Cold junction compensation with 34980A

[Option13]

Hello all,

I am trying to set up an Agilent 34980A to read multiple thermocouples. I am trying to use another set of thermocouples (channel 17, card 1, marked 1017 at the read thermocouple sub VI), wired to prevent cjc's, to account for cold junction voltages at the other thermocouples. 

I have attached images of the relevent panels. Wired like this, it as if the thermocouples 1001:1040 are uncompensated and that the read thermocouple sub VI is outputting a zero value. If I put the read thermocouple sub VI in a VI all on its own and configure it the same way, channel 17 read 70 degrees, when actual was around 73. The code in the loop, however, does not seem to see this. Is this due to the way I have it set up in the loop, or the rate at which the code runs?

Or, is there a cleaner way of doing this? Using an RTD instead of additional thermocouples is possible as well.

[nyc_(is_out_of_here)]

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@Option13 wrote:

Hello all,

I am trying to set up an Agilent 34980A to read multiple thermocouples. I am trying to use another set of thermocouples (channel 17, card 1, marked 1017 at the read thermocouple sub VI), wired to prevent cjc's, to account for cold junction voltages at the other thermocouples. 

I have attached images of the relevent panels. Wired like this, it as if the thermocouples 1001:1040 are uncompensated and that the read thermocouple sub VI is outputting a zero value. If I put the read thermocouple sub VI in a VI all on its own and configure it the same way, channel 17 read 70 degrees, when actual was around 73. The code in the loop, however, does not seem to see this. Is this due to the way I have it set up in the loop, or the rate at which the code runs?

Or, is there a cleaner way of doing this? Using an RTD instead of additional thermocouples is possible as well.

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In your other thread ( http://forums.ni.com/t5/LabVIEW/Agilent-34980A-Thermocouple-read-error/m-p/2925070#M846179)

about very similar topic, it was suggested that you actually verify the reading on the instrument's front panel which you have NOT done.

Why are you insisting on ignoring good advice?

Make sure that you are able to make good measurements BEFORE trying anything with LabVIEW.

[RTSLVU]

nyc has a good point, you need to understand how to use an instrument manually before you can expect to program it correctly. 

But that being said have you tried using the internal cold junction compensation? That's what I always use and have never ran into a situation where I needed anything else.

The 34980 is a lot like the 34970 I use. Here is what a configure for a 34970 looks like.

[Option13]

I have not measured it off of the front panel because we have recently capibrated the system, and I have less than an hour each day to play with the system. Knowing that our DAS was putting out good data made me hesitant to spend time on checking it from the front panel, but I will do so this afternoon.

[Option13]

RTSLVU, is that a file you made or supplied with the drivers from Agilent? I have also downloaded the driver for the 34970 and if that's available, perhaps I could replicate it.

[RTSLVU]

The 34970 VI's are from here http://sine.ni.com/apps/utf8/niid_web_display.download_page?p_id_guid=014F4CD7EF78136AE0440003BA7CCD...

This is only the initialise state. Using a 34970 properly is a two step process

1. Configure all channels and a scan list
2. Scan the 34970 to get the readings

So the case I am showing only runs once a the start of my program and I scan at regular intervals 

I did make my own "Scan" vi that has several output options, it is a modified version of teh 34970 scan example.

[Option13]

I checked the front panel temperature readings yesterday. 84 degrees in the Agilent for the reference channel, 73 degrees in the test space. Unfortunately I seem to have pressed a wrong button and now all of the previous setup has disappeared from the Agilent. The other DAS and the Agilent agreed before the settings were lost, confirming that our data is good.

Now the issue is that I cannot know for sure the reference junction temperature until the reference channel is properly set up again.