Do Null meters need to be Traceable? - Page 1
Forum section: electronics / general

author: ttttrigg3r
author_rank: Contributor
timestamp: October 13, 2022, 03:13:55 pm
content: Happy October all. I hope everyone is doing well.Is anyone using null meters for their calibrations? My question and scenario is this. I'm using a Null meter to do a comparison of a 10V power supply UUT against a 10V Standard (such as Fluke 742A). I know that the Fluke 742A is my standard traceable to NIST. Does the null meter also have to be traceable to NIST? i.e.does that null meter need to be a calibrated standard as well? When I read over standards such as 17025 and Z540, it says all standards need to be traceable.Thank you in advance.

author: Villain
author_rank: Regular Contributor
timestamp: October 13, 2022, 03:24:45 pm
content: Without giving a definitive answer:If the nullmeter contributes to the measurement uncertainty it is considered a calibration standard and needs to be traceable.If it does not contribute to the measurement uncertainty it is considered an accessory and does not need to be traceable.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 13, 2022, 03:37:43 pm
content: I am also not sure about a definite answer.However, I have bought 3 used Null-Meters over the years and interestingly, two of them had a sticker saying:"No calibration required"

author: alm
author_rank: Super Contributor
timestamp: October 13, 2022, 05:15:19 pm
content: If you use a null meter as they are primarily intended, namely adjusting something like a KVD until the difference between two voltage sources is zero, then I think you can call it a self-calibrating device like the Fluke 752A or 720A. This is what Fluke claimed about their 7105A system: the standard cell needed calibration, the null meter and Kelvin Varley Divider could be self-calibrated using nothing more than a stable voltage source, and these two could then be used to adjust other equipment to the standard cell.However, if you use it as a microvolt meter to measure a value other than zero, then you can't verify this using self-calibration, and I'd argue you'd need to calibrate it to check if it meets its mediocre absolute accuracy.Regarding if it contributes to measurement uncertainty, a divider like Fluke 752A contributes to measurement uncertainty, yet generally isn't seen as requiring external calibration. How would you even calibrate an adjustable device? Verify its temperature coefficient, thermal EMF and short term drift? I believe these are considered guaranteed by design.

author: bdunham7
author_rank: Super Contributor
timestamp: October 13, 2022, 05:28:25 pm
content: Quote from: alm on October 13, 2022, 05:15:19 pmThat's the question! Perhaps put some calibration tape over the big 'ZERO' knob on the front?

author: alm
author_rank: Super Contributor
timestamp: October 13, 2022, 05:42:58 pm
content: But the zero is only stable for up to 24h. It is designed to be adjusted before use, so you'd be increasing the uncertainty by taping that knob...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 13, 2022, 05:54:11 pm
content: Quote from: bdunham7 on October 13, 2022, 05:28:25 pmEven those instruments that are supposedly self calibrating like the Fluke 752 or 720 dividers would still from time to time need a check that they still work as designed. This would be things like the isolation resistance to the shield / case. For the null meter this would also be isolation, input bias current and it's zero (turn over or short). The tests could be part of the use procedure (e.g. zero adjust) or an extra calibration (check of proper function) step. This may not need a special cal lab or tracible instruments, but the functional check still needs to be done from time to time.Modern instruments may do some of the tests as a self test - still usually not 100% complete.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 13, 2022, 06:00:55 pm
content: Maybe you need a traceable short circuit to check the zero.

author: Ernesto
author_rank: Newbie
timestamp: October 23, 2022, 02:10:28 pm
content: Yes it needs to be calibrated or at least be characterized by you. Depending a bit on how much influence this has on the end result. The ISO 17025 is risk based. So you would need to assess the risks and consequences. On a more technical level, since the null detector will never really measure zero this residual could still need some correction. The calibration is rather simple since the meter will show some microvolts which you only need to know at the 1 - 10% level. (depending on your accuracy needs) Considering the zero knob, by doing the measurement twice, one connected normal and second where the voltage on the null detector is reversed. The mean of the two measurements is the "real" zero, the deviation from the mean is the "real" measurement. This way only the gain of the meter need to be calibrated.

author: mendip_discovery
author_rank: Super Contributor
timestamp: October 23, 2022, 04:18:46 pm
content: Quote from: Ernesto on October 23, 2022, 02:10:28 pmI thought I had replied to this thread. But I agree with Ernesto. I think the main ones are verifying that there is no bias in the zero of the null meter and the repeatability. If it is of a dial type I would expect checks on the deviation divisions. You would also have to check the source that it is giving out what you think it is, this is partly so others can repeat your tests and the measurements should be similar, also you could be sending out 2 A rather than 1 A therefor causing heating issues etc with the UUT.9001 and 17025 are both based on risk assessments. If you think it could have an effect on the results you need to document you looked at it, assesd it and included or excluded it in your calculations/Uncertainty/taceability.

author: TimFox
author_rank: Super Contributor
timestamp: October 23, 2022, 05:08:15 pm
content: Quote from: Ernesto on October 23, 2022, 02:10:28 pmEstimating the voltage uncertainty is straightforward, with a heavy short-circuiting conductor across the input.For many applications of a null meter, the input bias current may be important, requiring another measurement with an appropriate resistance across the input.Good null meters are designed to minimize any bias current, but the current can be important for a DVM used in place of a null meter to find a null.

author: KT88
author_rank: Frequent Contributor
timestamp: October 23, 2022, 07:42:31 pm
content: ...and not to forget: reduced isolation would cause common mode issues when floated at high potentials.

author: alm
author_rank: Super Contributor
timestamp: October 23, 2022, 08:13:28 pm
content: If you look at the calibration procedure for the Keithley 155, they test for all of those. But to me saying that makes it traceably calibrated is a bit misleading, because the important part is not that you checked that the dial is accurate within 2% compared to a traceable source (do you really care if a reading is 1 uV +/- 2%?), but that you did a functional check that the parameters like isolation are within (pretty broad) specifications. So qualitative rather than quantitative.