Questions according DMM

Martin Czech martin.czech at intermetall.de
Tue Nov 9 13:45:52 CET 1999


:::Hello,
:::
:::I'm new to the DIY World. I would like to buy a DMM, but I dont know
:::exactly what features are necessary and which not. Espacially I would
:::like to know what does 3, 3 1/2 or 3 3/4 display means.
:::
:::A further questions is the meaning of the "digit" error (e.g. 0,5% +-
:::3digits). Do I need a true RMS DMM?
:::
:::OK give me some hint.
:::
:::Thank you very much.
:::
:::Karsten Welskop

Metrix makes some reasonable DMM with 20000 Digits, not expensive.
I'm satisfied with that. Additional PC port support, but I didn't try.
Cap meter included, good to estimate those little caps I'm never sure
of the value. Frequency counter also. Hfe. It is sometimes hard to read the display (light).

The % is the scale error, it is of course proportional to the value.
If the manufacturer would use better resistors, or trimmers, most DMM
could have much better performance. Dual slope ADC are very linear,
and have automatic offset cancelation today. It is really the resistor
network that cripples the performance.

Additional you have an uncertainty, the is the meaning of the "digit"
spec.  3 digits would normally mean +- 3 counts uncertainty, additional
to the scale error.

So far I have not seen any desktop model, that could outperform my 
metrix handheld in price/performance.

I have posted a short report about that DMM when I testet it with
some of our ultra-precise lab DMM (HP/ high price). It 
must be in the archive.

This is also when I found out that so many useable digits are demanding:
The Ohm measurement makes really no sense when using clamps,
better use banana plugs, wire and solder for the lower Ohm ranges.

m.c.





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