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Temperature measurements wrong and errors

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PhD123
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Joined: March 2nd, 2020, 3:12 am
Country: France

Temperature measurements wrong and errors

Post by PhD123 » March 2nd, 2020, 3:22 am

I am using a Keithley 2750 with four switch cards (three 7708 and one 7700). I am scanning these channels for mostly temperature measurements (Type K thermocouples). I first noticed that temperature measurements were all higher than expected (+10 degrees celsius). And the variation was not equal across all cards (there was not an off-set equal on all cards). Unfortunately, the person who installed the Keithley system at my institution is dead, and nobody else knows how it works. In his written specification, he says that he wired an external PT-100 cold junction. However, when on the front panel and I choose the temperature options and click external junction, the values I obtain are ridiculous, -0.001 etc.. Then when I switch to the internal reference I see the values of 10 degrees higher. One year ago, these measurements were working, I am not sure what has changed between then and now.

Additionally, upon start up I see two errors +521 (card relay counts lost) and +513 (AC Calibration data lost).

If anyone could point me in the right direction it would be great, I am beyond frustrated and my lack of knowledge in this area really makes it hard for me to even know where to begin. Its for a very time sensitive project (My PhD).

FYI: I have called the tektronix support line, but that I should just ask the person who built it how they wired in the external junction, but since I cant do that, I have literally no idea what to do!

Andrea C
Keithley Applications
Keithley Applications
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Re: Temperature measurements wrong and errors

Post by Andrea C » March 2nd, 2020, 5:04 pm

For the error messages, your 2750 may be in need of a new LiIon battery. Has it been powered off for a long time?


For use of external RTD sensor for T/C measurements, the PT-100 should be connected to channel 1 of the card. Should be 4-wire connections, so wired to channel 1 of each bank.

If you do an RTD based temperature measurement from channel 1, does it seem reasonable? I suspect the PT-100 is under the clam shell and near the screw terminals of the 77xx cards. If that is the case, it will be warm. Does each card have it's own PT-100?

Once you successfully measure temp with the RTD, do the T/C measurements using that external reference get any better?

PhD123
Posts: 2
Joined: March 2nd, 2020, 3:12 am
Country: France

Re: Temperature measurements wrong and errors

Post by PhD123 » March 10th, 2020, 4:17 am

Yes this works for the front panel of the Keithley, thank you!

However, now when I go to scan the channels with my data aquisition (ExcelInx), the Keithley reads the internatl reference not external and so the temperature I collect are now showing the temperatures higher. Within Excelinx I cant find something which controls how to change the internal or external reference.. Is there anything I can do?

Andrea C
Keithley Applications
Keithley Applications
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Joined: October 15th, 2010, 10:35 am
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Re: Temperature measurements wrong and errors

Post by Andrea C » March 12th, 2020, 3:15 am

From memory, ExceLINX has a config sheet and a scan scan sheet. The scan looks to the config for some of the settings.
For the thermocouple channels, there should be a setting for the CJC. If it used to work, then it is in there.

Did the error messages about lost calibration go away?

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