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Keithley DMM6500 temperature issues

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Nigmose:
Trying to get to grips with the Keithley DMM6500 I ran into an issue with reading temperatures, that is the initial temperature would start off below the actual temperature, then drift even lower-see attached chart. I’ve tried numerous k type probes all with the same result. What is wrong.
Iced tap water - 18/08/2023      
Time   87V & Calibrated IEC Mineral Insulated Thermocouple   DMM6500 Fluke thermocouple
18:37   0.7   -5.526
18:40   -1.1   -4.757
18:41   -0.5   -5.695
18:43   -0.1   -6.261
18:44   0.1   -6.474
18:45   0.2   -6.689
18:45   0.2   -6.855
18:46   0.3   -7.014 ***
   *** Continues to drop.   

thm_w:
I asked to move this to the Test Equipment section: https://www.eevblog.com/forum/testgear/

- How long did you let the DMM6500 warm up? 30min is required for its accuracy spec
- Is the k-type probe sealed and for use in water? contamination may affect your measurement

Note that DMM6500 does not have a proper CJC measurement. RTD may be recommended for very precise measurements as mentioned here: https://www.eevblog.com/forum/beginners/reference-temp-for-dmm6500/

Simon:
If they read backwards it could be that the sensor is in backwards. Remember that they are voltage sources and the temperature is the difference between two of them.

Kleinstein:

--- Quote from: Simon on September 01, 2023, 11:19:21 am ---If they read backwards it could be that the sensor is in backwards. Remember that they are voltage sources and the temperature is the difference between two of them.

--- End quote ---
If the sensor were in reverse it would read way higher, like 40-50°C , like some 20-25 K up from the meter temperature instead of down.
The cold junction compensation in DMMs with banana plugs is pretty poor by design, but should still not be off that much. So there is a chance there is something more wrong (e.g. the cold junction sensor or a calibration constant).

A test would be using a copper short instead of the TC and this way effectively read the cold junction sensor and look if that is reasonable.

switchabl:
IIRC the DMM6500 doesn't have a cold junction sensor (unless a TC scanner card is installed?).

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