Calibration of P/I Convertor

What is P to I converter in process control?

The P/I signal converter converts pneumatic signals into electrical standard signals, such as 0.2…1 bar (3 … 15 psi) to  4…20 mA. As a result, it serves as a link between pneumatic and electrical/electronic systems. The force balance method serves as the foundation for the original signal conversion principle.

How does P to I converter work?

The DMS sensor measures the input pressure, and the downstream electronics unit correspondingly converts it to an analogue output signal. The electrical component is built using two-wire technology, which means that both the power supply and the output signal travel along the same pair of wires.

The controller, recorder, and other auxiliary equipment need to be looped into the electrical circuit. The power supply needs a direct current of 15 to 30 V.

What is P to I converter application?

Pneumatic signals are converted into standardized electric signals that are particularly useful as a transmission element between devices for electric and pneumatic measurement and control.

P/I converters are used, for example, to integrate pneumatic transmitters to electric controllers, computers, and process control systems. They act as an interface between pneumatic and electric measurement and control units.

A pneumatic signal serves as the input variable, and an electric DC current or DC voltage signal serves as the output variable.

How do we calibrate a P to I converter?

Purpose and Scope:

This procedure provides a comprehensive explanation of how to calibrate a P to I convertor using standards in the field.

Tools required for HART pressure transmitter calibration:

• Necessary hand tools.
• Standard low Pressure Calibrator.
• Standard Multimeter.
• Test leads and probes.
• Tubes and standard fittings 
• Soft Cloth for cleaning.

Safety

• Please visit the linked link to learn more about fundamental safety, general advice, and calibrating operations in process industries.

Basic Safety and General Concerns for the Process Industry Calibration Process

• While calibration is being done, inform the operator that P to I convertor measurements are unavailable.
• Request that the DCS or SCADA operator configure the controller in manual mode for the specific control loop and in MOS (Maintenance Override Switch) mode for the ESD loop.
• Locate the P/I converter that needs calibration. Verify that it is the appropriate P/I converter, and note any relevant data Tag number (e.g., the manufacturer, model number, signal range, etc.).
• Before calibrating the P/I convertor, the system may need to be depressurized, depending on the type of system.
• Pipe that connects the input of the P/I converter to that of the pneumatic transmitter should be sealed up or taken out. As a result, there won’t be any fair leak out and the P/I convertor will be isolated from the process.
• Keep in mind that based on the particular equipment and process location, this basic procedure for P/I converter may need to be modified.
• While working with P/I convertors or other process equipment, always adhere to local safety requirements and the manufacturer’s instructions.
• To avoid an unintended start, adhere to all applicable lockout/tagout protocols. Make sure the P/I converter is isolated from the process.

Calibration Setup

• It is essential that you position the P/I convertor and calibration equipment away from electromagnetic interference and vibrations. Also, the area needs to be well-ventilated and lit.
• Gather all the tools and equipment needed for P/I converter calibration.
• One end of the tube should be linked to the low pressure calibrator’s output, while the other end should be attached to the P/I converter’s input pressure port side. Check the connections to ensure they are tight and leak-free.
• Don’t turn off the P/I converter’s power supply. Check any nearby junction boxes or marshalling panels close to the control room using an instrument loop diagram to see if the power supply is present at the source.
• Between the junction box and the P/I convertor, create a series-connected analogue input loop to the multimeter (mA mode) using probes and a lead.
• The connections have been made and are currently being prepared for the P/I convertor’s calibration, as shown in the diagram.

Calibration procedure

• Check the tubing and wire connections on the P/I converters for stability.
• By removing the P/I convertor terminal cover, you can check if a 24 VDC power source is present.
• The instrument data sheet can be referenced to confirm a signal level parameter. Typical parameters are the tag number, the input signal range, and the output signal range.
• The low pressure calibrator must now be turned on, along with the multimeter. The mA mode must also be selected in the multimeter.
• Verify that the low pressure calibrator’s menu is set to the correct unit by looking at the datasheet for the P/I convertor.
• For example, the P/I converter’s datasheet states that its operating signal input range is from 3 to 15 psi and output is ranged from 4 to 20mA.
• Applying the 3 psi input (0%), using a low pressure calibrator, to the pressure port of the P/I convertor
• The next step is to ensure that the output of the P/I convertor is showing 4 mA on the multimeter display.
• If it does not show 4 mA, you should zero-adjust the P/I convertor until the output signal of the P/I convertor approaches 4 mA.
• Applying the 15 psi input (100%) to the pressure port of the P/I convertor using a low pressure calibrator,
• The output of the P/I converter should then show 20 mA on the multimeter display.
• If it is not showing 20 mA, you must adjust the P/I convertor’s span setting until the output signal of the P/I convertor approaches 20 mA.
• For information on where to make the zero & span adjustment, consult the P/I convertor calibration manual.
• In order to calibrate the P/I converter to the desired tolerance, the calibration operation must be repeated as many times as necessary.
• Depending on the specific low pressure calibrator and P/I convertor being utilized, the calibration procedure may change. So, before you start, make sure you follow the manufacturer’s instructions.

Recording calibration

• Check the output’s linearity at 0%, 25%, 50%, 75%, and 100% in both the upscale and downscale directions to ensure the P/I convertor is producing the correct output values.
• Calibration is needed if the output value of the P/I convertor does not fall within an acceptable range. If the output values have once more fallen outside of the permitted range, the P/I convertor needs to be serviced or replaced.
• The P/I convertor doesn’t need to be calibrated further if all output values (+/- %) fall within accepted limits.
• The blank calibration report as found/as left column should be filled in with the output data.

Completion of calibration

• Attach the calibration label on the P/I convertor once the calibration has been properly completed.
• Clean the P/I convertor, test instruments, when the calibration is finished, store them safely, and make a note of the calibration data for later use.
• Remove the connections for the P/I convertor, low pressure calibrators, and other calibration tools.
• The P/I convertor connections must be installed in the processing area.
• Verify the cleanliness of the workplace.
• De-isolate the equipment.
• Restore the signal level of the P/I convertor signal that has been bypassed or inhibited.
• Before employing the P/I convertor, make sure it is operating properly.

Sample calibration report

The figure below shows how a pressure calibrator and a multimeter were used as a reference to calibrate a P/I convertor sample report in a process area.

The Excel document that was used to construct the P/I convertor calibration report is available by clicking the link below.