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Temperature measurement big discrepancy

DMM6500 6½-Digit Touchscreen Multimeter
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Bravo9
Posts: 3
Joined: July 19th, 2020, 5:47 am
Country: New Zealand

Temperature measurement big discrepancy

Post by Bravo9 » July 20th, 2020, 4:37 am

Hi All,
I just bought a DMM6500. I tried to measure temperature with a type K thermocouple. I let the instrument warm up for an hour & then tried the test. The meter is showing a temperature about 15 degrees C higher than what is correct (at ambient of about 12 degrees C). This can be adjusted in the settings, but if you turn off the instrument & turn it back on, the same error occurs. This is using the front panel connections. The same thing happens when using the rear connections. Other measurements such as current & voltage, diode, capacitance seem to be good but I have not done an intense comparison with 2 other instruments which do match, being a Brymen 629 & a Siglent 4.5 digit bench meter. Although I bought this brand new from the New Zealand Tektronix agent Vicom on Thursday last week, the unit is about a year & a half old according to the calibration sheet, which does not impress me much, being a retired calibration technician.
So my question is: Is this unit faulty, or does it need re calibration, or am I not doing the test correctly, or does the reading have to be calibrated every time you turn on the instrument ?
Any help would be appreciated.
Thanks,
Gary

Andrea C
Keithley Applications
Keithley Applications
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Joined: October 15th, 2010, 10:35 am
Country: United States
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Re: Temperature measurement big discrepancy

Post by Andrea C » July 21st, 2020, 4:47 am

If it is 1.5 yrs old, I’d first check the firmware. Version 1.7.2 is current release.

For the thermocouple on front or rear terminals, you are providing a simulated reference junction temperature. That would be an estimate of the temp at those terminals or the room temp. I’d expect a room temp closer to 20 deg C rather than 12.

Replace the T/C with a shorting jumper HI to LO (ideally cooper wire). Does the meter now report the temperature of your simulated reference junction?

Bravo9
Posts: 3
Joined: July 19th, 2020, 5:47 am
Country: New Zealand

Re: Temperature measurement big discrepancy

Post by Bravo9 » July 21st, 2020, 9:13 pm

Hi Andrea,
Thanks for the info. I did the upgrade. There is a definite improvement. It still takes about 15 minutes for the temperature to stabilize. Is this correct ?
Best regards,
Gary

Bravo9
Posts: 3
Joined: July 19th, 2020, 5:47 am
Country: New Zealand

Re: Temperature measurement big discrepancy

Post by Bravo9 » July 21st, 2020, 9:26 pm

Hi Andrea,
After waiting for the temperature to stabilize & with 2 of the same type thermocouples placed next to each other, one plugged into the DMM6500 & the other into a Brymen BM869s meter, I adjusted the temperature reading. After this I placed a short on the i/p terminals of the DMM6500. The reading is 22.6 C, which I presume is correct.
Thanks for your help. I have more questions, I will post a separate topic.
Gary

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