PXI-4351 3-wire RTD measurements

[Mihai]

I want to use the PXI-4351 with the CB-68T to read temperatures from 3 of 3-wire RTDs. I connected the RTDs as shown in fig. 2-6 on page 2-16 of the User Manual. I read the whole PXI-4351 User Manual, as well as the Help for the corresponding LabVIEW driver - since I want to incorporate it in my application.

I still don't know how to instruct the instrument to interpret correctly the measurements, according to my configuration. What I mean is: how does the instrument know that I have one and the same RTD connected to both channel 2 and channel 3, and channel 3 is meant to be used to compensate for the error on channel 2, instead of just 2 RTDs in 2-wire configuration, connected separately to channels 2 and 3?

The other issue is: how long is it likely I will have to wait for a scan to complete in such a configuration?

Thank you,

Mihai Costache

Merit of Measurement Ltd.

New Zealand 

[Margaret F.]

Hello Mihai,

Thank you for posting to the NI Forums! First of all, I want to point you to the LabVIEW examples that install with the NI-435x driver. You can access these examples by opening LabVIEW and going to Help>>Find Examples. Once the Example Finder opens, you can double click on Hardware Input and Output>>NI-435x. Once there, take a look at the Multiple Transducers examples.

I want to ask for some clarification on what you mean by "channels 3 is used to compensate for the error on channel 2." Let me know what you mean by that and I will be happy to respond. Are you perhaps referring to the CJC?

In regards to how long you have to wait for a scan to complete, this will depend largely on your filter rate as well as your other settings which come into play for settling time. Take a look at page 2-3 and Table 2-1 on page 2-4 of the user manual. It will be difficult to determine an exact value, but this reference should give you a good idea. Hope this helps!

[Mihai]

Margaret,

Very glad to hear from you.

The example vi that you mention is precisely the one I adapted so it fits into my application.

In order to better explain what I mean, let's again refer to fig. 2-6 on page 2-16 of the User Manual. This is exactly how I connected my RTD. So my "measurement" wires are connected to channel 2, and my "error compensation" wires are connected to channel 3. If I understand you correctly, I need to read both channel 2 and 3 and then do something with the readings so that the results is corrected for the error introduced by wire resistance.

According to the note at the bottom of the figure, if I was reading resistance, then I would just subtract the two readings. But I want to read temperatures. Do I also just subtract the two temperature readings?

If this is still clear as mud, I'm happy to give you a call to explain further - if only I had a hpone number.

Kind regards,

Mihai

[Margaret F.]

Hello Mihai,

Please try the Measure RTD (with Accessory).vi found in the Example Finder under NI-435x>>Measurements. Just use one RTD and only define the first channel for the measurement. When measuring temperature, you will not have to subtract the readings. Let me know the results of using this example. Thanks!