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signal problem in 4-20mA loop
to eliminate the problem of signal drop in a 4-20mA loop.

we have a sensotech pressure sensor to measure refrigerant pressure. this is connected to a compressor. the compressor is driven by a motor connected to a freq drive to vary the speed. initially when the compressor is not running the pressure indicates 7 bar. the moment the drive is switched ON the pressure value drops to 4 bar when actually the pressure should be the same. the sensor is a three wire sensor giving 4-20mA output. this is connected to a eurotherm AI module. can anyone suggest checkpoints to resolve this problem?

By Tomy Zacharia on 22 October, 2005 - 1:50 pm

Dear Abhi,

The behaviour seems to be okay, with the assumptions as follows. The pressure transducer is installed upstream (inlet) of the compressor and the fluid is completely in gas phase. If my assumption is at variance from actual installation then more details may be supplied regarding the process.


Tomy Zacharia

Dear Tomy Zacharia,

We have installed sensors at all the points in the ref line. so some sensors are measuring liquid pressure while some are measuring gas pressure. also my observation is that all the values are dipping and not showing a sharp rise and fall as is usually the case when infuenced by noise. i need to know that if the sensor is a 0.05% accuracy sensor then how must the wiring be done over a distance of 15-20mtr without affecting the accuracy of measurement.


By Tomy Zacharia on 26 October, 2005 - 10:54 pm

Dear Abhi,

As some of the other contributors have rightly pointed out, the transmitter and the process needs to be individually checked out. The transmitter can be disconnected from the process and checked for wild swings
when the motor is switched on. Secondly a refrigerant service pressure gauge can be connected at the tapping for checking of any swings in pressure. In the extremely rare case of resonance or occurrence of pressure waves at the
point where the pressure is being measured, damping may be resorted through the use of variable orifices at the tapping points. If there are indeed pressure waves of sufficient magnitude (say due to reciprocating compressor)
the pressure transmitter can get permanently damaged.

Best of luck.

Tomy Zacharia

By S.Madan MOhan on 22 October, 2005 - 1:52 pm

one of the easy ways to solve the problem. is to install a Isolator with builtin power supply to be connected to the pressure Tx. the output is optically isolated so it can be connected to any device... In fact we can supply such units also if required.

S.Madan MOhan

We are looking for an isolator which will prevent the pressure sensor from picking up the noise. also i have been told that in a motor connected to a VFD if the ground is not connected in a particular way then the complte control loop of the VFD acts as an amplifier for noise which can create this problem. can you suggest where i can get the isolator?


You can get isolators in KRENEL

By Wayne Shimanis on 18 November, 2005 - 10:29 am

You may want to take a look at the Vortex Technologies model IP-420. The device provides isolation, EMI\RFI filtering as well as surge suppression. More information at

Wayne Shimanis

By ControlNovice on 22 October, 2005 - 1:54 pm

First guess is that the 4-20mA signal is too close to the VFD drive, or the motor cables. It's also possible the 4-20mA signal is not grounded properly (only one end of the shield should be grounded, usually in the final junction box at the reading device).

By Robert Scott on 22 October, 2005 - 1:59 pm

First determine if the problem is electrical or pneumatic. Do this by temporarily disconnecting the pressure sensor from the pump so you can be absolutely sure that the pressure doesn't change. Then observe the loop current when the pump runs. If the current doesn't drop in this test, then your problem is not electrical after all.

If it is electrical, then look for ways in which the sensor can be influenced by the pump motor. For example, if the sensor is not completely loop-powered, then there might be a local power supply that is drooping when the pump runs.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting