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PXI-6239 DAQ current increases when reading stable current

I am reading 2 Swagelok 4-20mA Pressure Transducers (1000psi and 3000psi) and the currents are correct for a long amount of time and then steadily increase. I have taken a measurement of the current across the connection and it does not change and stays at the correct value. This also does not happen every time. Is there a setting in the DAQ assistant that I need to set to fix this?

 

Thanks!

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Greetings, Amanda;

 

When you say that they steadily increase, by how much are you seeing the measurement change? Will it simply continue increasing for as long as you leave it measuring, or will it stop at some point?

 

You are using the DAQ Assistant Express VI in LabVIEW, Correct? What version of LabVIEW are you using?

 

Do you see the same behaviour if you simply measure within the Measurement and Automation Explorer? 

 

Furthermore, when you see this current rise, does it happen for both channels or do you sometimes see it happening only on one? If you switch channels do you also see the same behaviour?

 

Finally, did you take a look at our Field Wiring Considerations guide? It goes over what might be causing noise in measurements and is always a great starting point:

http://www.ni.com/white-paper/3344/en

 

Anyhow, I wish you a great day.

Cordially;

 

Simon P.

National Instruments
Applications Engineer
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The current measurement increases slowly, about 0.5-0.6 mA per minute until it reaches the maximum 20-21 mA. The transducer range is 4 to 20 mA. We first discovered the problem in the Measurement and Automation Explorer when I created a task to test the transducers, the problem occurs in the MAX and in Labview. Both channels increased when tested separately. But when both channels are in the same DAQ, only one increases at the steady rate.

 

The Labview version we are using is 2011 SP1. I am not sure it is a wiring problem. We read the current of one of the circuits and the current was constant while the current in the computer increased.

 

Thank you for your help!

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I believe I have solved my problem. I added a third signal, which is not connected to anything, and it worked perfectly. I stumbled across this when trying to switch the two signals but then wondered if subtracting the noise within Labview would work. I am not sure how adding a third signal fixed the steady increase of the first pressure transducer but it will be fine for our system.

 

Thanks again!

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