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Electrical Metrology | Measurement Tips & Products | Calibration standards => Electrical Metrology for enthusiast => Topic started by: Kiriakos GR on November 27, 2018, 07:15:05 PM
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I came across negative Ohms readings by testing calibration of three Keysight U1272A multimeters and I stayed puzzled for several minutes.
The answer came to me by looking these meters LCD which at top right side there was the indication of temperature.
Greece in November translates to beginning of true winter, and at that day room temperature was low at 16~17 Celsius.
All test and measurement equipment's they get calibrated at 23C room temperature for a good reason.
Negative Ohms Readings in the range of mOhm this is side effects influencing measuring circuit because the meter is not warm enough as it was at the time of calibration.
Zero Ohm value this is resistance at 23C by the law of Temco (Temperature coefficient).
When ppm of resistance decreases a regular multimeter will display negative Ohms value by design, because there is no other way to inform us that few ppm are missing from what we define as zero ohm resistance at 23C.
Basic Physic lessons at school it clearly teach us properties of metals, cold metals equals to less resistance, hot metals equals to higher resistance.
Temco (Temperature coefficient) at multimeter it can be more obvious when a cheap meter operates at 15C or bellow that point, and all that it is needed is you to sort your test leads ends at resistance smallest range.
Temco (Temperature coefficient) at multimeter at DC voltage this is a different story, no one can see a reading of missing micro-volts because HH multimeter does not have such high display resolution.
If you are calibration enthusiast and you are in need testing your DMM then you should wait to perform such tests in a warm day or in temperature controlled environment. ;)
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Five months later than this topic creation, and my room temperature this raising now close to 20C and just a bit above, soon it would be a good time to perform a new inspection of all three U1272A and also of my newcomer 6 1/2 precision multimeter.
Specific 6 1/2 precision multimeter this has voltage reference with active heater and this translates that it is immune to Temperature coefficient drift.
Therefore in the upcoming weeks I will have the opportunity to explore how immune really this is … At 23 Celsius and higher than that. :)