Accuracy Claim on test instrument

[lyledunn]

I have a new digital ohmmeter that claims an accuracy of 2%+-2digits on the 20ohm scale (19.9). The resolution is 0.01ohms.
Does this mean that a reading of say 0.4ohms could be 0.18 or 0.62 resulting in a deviation from the measurand of circa 55%?

Regards,

Lyledunn

 

[davidbeach]

I would say it could range from 0.4*1.02=0.41 + 2 digits = 0.43 on the top end to 0.4*.98=.39 - two digits = 0.37. So 0.4 ohms could show up on the meter as anything from 0.37 to 0.43.

 

[IRstuff]

That would depend on whether the specification is 2% of FS or 2% of reading.

TTFN

 

[davidbeach]

Well, if it was 2% of full scale, a resistance of 0.4 ohms could read anywhere from 0 to 0.82 ohms. I would hope that it would be 2% of reading, but who knows?

 

[lyledunn]

Yes, thanks David, as pointed out by IRstuff there is ambiguity in the claim. My little knowledge of error theory recalled that accuracy is referenced to f.s.d. However, I read an article in a trade mag that alluded to the almost proposterous margin of error in some instruments designed for field work. I have of course asked company to clarify.

Regards,

Lyledunn

 

[IRstuff]

Just as a general comment, 0.4 ohm should NOT be attempted on a standard ohmmeter. The probe contact resistance is a sizable portion of 0.4 ohm. You should and must use a micro-ohm meter or a Kelvin connection for measurements that low.

TTFN

 

[itsmoked]

IRstuff you can use a standard meter with acceptable accuracy if you ZERO it on one side of the unknown. That works very well.

 

[IRstuff]

True, if you've got a zero function ;-). And, if you remember to set it back to zero every time you use it. None of my DMMs have that particular feature.

TTFN

 

[itsmoked]

You should get a meter made this century. You know... the kind without a needle.







Actually I have to teach my daughter about magnetic fields verses current and wish I had a needle based meter Now....

 

[IRstuff]

I'll assume that you missed the "DMM" ;-)

TTFN