DC Current Accuracy on 6 1/2 digit meters, what's the point? - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: October 15, 2015, 04:42:20 pm
content: I am a bit confused by the fact that many 6 1/2 digit DVMs have accuracy specs of something like approx 0.1% for current and yet report a current reading with 6 1/2 digits of precision. For example, on my HP 34401A, the 3A range is documented as having 0.100% of reading plus 0.020% of range accuracy at the 24 hour mark (i.e., pretty much best case after calibration). If it is so inaccurate, why provide a 6 digit reading at all (i.e., 3.00000 A)?So, for a middle of the road current at this range of 2A, we have 0.1% of reading (0.002 A) plus 0.020% of range (0.0006 A)=0.0026 A of uncertainty. Doesn't that mean that the reading pretty much meaningless beyond 2 (well 2.4) decimal places (e.g., 2.00 A) for high currents at this range and is basically 3 1/2 digit at best? It seems to me like I might as well always use the 4 1/2 digit measurement accuracy setting and throw out the last digit completely.Am I missing something?

author: Len
author_rank: Frequent Contributor
timestamp: October 15, 2015, 04:48:42 pm
content: Often you are not so concerned about absolute accuracy but want to see if there's a small difference, e.g. 1.00001 vs. 1.00002.

author: SharpEars
author_rank: Unknown
timestamp: October 15, 2015, 04:50:06 pm
content: Quote from: Len on October 15, 2015, 04:48:42 pmSo you are concerned in relative moves in the current drawn, rather than the actual real amount? Then how do you reconcile this across multiple meters, if they have huge offsets with respect to each other?

author: djacobow
author_rank: Super Contributor
timestamp: October 15, 2015, 04:50:43 pm
content: I think that commonly the best spec for a meter is on a lowish range DCV and that's where all those digits really shine. Other modes might do more poorly, but I guess there's no reason to turn off digits just because.That said, accuracy and precision are different things, and though it's nice to have them together, but even when you have one without the other, there are situations when that is still useful. For example, let's say the current measurement is totally inaccurate but has high precision. That means it can still tell you about small changes relative to some baseline, even if it can't tell you with good accuracy what that baseline was.

author: tszaboo
author_rank: Super Contributor
timestamp: October 15, 2015, 04:53:13 pm
content: Transfer accuracy, and short term stability is good enough to do useful stuff with it. But yeah, try making a current measurement with air cooled shunts and you will quickly realize they are trying their best.

author: SharpEars
author_rank: Unknown
timestamp: October 15, 2015, 04:55:34 pm
content: Quote from: NANDBlog on October 15, 2015, 04:53:13 pmAnd yet the meter itself uses an air-cooled (temperature compensated?) shunt and is fairly stable to the last digit in its reading (after stabilizing), given a good stable constant current source - I've done this.

author: SteveLy
author_rank: Regular Contributor
timestamp: October 15, 2015, 05:51:18 pm
content: The extra digits are no use for absolute precision, but they are still useful for seeing how things are changing, whether going up or down and give you an indication how fast. Also useful for comparing separate measurements that differ by small amounts - as long as those final few extra digits on the DMM are stable.

author: tszaboo
author_rank: Super Contributor
timestamp: October 15, 2015, 06:06:55 pm
content: Quote from: SharpEars on October 15, 2015, 04:55:34 pmLow value shunts will drift much more on the long term. It is not the short term accuracy which is bad. Also, self heating will make nonlinearity, the smaller the shunt the bigger the error. Low value shunts (from the same series) also have higher tempco.

author: eas
author_rank: Frequent Contributor
timestamp: October 15, 2015, 07:56:35 pm
content: Quote from: SharpEars on October 15, 2015, 04:42:20 pmIn a similar thread you started about DCV uncertainty, I asked about why <10ppm long term accuracy was so important to you. I ask a similar question here.Why do YOU need the kind of precision you think is missing? I ask that not to try and somehow put you in your place, but because a concrete problem that is meaningful to you could provide a good opportunity for investigating the point of relatively low DC current accuracy on 6 1/2 digit DMMs.Without that info though, I think you are getting hung up on these issues because you have preconceived notions about the accuracy of 6.5 digit DMMs that aren't shared by the designers and original buyers of such instruments.The DC voltmeter comes first, both in the evolution of measurement equipment, and the design of a DMM. The buyers of early precision DMMs were likely accustomed to using precision voltmeters as the basis of volt, current, and perhaps resistance measurements. So, when considering their first precision DMM, they would naturally have understood the importance of the basic precision and accuracy of the DCV function.Similarly, because all the other functionality of DMMs rests on the DCV function, everything else about the design is going to rest on that foundation. It may be economically unfeasible to have a current shunt for the DCI section that approaches the precision and accuracy of the underlying DCV function, but because that precision and accuracy already exists for other purposes, there are good reasons for making the added precision available, and few good reasons for withholding it. I note though, that, at least for the Keithley DMMs I have, the digits displayed by default varies between functions, based on the the usable precision of the function for typical use.So, what I think you are missing is that:1. It is useful to have precision greater than basic accuracy for, among other things, short term relative measurements and also tracking and correcting longer term drift.2. Many of those who have a specific need for maximum precision and accuracy can make good use of "extra" precision.3. People who need more precision and accuracy than these units can offer will, I would hope, dig far enough into the specs to know that these units won't meet their need.Maybe I'm missing something though. Maybe there is a plausible scenario where someone could require greater accuracy than a typical 6.5 digit meter offers without also knowing enough to know that they should dig into the specs. If there is, I'd be interested to hear it. I'd be even more interested to hear of a scenario where such a person wouldn't still end up being burned by the effects of tempco, or induced noise, or thermoelectric effects, or contact resistance, or any of the other things that intrude when you get near ppm measurements.