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Products => Test Equipment => Topic started by: ballsystemlord on October 26, 2023, 02:33:22 am

Title: [Sloved] Percent of range, how is it calculated?
Post by: ballsystemlord on October 26, 2023, 02:33:22 am

Hello,
I was reading some specs for DMMs and instead of the typical xx% + xxDigits, I came upon xx% + xx% of range.
So if the range is 1000.00V, and I'm measuring 1V, and the accuracy is 0.1% + 0.01% of the range, then I could get a reading of up to 101.001V?!
(1V*1.001%)+(1000(range)*0.01%) == 11.001V
And that's assuming I'm using only the portion of the range after the period. If not, then 1V could become:
(1*1.001)+(100000*0.01) == 1,001.001V!!!

Now I suspect I'm making a mistake somewhere, please point me in the right direction.

Thanks!

Title: Re: Percent of range, how is it calculated?
Post by: Fungus on October 26, 2023, 02:40:09 am

I assume you mean when measuring AC voltage.

Many TRMS converters are inaccurate at the low end of their ranges.

That's why we have auto-ranging meters - to switch to a lower range on demand and avoid these problems.  :)

Title: Re: Percent of range, how is it calculated?
Post by: ballsystemlord on October 26, 2023, 03:06:33 am

No, not AC. This is the DC range.

Title: Re: Percent of range, how is it calculated?
Post by: mwb1100 on October 26, 2023, 03:10:16 am

You’ve got a decimal point in the wrong place. 

0.01% of the range would be:

    (1000V * 0.0001) = 0.1

Title: Re: Percent of range, how is it calculated?
Post by: ballsystemlord on October 26, 2023, 03:24:38 am

Which if we took the above datasheet picture and applied that for 1V at the 1V range, we'd get ((1*1.01)+(1*0.00005)) = 1.01005. Or the reading could be off by as much as 1% even though the meter's accuracy is 0.01% in the 1V range... Which means that absolute accuracy specs are a bit misleading?

Title: Re: Percent of range, how is it calculated?
Post by: J-R on October 26, 2023, 04:02:07 am

(1V*0.0001)+(1V*0.00005)=0.00015V
So in spec is between 0.99985V and ‭1.00015‬V

Title: Re: Percent of range, how is it calculated?
Post by: bdunham7 on October 26, 2023, 04:02:51 am

I don't know how you are doing that math?  0.01% means you multiply by 0.0001 to get the tolerance amount.

So for 1V on the 1V range you have (1V x .0001 + 1V x .00005) = 0.00015 allowable error.  So your 1V input would be guaranteed to read between .99985 to 1.00015.

Now 1V on the 1000V range you have (1V x .0001 + 1000V x .00005) = .0501 allowable error.  Your 1V input is now guaranteed by spec to read betwen .9499 and 1.0501, except that you only have a 5.5 digit meter so in the 1000V range, or the 1000.00V range with all the digits shown, your 1V can only read 000.95 to 001.05.  Of course the leading zeroes may be suppressed.

Title: Re: Percent of range, how is it calculated?
Post by: ballsystemlord on October 26, 2023, 04:27:31 am

I Sorry about that, I keep forgetting that a 0.XX% means to multiply by 0.00XX to get the correct result.

Title: Re: [Sloved] Percent of range, how is it calculated?
Post by: AVGresponding on October 26, 2023, 05:21:36 am

AC in particular will usually specify the accuracy is only valid for readings within 20-80% of range (or thereabouts) as well.