How to Calibrate a Current to Pressure Transducer ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Calibrate a Current to Pressure Transducer

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Basic Operation
The I/P transducer receives a 4‐20 mA DC input signal from a control device and transmits a proportional field‐configurable pneumatic output pressure to a final control element usually a control valve. The I/P converter is typically used in electronic control loops where the final control element is a control valve assembly that is pneumatically operated. In most applications the I/P transducer is mounted on a control valve or very close to it in a mounting bracket.
See How a Current to Pressure Transducer Works
Typical Input/output configuration for the I/P Converter
It is possible to configure the I/P transducer for various input – output configuration. For an input of 4 – 20mA, we could have outputs of 3 to 15psig (typical), 6 to 30psig and 2 to 33psig all depending on the field application. Similarly, we have inputs of 4 – 12mA and 12 – 20mA both corresponding to an output of 3 to 15psig

Calibrating the I/P Transducer
I to P transducers are calibrated in much the same way analog DP transmitters are calibrated howbeit with a little difference. A DP transmitter senses process pressure and outputs a standard 4 – 20mA signal. In calibrating them, we apply pressure within the calibration range of the transmitter and confirm that the output of the transmitter is within the 4 – 20mA current range throughout its calibration range with little or no deviations by making ZERO and SPAN adjustments – See How to Calibrate Your DP Transmitter.

In an  I to P converter, current in a specific range e.g 4 – 20mA is supplied to it and the device outputs a pneumatic output of a specific range commonly 3 – 15psig in most applications. Therefore to calibrate the I/P transducer, we simply supply it current in a specific range with  4 – 20mA the most common and we confirm that the output of the transducer is within the range 3 – 15psig by also making ZERO and SPAN adjustments.

Calibration Procedure
As already noted, calibration of the I/P converter involves basically a ZERO and SPAN adjustment.

Safety Precaution
The steps and guideline provided here are meant as a general guide. Before embarking on an actual calibration, consult your manufacturer’s manual for the I to P converter. It should be your de facto guide. Make sure you are wearing all the necessary safety gear and that you are complying with all the requirement for Hazardous Area classification and any other requirement that will ensure the exercise is done safely.

Materials Required:
  • Current Source – capable of delivering 150% (30m A) of maximum output of transducer with the required compliance voltage.
  • Supply pressure of at least 20psig.
  • A milliammeter (In the absence of a milliammeter, a 0 – 10VDC voltmeter with a 250 ohm resistor connected across it can serve the same purpose – 1VDC indication for 4m A and 5VDC indication for 20m A)
Connect the equipment setup as shown below:
Calibration Steps:

Step 1
To access the transducer terminal, remove the electronics module cover.
Step 2
Confirm that the switch settings for different input –output configuration is set to achieve the desired input/output range
Step 3
If the control device in the control loop is being used as the input current source, connect the control device to the I/P transducer as shown above otherwise, disconnect the device and connect the current source to the transducer incorporating a current meter to monitor the calibration operation as shown above

Step 4
Confirm that the supply pressure to the I/P transducer is at least 20psig

Step 5
Adjust the input current to the low mA DC value e.g 4mA in the case of a 4 – 20mA output calibration.

Step 6
Confirm that the output pressure of the  I/P transducer is now 3psig (0.2barg). If not, locate the ZERO adjustment screw and adjust the ZERO potentiometer until the output pressure is 0.2 bar (3 psig). Note ZERO and SPAN adjustment screws are conspicuously visible and labelled. They can be easily seen after removing the electronics module cover.

Step 7
Next adjust the input current to the high mA DC value e.g 20mA in the case of a 4 – 20mA input calibration.

Step 8
Confirm that the output pressure should be 1.0 bar (15 psig). If not, locate the SPAN adjustment screw and adjust the SPAN potentiometer until the output pressure is 1.0 bar (15 psig).

Step 9
Repeat steps 5 through 8 until the output pressure of the  I/P converter is within the referenced accuracy requirements stated by the manufacturer without further adjustment.

Step 10
If you used a current source other than the control device, disconnect the current source and reconnect the control device.
Your I/P converter is now ready to be in operation