Author Topic: Keithley DMM6500 temperature issues  (Read 1983 times)

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Offline NigmoseTopic starter

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Keithley DMM6500 temperature issues
« on: August 31, 2023, 01:54:39 pm »
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.   
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Offline thm_w

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Re: Keithley DMM6500 temperature issues
« Reply #1 on: August 31, 2023, 10:17:04 pm »
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/
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Offline Simon

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Re: Keithley DMM6500 temperature issues
« Reply #2 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.
 
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Offline Kleinstein

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Re: Keithley DMM6500 temperature issues
« Reply #3 on: September 01, 2023, 11:48:07 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.
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.
 

Offline switchabl

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Re: Keithley DMM6500 temperature issues
« Reply #4 on: September 01, 2023, 03:32:03 pm »
IIRC the DMM6500 doesn't have a cold junction sensor (unless a TC scanner card is installed?).
 
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Offline NigmoseTopic starter

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Re: Keithley DMM6500 temperature issues
« Reply #5 on: September 06, 2023, 06:27:39 pm »
Thanks, the DMM6500 had been on for hours and I’ve run tests in a thermally stable air chamber to eliminate drafts etc and in open air as well as iced water, all showed the same issue.
Finally got an answer from Keithley who stated that basically you cannot just plug a probe into the meter and expect stable readings. It is suggested that I fit the 2001 card, but at over £1000 aears excessive for thee occasional measurement.
However thanks for your input.
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Offline thm_w

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Re: Keithley DMM6500 temperature issues
« Reply #6 on: September 07, 2023, 01:00:27 am »
As was linked above, PT1000 can be used, cost maybe $10, seen here: https://www.mdpi.com/2227-9040/10/8/309
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Online Hydron

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Re: Keithley DMM6500 temperature issues
« Reply #7 on: September 07, 2023, 06:47:58 am »
Most bench meters cannot use a thermocouple out of the box. As others said, use a PT100 or PT1000, preferably 4-wire if the former.
 
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Online ch_scr

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Re: Keithley DMM6500 temperature issues
« Reply #8 on: September 07, 2023, 07:47:57 am »
The "fake" Keithley 2110 (aka Picotest M3510A) has the proper thermocouple socket with CJC on the front panel. 1Y accuracy still only 1°C / 1.5°C depending on type. Not the same class of meter as the DMM6500 though, obviously.
 
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