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Is the resistance of digital DMMs a concern under normal measurement conditions?

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shapirus:

--- Quote from: coromonadalix on December 17, 2023, 03:45:58 pm ---again with the 500k count  loll   its a gimmick, more resolution  is not precision   :palm:

--- End quote ---
they don't claim anywhere that it's about precision.

Kleinstein:
There is no extra difficulty for the scale factor for the lowest range.  The difficulty is in the offset part, so getting really a zero reading for a short. This starts with thermal EMF from the test leads. So holding the probes with the hands in usually a no go if one cares about µV.  Thermal fluctuations can also be a relevant noise source at the low end.
The extra resolution could still be useful for low voltages, like thermocouples or short finding from residual voltage at ground points, even if it does not improve the accuracies in the more normal ranges.

For checking the meters reference and stability of the scale factor it is usually better to use a higher range (e.g. 2 V) to avoid those low voltage offset problems.
A battery if undisturbed can be a low noise  and short time reasonable stable voltage source for some tests. However batteries are still sensitive to temperature and mechnical stress that can cause the voltage to make jumps.
One such test with a battery could be the turn over test: measure a battery voltage with both polarities of the meter. The difference is from the meters offset and INL error.
With a meter with 10 M input resistance even that can be enough load to a battery to get it slowly drift (usually down). So a test may want to check a few cycles (pos, neg and zero).
The noise when measuring the battery voltage over some time is a mix of noise and drift of the battery and the meters internal reference. When handled with care the battery is usually at least lower noise.

Normally the lead resistance should be way lower than 4 ohms, usually more like 0.1 - 0.2 ohm or so as the reading for a short with 2 wire ohm. Probes with 4 ohms would likely blow when used with a higher current. If the contacts are that bad (e.g. chrom coated) it would be a reason to through them out or grind down the tips.

joeqsmith:

--- Quote from: shapirus on December 17, 2023, 03:09:24 pm ---This is Brymen 869s.
... However the 6th digit, where accuracy is not specified, can still be used to monitor changes of measured values over time. ...

--- End quote ---

I've used that 500k count mode for trending drift.  I wouldn't say it's a gimmick but useful in the right hands.  Following shows mine attached to a reference. 

https://youtu.be/dnnPDajITqM?t=141

shapirus:

--- Quote from: Kleinstein on December 17, 2023, 04:04:25 pm ---There is no extra difficulty for the scale factor for the lowest range.  The difficulty is in the offset part, so getting really a zero reading for a short. This starts with thermal EMF from the test leads. So holding the probes with the hands in usually a no go if one cares about µV.  Thermal fluctuations can also be a relevant noise source at the low end.

--- End quote ---
It displays values changing in the -0.004..-0.007 mV range when the inputs are shorted right at the jacks with a short piece of (thin) copper wire. Stable -0.00 mV in the 50k-count display mode.

Of course I wouldn't care to measure absolute µV values with a DMM like this, but it can work just fine to monitor a slowly changing value.

Fungus:
I think the gold leads is a red herring. Try swapping the leads around and see.  :popcorn:

The resistance of DMMs is a concern in proportion to the impedance of the voltage source.

Some of them have high impedance modes for measuring delicate circuits.


--- Quote from: bdunham7 on December 17, 2023, 05:48:43 am ---Which DMMs have µV ranges??

--- End quote ---

Lots of them. Even some cheap Anengs.


--- Quote from: bdunham7 on December 17, 2023, 05:48:43 am ---I didn't really understand the part about disconnecting the gold-plated probes.

--- End quote ---

I assume the meters were connected in parallel.

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