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3 Ways to Calibrate a Pressure Transmitter That You Can Easily Apply
Source: | Author:J | Published time: 2025-05-23 | 17 Views | Share:
During my learning days, when I hear a pressure transmitter (and other transmitters), it seems it is a very complicated instrument to calibrate (How about you?).
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But when I already learned and experience calibrating it, I can say that it is just calibrating a simple pressure gauge. the only difference is that the applied pressure is now converted into a current output, the  4 to 20 mA signal. In order to be understood by the user, it will be converted back to pressure units by the PLC display.
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We do not need to know how to operate a PLC (but it is better if we do), what we need is, during calibration, it is important that the operator of the PLC is with you, make the user operate while you perform the calibration.
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This simple pressure transducer is a transmitter because it is used from a remote location going to a control panel (PLC) to monitor and control the process involving pressure. The signal is transmitted through a wire in the form of a standardize  signal which is the current, the 4 to 20 mA signal.
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In transmitter calibration, not just pressure transmitter, an additional knowledge about linear interpolation is needed to understand the conversion of current output to pressure units or other units or vice versa. I get to be confused about this conversion when I was newly exposed to this so I will include the conversion formula here (just in case you are confused too). Just read on.
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In this post, I will present the following:
  1. 3 ways to set up a pressure transmitter for calibration.
    • Loop calibration set-up

    • Individual or Isolated Instrument Setup

    • Current Meter In-Series Connection Setup

  2. 3 types of pressure transmitter connector

  3. what standard to use in calibrating a pressure transmitter and how it is connected and configured

  4. The conversion of pressure to current using manual computation ( or by using a method called linear interpolation)
  5. Calibration procedure
Instrument Classification

Sometimes, we are getting confused whether an instrument is a transmitter or a transducer. For us to see the difference, below are the instrument classification directly taken from IEC 60770-1:2010.

(–) stands for any physical, electrical or chemical quantity to be measured and processed, such as pressure, temperature, level, flow rate, density, pH, composition, as reported below:

a) (–) transmitter a measuring transducer whose output is a standardized signal
b) (–) meter an instrument intended to measure a physical quantity
c) (–) indicator an instrument intended to visually indicate a physical quantity
d) (–) switch a measuring transducer whose output is a binary signal (ON/OFF or 0/I)
e) (–) transducer a device which accepts information in the form of a physical quantity and converts it into information in the form of the same or another physical quantity according to a definite law
f) (–) sensor an electric signal transducer that converts a signal of any kind into an electric signal
Pressure Transmitter Calibration Set-Up
There are 3 ways that we can set-up a pressure transmitter calibration.

  1. Loop Calibration Setup
    What is loop calibration?

Loop calibration is a calibration setup where the instrument is calibrated as a whole involving the pressure sensor and transmitter inter-connected to the PLC display. It is a calibration method where the instrument is calibrated in its actual location where it is installed and in operation, but it is isolated in order not to affect other processes.
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The advantage of this is that you are getting the overall performance of the instrument. you can determine the overall functionality of the loop.
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The disadvantage is that if there is an error, you can not tell immediately what is causing it and troubleshooting may take longer.
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Loop calibration involves a 4 to 20 mA signal. Readings can be taken as current units or pressure units. Current is the signal that is being sent to the PLC Display but it is converted back to pressure units.
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If you want to get the current readings, you need to connect a multimeter that is in series with the wire going to the PLC.
 See no. 3 set up below.
  1. Remove the installation of the pressure transmitter on its location (to expose the pressure input). Do not remove cable or wiring connections.
  2. Using the proper fittings and connectors, connect the calibrator pressure output to the input of the instrument.
  3. See to it that there is no leak.
  4. Check and ensure correct tagging or ID in the PLC display to match with the transmitter in order to see the readings once the pressure is applied.

  5. If you want to check the current value while applying pressure, you may do simultaneously the setup no 3. below.

  6. It is now ready for calibration.

  1. Individual or Isolated Instrument Setup

In this setup (which is also called Bench Calibration since it is removed from its installation), we will isolate the main instrument to other connections, only the main sensor that is directly tapped to our reference standard and reading is taken directly on the display of our transmitter calibrator.
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This method is more accurate because no other outside sources of error are present. Troubleshooting is easy, you can directly determine the problem because you are just concentrating on the instrument itself.
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But the disadvantage is that mistakes may occur during the re-installation. So be careful not to interchange connections. Also, you need a special calibrator with a loop power to perform this. This is one of the special capabilities of Fluke 754 as a transmitter calibrator.
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Instrument-only calibration involves the output signal which is directly displayed as 4 to 20 mA. You need to apply interpolation in order to determine or convert it to pressure value or units.
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How to use interpolation or to convert pressure to current? Continue reading further…
  1. Remove the instrument from its location including its connecting wires. Ensure there are no other power lines to be disturbed.
  2. Connect the calibrator supply pressure to its input or instrument sensor. This is the same as the loop connection setup. But this time, the instrument is isolated.
  3. Since the connecting wires are removed, the output ports are exposed. Check the transmitter connection diagram or label to ensure proper functioning and output signal detection.

4. Since the transmitter is disconnected to its line, there is no power to it, in this case, we will enable the loop power of Fluke 754 pressure transmitter calibrator to supply it in the transmitter.


5. At the same time and connection, we can read the output current that is based on the supplied pressure.
6. Through linear interpolation, we can determine the output in pressure units.
7. Compare computed value on the displayed value if within the expected tolerances.

  1. Current Meter In-Series Connection Setup

This setup is the easiest and most simple to perform. Just connect the Fluke 754 probe (or simply a calibrated multimeter) in series with the circuit or the transmitter current path and directly measure the actual current.
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This procedure may not be a stand-alone or sufficient procedure, it is preferred to perform together with setup no 1, but if there is no other option and the user approves this procedure, then it may not be a problem.
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This set up does not need any access or connection from the transmitter itself, only verifying the actual current then compute for the converted pressure value. The only thing you can measure is 0 pressure (4mA) and the actual pressure that is used in the process.
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 Some instruments have easy access where you can directly tap the current meter to a dedicated current-output port to get the current reading.

The main standard used in Pressure Transmitter Calibration- Pressure Transmitter Calibrator

I believe we are now familiar with FLuke 754 as a reference standard for pressure calibration 

especially transmitters. There are other process calibrators out there but this is what I always use since I started pressure calibration. It has pressure modules that have different ranges depending on your needs.
If you have a lot of instruments involving pressure and temperature instruments, this calibrator is a must.

3 Types of Pressure Transmitter Fittings/Connector

The pressure transmitter has different connections and design depending on its use or purpose.  Below are some types of pressure transmitter connectors that I encounter:

  1. Diaphragm type connections– mostly used in a pharmaceutical company. Used in a liquid that has a solid particle that can clog on a simple NPT connector. I always like calibrating this type of connector because you can easily remove and return the unit. But sometimes, you need a dedicated fitting in order to connect it to the calibrator. For us, we need to have it manually fabricated in order to use it.

2. NPT type connector– the most common type of connectors being used for general purpose. This is a 1/2 size NPT which is what we usually encounter.

3. Vinyl Tube or Rubber Fittings for differential pressure– used for low-level pressures and mostly air is the medium used. As for its calibration, it is the same as a regular differential pressure gauge,  just supply pressure to its positive side and let the negative side exposed to ambient.