Problem with 3-wire transducer on analog input

olloy

sory for late answer.

please see attachment

No problem and thanks olloy, but that's exactly how it's wired at the moment. The problem must lie somewhere else.

I was thinking maybe it has something to do with the max. load the transmitter can handle? It's 1kOhm, I doubt the AI has a greater load to be honest.

Do you have any idea what's the impedance of the AI? I can't seems to find it anywhere.
Or maybe you know what else could be the problem?

Thanks in advance for the efforts.

regarding your question: usually, for siemens ai input impedance is 250 Ω.

lets check other ideas:

1. interference frequency supression (should be set according with your AC voltage frequency);

2. lengh of the cable;

3. are you sure with connection diagramm? which transducer you use?

olloy

regarding your question: usually, for siemens ai input impedance is 250 Ω.

lets check other ideas:

1. interference frequency supression (should be set according with your AC voltage frequency);

2. lengh of the cable;

3. are you sure with connection diagramm? which transducer you use?

Dear olloy,

first of all, thanks again for the time and effort you are putting in to this.
The ai imput impedance can't be the problem then.
1. The interference frequency suppresion is set to 50Hz which is the line frequency here; So it's correct, but I don't think a wrong configuration fo this parameter can cause such a deviation on the mA value.
2. Length of the cable is very short, less than 5 meter, so impedance should be fine, and all signals cables are seperated from power cables to avoid interference.
3. It's a 3-wire electrochemical transducer, output is 4-20mA, it measures chlorine. I'm pretty sure about the wiring because when I replace the AI card of siemens with a Ampere-meter I get the correct current value.

I'm pretty stuck here, I don't really know what causes this problem. Your help is greatly appreciated.

Kind regards,
Jens

Hmm..

Havent other ideas..

Ground loop??

olloy

Hmm..

Havent other ideas..

Ground loop??

Dear olloy,

we have connected the same transmitter to another AI card and saw we had the same problem, but a much smaller deviation on the analog value.

Instead of a drop from 4mA to 2.6mA, we had a drop from 4mA to 3.4mA on that card.

We measured the input impedances of both cards and noticed they are not the same, first card had approx. 110 Ohm and second card approx. 35 Ohm.

We noticed how more we increased the impedance how more the analog value dropped.

It's like the transmitter can't handle the load. We think something is wrong with the electronics of the transmitter, since its specifications mention it should be able to handle a load up to 1kOhm.

We are now waiting for a new transmitter and hope replacing it will solve the problem.

Kind regards,
Jens