PLC and HART communicator showing different values? Reason?

User Question:

I wanted to reach out because I have been having some challenges with figuring out the value and scaling that read from a differential pressure transducer.

It is a Yokogawa EJA110A-EM and the value I see with using a HART communicator is 5 inH2O where as the input value from the PLC is about 700 and I don’t know what units they and factors they used to get this number.

My LRV is 1.411 inH2O and URV is 165.680 in H2O. The Analog output reading was 4.347 mA. The reason why I am looking into this is because the level readings we are currently calculating are not as accurate as they could and want to find a better to calculate the level more accurately.

Do you know why the value I am seeing on the PLC is different from the pressure value I see from using a HART communicator and how I could scale the pressure properly?

Any feedback would be greatly appreciated.


First, check the configured transmitter range in the PLC.

You have transmitter LRV, URV, mA (from the PLC side)

Now calculate the Process variable PV using the above values.

You can use this instrumentation calculator also.

The calculation result must match the PLC reading. If readings are matching then the PLC shows the correct value here. But If you feel this value is wrong then you have to proceed with transmitter calibration. If readings do not match, then check the configuration in the PLC.

Also, check the transmitter display range if available. The transmitter display range and PLC-configured ranges must be the same.

If all the above checks are normal, then re-calibrate your transmitter and also verify its range from the datasheets.

Sometimes the transmitters show the correct value but the assumption of the process parameters is wrong. There may be some problems with the process itself.

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Well explained. Infact the configured Transmitter range in the plc might be the main issue .

Also seem tiny differences between HART handheld and Transmitter as well.

The value seen on the hart communicator is equivalent to the transmitter output (in mA). So there is no issue from the transmitter side.

The issue is from the PLC side where it shows 700 (unknown unit ) for 4.374 mA input.
this means the PLC configured range is 0 to 29946.( assuming it’s linear).

However, transmitter calibration should be curried ou.

for the transmitter calibration. you need to make zero trim first in order to get the accurate pressure value.

once you get the correct PV, set the LRV and URV as per the installation and measuring range.
then configure the display to show the value in percentage.

You should get the accurate level in the transmitter display. Mach the reading to the actual level ( see the level gauge if applicable) or use any means of verification.

then back to PLC side make sure it’s properly renged and configured.

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