Calibration of I to P Converter (Current to Pressure Transducer)

Basic Operation of Current to Pressure Transducer:

• The Current to Pressure (I-P) Transducer accepts a current signal of 4 to 20 mA as an input signal from the controller and transmits a proportional field configurable pneumatic output pressure signal 3 to 15 PSI final control element called a control valve.
• Generally, the Current to Pressure (I-P) Transducer is used in an electronic control loop along with a pneumatically operated final control element called a control valve assembly.
• In most process applications Current to Pressure (I-P) transducer is fixed on a control valve with the help of a mounting bracket.

Typical Input and output configuration for the I to P Converter:

• Configuration of an I-P transducer for various inputs and outputs is possible.
• Typically, the input for an I to P transducer is 4 to 20mA an electrical quantity and produces an analogous pneumatic signal of 3 to 15 PSI as an output signal as a physical quantity.
• Depending upon the requirements we can have outputs 6 to 30 PSI and 2 to 33 PSI respectively,
• Similarly, we may have inputs of 4 to 12mA and 12 to 20mA analogous to an output of 3 to 15 PSI.

Tools required for performing calibration:

The tools required for calibrating I-P transducer is shown below.

1. Instrument air Supply about 20 PSI.
2. Source meter with FINE and COARSE tuning capable of delivering a current of 30mA.
3. Pressure Gauge.
4. Multimeter.
5. Connecting cables.

Equipment involved in calibration:

1. Current to Pneumatic Transducer.
2. Air filter and regulator.
3. Control Valve.

Things to consider before performing calibration in process area:

• Confirm the availability of the equipment from plant operators.
• Obtain a work permit to perform the task
• Do a job risk assessment in the field where the task will be performed
• Do a tool box talk with the crew members performing the job in the field
• Isolate the respective control valve from process line.
• Verify the tag number of I to P convertor and Control valve in the field.

Safety Precaution during calibration:

• The steps and guidelines provided here are meant as a general guide.
• Before commencing an actual calibration, refer to the manufacturer’s manual for the I-P converter.
• Make sure that you are wearing all the necessary safety gear and that you are following all the requirements for dangerous Area classification and any other requirements that will ensure the exercise is done safely.

In calibrating the I-P transducer:

• We must provide accurate input and check the appropriate output with a test gauge or any other test device.
• Check for ZERO, SPAN, and Linearity, and make an adjustment as required.
• An accurate current generator or an accurate voltage generator with a compulsory precision resistor of 250 ohms and a power rating of 0.5 Watts can be used.

I to P Convertor Calibration Steps:

• Detach the transducer from the process.
• Remove the module cover to flare up the transducer terminals.
• Ensure the switch setting for various configurations of input or output is set to perform desired input and output range.
• Link the control device to the I-P transducer, in which the device is used as the current source in that current loop.
• If not unlink the control device and connect the current source to the I-P transducer including a current meter for monitoring the calibration.
• Ensure that the pneumatic supply pressure to the I-P transducer must be 20 PSI.
• Tune the input current to the minimum value of 4mA in case of 4 to 20mA input calibration.
• Ensure that the output pressure of the I-P transducer is 3 PSI or 0.2 bar, if not spot the ZERO adjustment screw and tune the ZERO potentiometer to get an output of 3 PSI or 0.2 bar.
• Note that the ZERO and SPAN adjustment screws are labelled and clearly visible after removing the module cover.
• Now tune the input current to a maximum value of 20 mA in case of 4 to 20 mA input calibration.
• Ensure that the output pressure of the I-P transducer is 15 PSI or 1.0 bar, if not spot the SPAN adjustment screw and tune the SPAN potentiometer to get an output of 15 PSI or 1.0 bar.
• Perform linearity check at 25%, 50% and 75% to make sure the I-P convertor is producing desired output as shown in the below table

• Record the values during each step for preparing the calibration report
• Repeat steps 7 TO 13 until the output pressure of the I-P converter is within the cited accuracy requirements defined by the manufacturer without making any further adjustments.
• Unlink the current source and re-link the control device in case we are using a current source other than the control device so that the I-P converter is now ready to function.

Actions to perform after carrying out calibration:

• Do a thorough housekeeping in the field, remove all the waste materials got generated as part of the task and dispose them in designated bins in the field.
• Hand over the work permit to operator and give clearance to operators to bring the control valve to operation.
• Prepare a calibration report as per the readings noted in Step 13 and keep it as record of calibration.

Sample Calibration Report:

Sample calibration report of I-P transducer is shown :