Two multimeters, two different voltages. Now what?

[ComfyChair]

This is the EE equivalent of the old saying "A man with a watch knows what time it is. A man with two watches is never sure."

The version I've heard is:

Someone with one clock knows exactly what time it is.  Someone with two clocks has no idea what time it is.  Someone with three clocks has a slightly better idea than the person with two clocks.

Suspecting an issue with my old Mastech multimeter, I got a BM235. Now I have three multimeters to compare . The Aneng and The Brymen are practically in agreement. The outlier is the Mastech 

Does it make sense to have it calibrated? If so, how?

[coromonadalix]

mmm  my 2 cents,   

your meter have low resolutions   and the dmm ic's  can round values

the aneng is 20k count,  but resolution ... meh

get a better meter for precision stuff,   resolution is not precision .... not to start a huge debate

you have better meter(s) with at least 40k counts or more who doesn't cost an arm / leg  and do have better precision

find ones  in the dmm thread / compare ...

i would not even try to calibrate them, but if you are able ...

[J-R]

DC accuracy specs on the BM235 are 0.3% + 2 counts.  So 1.503V could read between 1.496V and 1.510V and still be just within specification.  So combine that with the 1.491V reading on the Mastech and you can see that without having any calibrated equipment you are having to make assumptions about which is correct, and by how much.

However, it's somewhat reasonable to assume that the Mastech is the one that is incorrect.  But adjusting it will not be reasonable.  You don't have any proper way to DIY it, and paying someone to do it is probably either not possible or will be $100USD or more.  It's probably not a closed case calibration either, so many of the range selections could still be off a bit (but within tolerance).


Sorry to say, but the BM235 was a mistake to purchase, since it doesn't add much of anything to your situation.  The minimum probably would have been a BM785 or BM867s with a calibration certificate, (or better to get the BM789 or BM869s).

But to rehash some points perhaps, you really want a bench DMM that you can ship out for calibrations every so often.  A typical bench DMM is far more stable and accurate than a typical handheld, and is worth the money spent for the calibrations.

[mendip_discovery]

This is the EE equivalent of the old saying "A man with a watch knows what time it is. A man with two watches is never sure."

The version I've heard is:

Someone with one clock knows exactly what time it is.  Someone with two clocks has no idea what time it is.  Someone with three clocks has a slightly better idea than the person with two clocks.

Suspecting an issue with my old Mastech multimeter, I got a BM235. Now I have three multimeters to compare . The Aneng and The Brymen are practically in agreement. The outlier is the Mastech 

Does it make sense to have it calibrated? If so, how?

Open the mastec up, and have a look to see if there are trim pots that can be adjusted. Hopefully there is and you can just adjust it to be similar to the others.

[tooki]

Suspecting an issue with my old Mastech multimeter, I got a BM235. Now I have three multimeters to compare . The Aneng and The Brymen are practically in agreement. The outlier is the Mastech 

I'm not even slightly surprised. At my old work, they had a few Mastechs --  I'm pretty sure they were the MS8268 -- and they weren't that great. I never got around to trying to calibrate the worst one of them before I left.

Does it make sense to have it calibrated? If so, how?

Paying someone to calibrate it would cost far more than that meter was worth when new. The MS8268 is calibrated by internal potentiometers, so I imagine the lower-end MS8264 does, too. See https://www.eevblog.com/forum/testgear/mastech-ms8268-calibration/

If you can access a good bench multimeter as a reference, use that, but otherwise I'd just twiddle the pots until the measurement more or less agree with the BM235 and AN870.

(For what it's worth, the Anengs have pretty decent accuracy for their price. I'd trust their measurement over a Mastech...)

[J-R]

Since we're in the Metrology forum, I'll point out I don't think it's proper to adjust the Mastech without at least a calibrated DMM or reference.  Yes you can make an assumption that it's incorrect, but just making it match the other two is bad form.

Can you find a friend with a bench DMM to at least add some additional confidence to the situation?

[tooki]

Since we're in the Metrology forum, I'll point out I don't think it's proper to adjust the Mastech without at least a calibrated DMM or reference.  Yes you can make an assumption that it's incorrect, but just making it match the other two is bad form.

Why? Mastech gear is mediocre on a good day, so IMHO going beyond "match it to a new decent DMM" is just putting lipstick on a pig. There's no point in applying metrology traceability principles to such a poor tool.

[J-R]

Since we're in the Metrology forum, I'll point out I don't think it's proper to adjust the Mastech without at least a calibrated DMM or reference.  Yes you can make an assumption that it's incorrect, but just making it match the other two is bad form.

Why? Mastech gear is mediocre on a good day, so IMHO going beyond "match it to a new decent DMM" is just putting lipstick on a pig. There's no point in applying metrology traceability principles to such a poor tool.

There's a solid non-zero chance that the Mastech is correct and the other two are incorrect.  I think it's a terrible step to go from "I think/assume this one is wrong" to "I'll adjust this one to match" even if you're not a stickler for the Metrology "principles".  The OP can do what they want, but we shouldn't condone that.

[tooki]

Since we're in the Metrology forum, I'll point out I don't think it's proper to adjust the Mastech without at least a calibrated DMM or reference.  Yes you can make an assumption that it's incorrect, but just making it match the other two is bad form.

Why? Mastech gear is mediocre on a good day, so IMHO going beyond "match it to a new decent DMM" is just putting lipstick on a pig. There's no point in applying metrology traceability principles to such a poor tool.

There's a solid non-zero chance that the Mastech is correct and the other two are incorrect.  I think it's a terrible step to go from "I think/assume this one is wrong" to "I'll adjust this one to match" even if you're not a stickler for the Metrology "principles".  The OP can do what they want, but we shouldn't condone that.

Nonzero? Yes. "Solid"? No. Nonzero but vanishingly small.

If we were talking about a single Fluke disagreeing with the Mastech and Brymen, it'd be one thing. But Mastech is a cheap brand that never performs that great.

[ComfyChair]

Thank you all again for the very insightful comments once again.

For those telling me the BM235 was a poor choice, well I must adimt I already had it but it was stored at another place. Having two labs is never a good idea but that's another story... 

Anyway, I think that for this power supply project I'll "calibrate" the output aganst the Brymen / Aneng and call it a day. The Mastech will be kept for general troubleshooting duties like continuity testing and the likes.

For the future I'll try to get an accurate reference and/or a reliable benchtop multimeter.

[Smokey]

At least you aren't working with IMUs.  Apparently you need to average the readings from 32 of those to get the confidence level up
https://hackaday.com/2024/10/02/easily-build-this-imu-array-sandbox/
https://github.com/will127534/IMU_Array

[unseenninja]

At least you aren't working with IMUs.  Apparently you need to average the readings from 32 of those to get the confidence level up
https://hackaday.com/2024/10/02/easily-build-this-imu-array-sandbox/
https://github.com/will127534/IMU_Array

This project only really improves IMU data when there is no other data to help - like in space applications. I've flown quadcopters for many years which only use a single IMU. To get good longer term accuracy with just IMU data is very difficult. However, when you add other sensors into the mix, like GPS and barometric sensors and use all the sources of information available, you can easily run a completely programmed flight path then return to home and land very close to your initial take off point.

While averaging the results from 32 IMUs is a great approach for off planet applications, there are better ways to do it here on earth.