I received my DMMCheck Plus -- a quick test - Page 1
Forum section: electronics / general

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2018, 12:49:23 am
content: My DMMCheck Plus arrived this week, so I used it to check a few of my DMMs while it is still recently calibrated.The table compares my DMM readings with the expected values. Green cells are where the reading is within the expected uncertainty of the DMMCheck, red is outside the range (but not necessarily outside the expected accuracy of the meter under test).The BM869s continues to look like a nice meter. (It was purchased in October 2016, so it is about 2 years old at this point.)

author: xrunner
author_rank: Super Contributor
timestamp: November 18, 2018, 01:59:42 am
content: Good data. I also received mine and although I didn't make a compilation of tests I didn't find my main DMMs readings to be of any concern. I might compile some data tomorrow.

author: alm
author_rank: Super Contributor
timestamp: November 18, 2018, 03:27:57 pm
content: It is probably a decent first approximation to consider the uncertainty in the reference and the DMM independent, although there will obviously be covariance due to common external effects like environment temperature. Then you could naively say that if the reference is in spec and the DMMs are in their one-year spec, in 95% of the cases the measurement of the DMM should be within the reference +/- RSS(uncertainty reference, uncertainty DMM), which probably for all but the 3456A would mean the error of the DMM dominates, at least for DCV/R. If you assume the reference to be in spec, then any meter reading outside this range would be likely to be outside it's 1 year tolerances, though the opposite may not be the case, especially if the test uncertainty ratio is low.For ACV I would argue that the fact that the DMMCheck Plus uses a square wave adds extra error depending on the bandwidth of the DMM. The Protek with its 20 kHz bandwidth, compared to the 3456A with its 250 kHz bandwidth, could make a difference there.

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2018, 04:11:58 pm
content: On thinking about it, I suppose the uncertainty in the reference resistor values is not 0.1%, but should be the measurement uncertainty of the 3458A used to measure them, which is about 10 ppm. I should adjust the table accordingly.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 18, 2018, 04:58:32 pm
content: It depends on the stability and TC of the resistors. If they are not very low TC and good stability the resistors won't hold the 3458 accuracy very long. So after some months it could be close to the nominal 0.1 %, maybe a little better.Using only square wave is really limiting the value of the AC part. It would be nice to at least also have some low pass filtered square wave to remove the very high frequency part before the DMM to check. Even with an additional uncertainty due to the filter this would be much more useful for a DMM with limited BW. The effect of having a square wave is worst, as one hardly knows how the higher frequency roll of of the DMMs is.The way the higher frequencies roll off can be tricky and can be amplitude dependent with analog true RMS chips.

author: alm
author_rank: Super Contributor
timestamp: November 18, 2018, 04:59:15 pm
content: It is a combination of the uncertainty of the measurement plus the stability of the resistor since then.The uncertainty of the measurement would be a combination of the uncertainty of the 3458A and the stability of the setup. If the measurement was unstable due to temperature fluctuations in the resistor (as I've observed with cheap 1/4 W metal film resistors in the past), it's not going to be accurate down to 10 ppm. This should be specified in the specs of the reference.The stability of the resistor would be a combination of age drift, shift due to mechanical stress, temperature coefficient, etc. This may be better than 0.1%, but I'm not sure how much. Measuring the resistor 1 year later at a temperature and humidity far from the value it was measured at is not likely to be within 10 ppm of the value measured by Doug.

author: IanB
author_rank: Super Contributor
timestamp: November 18, 2018, 05:11:18 pm
content: The resistors are stated to be Vishay PTF56 0.1% 10 ppm/°C parts, so a temperature change of 1 degree will already move the value outside the accuracy of the original measurement.