4-20mA Signal Output Intermittent


Thread Starter


Hi Guys,

I need your expert advice on my issue below.

Currently we did an upgrade to change 1 unit of General Monitors Open Path IR5000 to General Monitors Open Path IR5500 (by using the same cable as the existing). The existing installation is using a 4 wire system, and the new unit is using a 3 wire system. As the whole DCS system is actually design to a 3 wire system, it's not an issue for us to convert it from 4 wire to 3 wire system.

After 2 month of installation, the DCS begin to get an intermittent signal from my instrument, it is something like the signal will become more than 20ma, then 4 ma, then rise again to 20ma. It all happens in a 1 second interval between each signal. Sometimes I will get more than 1000 activity in a single day. the time this happens also varies, sometimes at night, and sometimes during the day.

As the instrument purpose is to detect hydrocarbon gas, there is no trace of this gas even when it triggers to more than 20ma.

The signal is in normal condition normally (4ma), and when it shoots to more than 20ma, it will trigger several hundreds of time, then it become normal again for several hours.

We did swap the new problematic instrument with an old working instrument, and no signal intermittent observed. Firstly I suspect that my new instrument is problematic, and then I bought another unit, and the same problem occurs.

I also did check on the cable for any leak, and none can be found.

Is it because of the output signal load difference?. the new one is having maximum 700 Ohm, while the old one is 600 ohm (personally i don't think this is the issue, as the new model is having higher load resistance).

One more thing, the old unit power consumption is 25W @ 24vdc, while the new one is 10w @ 24vdc.

The DCS is from Yokogawa (I will update on the model once I got the details).

Do you guys have any experience regarding this from before? if you do, I really hope you can share it with me.

Thanks guys,


Curt Wuollet

It always helps to find out which one is lying. Put a current meter in the loop and see if those extremes are really happening. And whil you're at it find out if you are within the compliance limits for voltage. Some current loop gear does strange things when it runs out of voltage on either end. This sounds like it could be that sort of problem. Wire issues can pull the voltage out of spec without necessarily affecting the current. A constant current source operates at high impedance.

A pretty good check list -<p>

Any wire nuts or terminal blocks in the path? Tighten.

Other wires sharing the same run inducing into the current loop. (Depends how fast the receiver is - and if there is any filtering. Sometimes a cap across the receiving end will do it.)

If the receiver is powering the loop - it is possible that the loop power supply has noise.

Is there a ground connection at both ends? (bad idea for 4-20mA loops)

Not sure of your setup - a 4-20mA loop by definition is 2-wires (there can be a cable shield).

A lot of times, inappropriate grounding can induce noise into a loop. Isolating the sensor/transducer from ground if possible can be a fix.

See -