ITTSB.EU Blog Forum
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. :)
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This is news even for me, that even at 6 1/2 precision multimeter, they are two major factors these influenced by Temco.
No1: Internal voltage REF, this responsible for measurements accuracy.
No2: Internal current source chip, for Ohms mode, this must be a stable supply of 5mA at 10 Ohms range.
Internal current source chip, this is a regular component, not shielded and not specially protected.
In theory, Voltage REF and Current sources, they require a Burn-In process, so to obtain maximum stability.
At my 8846A, Voltage REF this must counts approximately 6000 Hours in operation or more.
At my 8846A, current source chip, this operates occasionally, and it might worked no more that 48 hours so far.
Its thought this drives the next, its possible an fresh calibration adjustment, this targeting more at any state changes of this current source chip.
I am currently running overnight test and data log, so to inspect current source chip behavior and performance.
Thanks to TsDMMViewer, I can run this task, with 8846A display set to OFF. :)