: FM SIR (self inspection report) for the transmitters.

Present testing and calibration method Power supply connection

Present testing and calibration method Open collector output connection

Present testing and calibration method

Testing and calibration of conductivity transmitter

SMART Pro pH Module

Present testing and calibration method Testing and calibration of pH/ORP transmitter

Present testing and calibration method Testing and calibration of DO transmitter

Drawbacks of present method Complicated and difficult.

It is time consuming.

There is a great possibility of loose connections, which may lead to wrong calibration and testing errors may occur, also there may be wrong connections that can damage the device being tested.

In the present method of calibration there may be open or short-circuiting in the circuit, which results in testing and calibration error.

The wiring is in direct contact with the person who is testing the device, so this method of testing is quit dangerous.

Drawbacks of present method It takes time to adjust it to the exact values also the exact required

values are difficult to obtain.

The jig used for DO and Conductivity are exposed to the external environment. So, external factors may cause variation in the values of the components mounted onto the jig, resulting in variation in the calibration counts and testing values.

Setting the resistance values for the temperature sensor setting and calibration via the decade box is time consuming.

For HART connection the power supply connection is removed and HART is connected with a load resistance in series. Hence, it is time consuming.

Proposed method

It was proposed to make a gadget which is capable of testing all the transmitters and can overcome all these drawbacks. In this design all the instruments that are used for the testing and calibration are

integrated together in one device. Easy Accurate Less time consuming Efficient Increased productivity

Conceptual design(Front plate design)

hu SMART Pro COND/DO CALIBRATOR

Input Design

Power supply: 12, 24, and 36 V DC.

Conductivity simulator: Values selected for the conductivity calibration are 100E, 0.95K, 9.5K, 95K, 950K, 10M and for testing the resistance values selected are 100K, 182K and 10M which corresponds to approx. 10 µS/sec, 5 µS/sec, 1 µS/sec respectively for cell constant K = 1.

PT100 simulator: Temperature is calibrated for the values -20, 0, 25, 50 and 100 oC.

Input design

Input design

DO simulator: calibration and testing of DO are 1.5M, 4.5M, 302K, 22K, 1M and 2M.

NTC simulator: The temperature calibration is done with the resistance values 8.2K, 100K, 570E and 22K for -10, 50, 150 oC and open temperatures respectively.

mV simulator: For calibration the mV values to be generated should be 414.4 mV, 296 mV, 0, -296 mV and -414.4 mV.

Output design

Power supply

I/P O/P

Regulator design Using 78xx family of regulator The SMARTPro module work over range of 12-36V supply, and the testing is done for 12V,

24V and 36V of supply. Hence, we need a regulated supply of 12, 24 and 36 Volts.

Transformer Rectifier Filter Regulator

OUTPUT VOLTAGE 78xx5V 78056V 78069V 7809

12V 7812

15V 7815

24V 7824

Output design

Power supply

General circuit diagram of power supply using 78xx:

Drawback of using 78xx family

This series of products have very limited scope and can only be used for generating some particular fixed voltages as mentioned in the above table. This approach increased inventory and also proved to be very costly.

Output design

Power supply

Using LM317

3-Terminal Adjustable Regulator

Improved system performance by having line and load regulation of a factor of 10 or better.

Improved overload protection allows greater output current over operating temperature range.

Improved system reliability with each device being subjected to 100% thermal limit burn-in.

They are easy to use and require only 2 external resistors to set the output resistor.

Provide full over load protection available in IC’s.

Output design

Power supply

Features

■ Guaranteed 1% output voltage tolerance (LM317A) ■ Guaranteed max. 0.01%/V line regulation (LM317A)

■ Guaranteed max. 0.3% load regulation (LM117)■ Guaranteed output current from .10 to 1.5 A■ Adjustable output down to 1.2V■ Current limit constant with temperature■ 80 dB ripple rejection■ Output is short-circuit protected

Output design

Power supply

LM317 requires only two resistors to set the output voltage

The current set resistor R1 is usually 240 Ώ

Output design

Power supply

The circuit diagram of regulated power supply of 12 V, 24V, 36 V DC using LM317

Output design

Power supply

Calculation for values of resistor R2, R3, R4 for 12 V, 24 V and 36 V output voltage respectively

We get,

R2 = 2.03 K

R3 = 4.33 K

R4 = 6.5 K

External capacitors An input bypass capacitor C1 is recommended. A 0.1 µF disc or 1 µF

solid tantalum on the input is suitable input bypassing. Also, an output capacitor C4 is added to improve transient response. Output capacitor in range of 1 to 1000 µF of aluminum or tantalum electrolytic is used to provide improved impedance and rejection of transients.

Output design

Power supply

Protection Diodes

When external capacitors are used with an IC regulator it is sometimes necessary to add protection diode to prevent the capacitor from discharging through low current points into the regulator.

D1 protects against C1

D2 protects against C2

Output design

Power supply

Output design

Power supply

Output design

Power supply

Heat sink requirement

Under all operating conditions, the junction temperature of the LM317 should not exceed the rated maximum junction temperature (TJ) of 150° C for the LM317 or 125°C for the LM317A and LM317. A heat sink may be required depending on the maximum device power dissipation and the maximum ambient temperature of the application. To determine if a heat sink is needed, the power dissipated by the regulator, PD, must be calculated:

PD = ((VIN − VOUT) × ILOAD) + (VIN × IG)

TR(MAX) = TJ(MAX) − TA(MAX)

θJA = (TR(MAX) / PD)

Output design

Power supply

If the calculated maximum allowable thermal resistance is higher than the actual package rating, then no additional work/heat sink is needed.

Since, for LM317 T-220 regulator θJA is 50 °C/W.

Here, θJA = 35.663 °C/W for VOUT =12 V less then is 50 °C/W. after calculation

Hence, heat sink is required.

Output design

Power supply

Transformer specification

The primary winding voltage is 230 VAC ±10%, 50 Hz frequency; now the secondary winding for secondary current rating 500 mA.

Since maximum output required current is 36 VDC. Hence, a suitable value for the regulator input voltage will thus be in the range 37.2 V (min) to 38 V (max). Since, the voltage drop across LM317 regulator is 1.2 V (36 +1.2 =37.2 ≈38 V). And here a full wave rectifier arrangement is used using 4, 1N4007 diodes each having drop of 0.7 V (0.7× 4 =2.8 ≈ 3V)

So, the DC voltage required at rectifier output is 38+3 = 41.00V DC Vac = Vdc ÷ 1.1 Vac ≈ 39 V AC Hence, the voltage requirement at the secondary winding is 39 V

AC.

Output design

Power supply

Rectifier specification

Here full wave rectification is required. So, 4 diodes 1N4007 are used arranged in a bridge type configuration.

Specification:

PIV (Peak Inverse Voltage) 1000V

Ifav 1A

Vf 1.1 V

Ir max 10 µA

Output design

Power supply proto building

Output design

mV Source

Basic circuit design for mVolts source is shown below:

Using Kirchoff’s voltage law in loop one, Assume current 10 µA

R1= 680 K

Output design

mV Source

R2 = 2M (theoretically) ; R2= 2.3M (practically)

R3 = 0.5M

Since, R3 is a series combination of 7 resistances of equal value.

Hence, value of each resistance is ≈72K

Layout design

Power supply

Layout design

Conductivity Simulator

Layout design

PT100 Simulator

Layout design

DO/NTC Simulator

Design validation

Bibliography

• SMARTPro 8966, 8967, 8968 transmitter’s manual.

• Op-amp and Integrated Circuits- Ramakant Gaikwad SE INSTRUMENTATION AND CONTROL OP AMP

• http://www.sensorex.com/products/conductivity_sensors/accessories/conductivity_transmitters.html

• http://www.baseelectronicsandsystems.com/ph-tds-conductivity-meter-transmitter.html

• http://www.national.com/mpf/LM/LM317.html#Overview

• http://en.wikipedia.org/wiki/LM317

THANK YOU

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Electronic Calibrator