U1282A microvolts drifting - Page 1
Forum section: electronics / testgear

author: tgetec
author_rank: Newbie
timestamp: October 23, 2021, 08:50:13 pm
content: Hi all,have an U1282A here that gives me headache; the millivolt range is drifting up to +/- 50 microvolts within hours. I can reproduce this with the inputs shorted, and let it sit there. It looks to me the slightest environment temperature variation causes the meter to start drifting. Even changing the position from laying flat on the bench to have it upright on the tilt stand will end up in drifting values.Testing the same setup with an U1252B or an Brymen 869s, these both do not drift at all.Giving the Specs it could be arguable that the "short input" measurement could be within spec, the question is: Is "drifting" within spec?I noticed also that shorting the inputs almost always starts at "0.000" an than something starts counting up or down.MicroAmps are stable as expected.Just an example below, have seen "0.057" in this configuration also.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2021, 02:36:02 am
content: Could it be a thermocouple effect?

author: exe
author_rank: Unknown
timestamp: October 24, 2021, 08:00:04 am
content: All DMMs were tested with the same leads?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2021, 08:09:20 am
content: Are the aligotor clip leads really making contact ? I have similar leads and the plug part is relatively short, too short for some shrowded terminals.Thermal EMF can be up to some 50 µV with such leads, just after handling the cables with the hands. For a steady state fixed temperature this would be a bit on the high side.There is a slight chance to get some internal thermal EMF, e.g. from the protection part, though 20 µV would be a bit on the high side for a good meter. This would be mainly from temperature gradients, so not much depending on the external temperature.

author: electr_peter
author_rank: Unknown
timestamp: October 24, 2021, 10:08:18 am
content: U1282A 60mV range specs: 0.05 % + 10 (Accuracy ± (% of reading + counts of least significant digit). Thus, +-10 µV change is expected. [source:U1280 Series Handheld Digital Multimeters (.pdf 2.73 MB)]Photos show shorted 60mV range went from 0 µV to -20 µV while internal temperature (showed in top right corner) went from 18.5 C to 19.1 C. This gives 20 µV/ 0.6 C = 33.3 µV/C sensitivity which is in the range of typical thermocouples.It is likely combination of DMM reading drift due to specs plus thermal effects of the leads. You can't expect to see no changes in mV or µV ranges when temperature changes and different metals are involved.

author: tgetec
author_rank: Newbie
timestamp: October 24, 2021, 03:07:12 pm
content: Thanks for looking at this.Basically I had similar thought on this, thermocouple effects, material combinations, DMM internal temp differences... I was not sure about the "in spec" temperature caused deviations, thanks for pointing a this.Btw. switching the direction of the shorted leads makes no difference.(If its a thermocouple effect of the leads, it should switch the sign in the reading also, right??)I see that these effects make the µV range kind of unpredictable, (see below) when temperature is changing.A bit unpleasent that on the "high-end" meter the effect was so obvious noticeable, where the older/cheaper DMMs needs to be treated much harder to show similar effects:Just done a quick test again:U1252B: (btw it is an Agilent)20.5°C::0µV17.1°C::5µVU1282A21.8°C::2µV17.1°C::37µVAnd meanwhile the U1252B is at 18.7°C::-4µVthe U1282A is at 18.3°C::-39µV it has drifted 76µV

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2021, 03:18:38 pm
content: With temperature below room temperture one should be careful with condensation. This can upset things a lot.A point to check may be the meters internal input bias. After ESD damage of some dirt/ surface deposits there may be higher than normal input bias and that can cause some offset from a dop on the protection part. So how much does the meter read with an open input (I assume 10 M impedance for a handhelp) ? This may need some AC shield around the DMM, commected to the COM terminal.

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2021, 02:23:32 am
content: Quote from: tgetec on October 24, 2021, 03:07:12 pmIt would not matter if the leads are made of the same materials.The umbalanced thermocouples are likely to be internal anyway.