Keithley DMM6500 temperature issues - Page 1
Forum section: electronics / testgear

author: Nigmose
author_rank: Newbie
timestamp: August 31, 2023, 01:54:39 pm
content: 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 - 18/08/2023Time 87V & Calibrated IEC Mineral Insulated Thermocouple DMM6500 Fluke thermocouple18:37 0.7 -5.52618:40 -1.1 -4.75718:41 -0.5 -5.69518:43 -0.1 -6.26118:44 0.1 -6.47418:45 0.2 -6.68918:45 0.2 -6.85518:46 0.3 -7.014 ****** Continues to drop.

author: thm_w
author_rank: Super Contributor
timestamp: August 31, 2023, 10:17:04 pm
content: 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 measurementNote 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/

author: Simon
author_rank: Unknown
timestamp: September 01, 2023, 11:19:21 am
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2023, 11:48:07 am
content: Quote from: Simon on September 01, 2023, 11:19:21 amIf 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.

author: switchabl
author_rank: Frequent Contributor
timestamp: September 01, 2023, 03:32:03 pm
content: IIRC the DMM6500 doesn't have a cold junction sensor (unless a TC scanner card is installed?).

author: Nigmose
author_rank: Newbie
timestamp: September 06, 2023, 06:27:39 pm
content: 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.

author: thm_w
author_rank: Super Contributor
timestamp: September 07, 2023, 01:00:27 am
content: As was linked above, PT1000 can be used, cost maybe $10, seen here:https://www.mdpi.com/2227-9040/10/8/309

author: Hydron
author_rank: Super Contributor
timestamp: September 07, 2023, 06:47:58 am
content: 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.

author: ch_scr
author_rank: Frequent Contributor
timestamp: September 07, 2023, 07:47:57 am
content: 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.