121GW not passing calibration

[mbd16]

Hello everyone,

I have a 121GW that I sent off for calibration and the calibration team sent me the following details:

"This unit’s ACVs is spec’ed to perform up to 5 kHz. However, it would not pass calibration in accordance with the manufacturer’s specifications. I was able to calibrate this unit from 5 VAC to 600 VAC at 45 Hz to 1 kHz. Please let me know if these ranges will be acceptable to your end user."
 
(see attached image)


I will admit, i'm not a calibration nerd or very well versed in what these values mean, and how far out of spec this is. i suspect its just barely off.

121GW is on the latest 2.05 firmware. purchased in 2021.

Let me know what you guys think about this.

Thank you!

[mendip_discovery]

So they tried to do 5 kHz and the unit was outside of spec. So they did it at 1 kHz and it passed.

Not got the spec here in front of me but does it state it does 5 kHz in all those ranges?

Do you use it to measure AC voltages at >1 kHz.

[mendip_discovery]

So the spec does say it does 5 kHz in a fair few if the ranges.

See attachment.

I wonder if they were unsure if adjusting would mess up the lower frequency ranges for no real gain. I will take mine into work and see what it's like at 5 kHz.

[J-R]

Without seeing the failed test data, I don't think we can form a specific conclusion just yet.  Are they aware of the reduced accuracy specs of 1.5%-2% at 5kHz?

[thm_w]

Without seeing the failed test data, I don't think we can form a specific conclusion just yet.  Are they aware of the reduced accuracy specs of 1.5%-2% at 5kHz?

In the table they show: AC 500mV 5kHz 7.51mV

Which is 1.5% (7.5mV) + 10 counts (10uV).
So I think they are calculating that correctly.

[J-R]

Do we know the official frequency for the AC calibrations?

[alm]

If anything the limits for calibration should be tighter than in the datasheet. If the unit does not pass calibration (=comparing the DUT to a reference), it might need adjustment. If even after adjustment it still fails calibration, then it's broken and needs repair, or maybe it's a design flaw that makes it fail to meet specifications.

It sounds to me like they have not done adjustment. But as J-R says, the manual is lacking in that it does not say what frequency the ACV and ACI ranges should be adjusted at, which could certainly influence the performance at the bottom and top of its frequency range.

[mbd16]

Thanks for all the input everyone.

I don't believe the calibration lab has done any adjustment on the device. And these specs really are at the edge of the multimeter's performance.

I personally do not use it for measuring anything more than simple DC voltage and continuity tests. However, this device is a company device where anyone could use it for anything within the meter's design spec. Though, I suspect anyone doing those kinds of measurements would use a much higher end meter that we certainly have available.

When we use test equipment for testing hardware, the equipment and its calibration date must be valid to meet compliance. Although i'm sure we could just document this device being slightly out of spec at the high end and justify that the meter is of sufficient performance to give a valid test result in the case of an audit.

Just wanted to see what the masses thought about this, and try and get more useful feedback for the calibration lab.

Thank you!

[bdunham7]

When we use test equipment for testing hardware, the equipment and its calibration date must be valid to meet compliance. Although i'm sure we could just document this device being slightly out of spec at the high end and justify that the meter is of sufficient performance to give a valid test result in the case of an audit.

This is the issue with impressive but less conservative specifications and a good example why it is dangerous to compare products from different manufacturers based on their published specs.  Your problem now is that the meter is likely more than good enough for your measurement purposes but not adequate for your compliance purposes.  If the manufacturers specifications were worse (more conservative) you wouldn't have a problem!

In theory, the user can specify their own procedures and tolerances, but then you have to justify them.  The easy way out is to simply adopt the manufacturers specifications, procedures and calibration intervals.  I doubt it is worth your time to sit down and do the paperwork you would need to accept an alternative calibration spec and procedure just to keep using one $200 meter.  People do stuff like just putting a label on the meter saying "1kHz AC MAX", but I'm not sure how legitimate that is. It is possible a clever calibration tech might be able to recalibrate the meter so that it passes all of the tests, but there appear to be no separate HF trimmers or other method of adjusting the 5kHz response.  It would be interesting to know if the 5kHz ranges that were out of spec were high or low. 

[mendip_discovery]

The lab get out is to put a sticker on saying "limted calibration see certificate".

The lab may not have known about the 121gw and just assumed it was another rebranded chinese meter with romantic/overhyped specs. So the moment it didn't hold the 5 kHz spec they just dropped it to 1 kHz and just told you as it maybe fine for most measurments.

From a 9001 PoV, you just need to do a risk assessment. What are the chances it will be used for a >1 kHz AC measurement? What would you do to reduce the risk?

Though without a reading for the 5 kHz AC range you can't tell if it's just a digit outside of spec or 20%.

[J-R]

I experimented with calibrating the ACV ranges of the 121GW but the documented procedure is blatantly incorrect (even with the 10% offset).  I am trying to track down the correct, full procedure.  I have one Kickstarter 121GW that is not so great at ACV and another newer one that is very good (2021-ish).

[mbd16]

Thanks for looking into this! Are you suggesting the calibration procedure in the 121GW documentation is not correct?

My 121GW was purchased September of 2021 from the EEVBlog store for reference.

I think at this point I'm just going to have them label this unit as uncalibrated and not to be used for official testing. We've got plenty of calibrated multimeters in back I can use, and just keep this around for testing simple prototypes. Can always reference it with other calibrated meters every now and then just to see where it's at.

But i'll stay posted and check back here for updates (if any).

Thanks again!


Edit: got the full calibration report back. See attachments below

[mendip_discovery]

I would just put a label on that says do not use on AC above 1 kHz

Its a good meter and would be a shame not to use it just because of an edge case that someone might be measuring AC voltage at 5 kHz with a handheld multimeter.

[J-R]

The 121GW calibration procedure from the manual is somewhere between incomplete and blatantly incorrect.  It makes no sense to not have this information all these years later after release so it seems clear UEi is playing some games here.

I adjusted/calibrated at various specific frequencies using different methods based on the manual, community information and other info gleaned from a similar DMM sold by UEi.  If you test afterwards at that same frequency, the readings are perfect, but simply select another frequency and they are very far off.

I reached out to the company I purchased the unit from and they said they would contact the manufacturer, but I have not heard anything since.

[J-R]

Got my hands on this.  I tried the 5V range and it worked.  Back up your calibration first!

[J-R]

AC current calibration setpoints and inputs:


uA switch and jack position
50uA range
5uA/50Hz
50uA/50Hz

500uA range
50uA/50Hz
500uA/50Hz
50uA/10kHz
500uA/10kHz


mA switch and jack position
5mA range
500uA/50Hz
5mA/50Hz

50mA range
5mA/50Hz
50mA/50Hz
5mA/10kHz
50mA/10kHz


A switch and jack position
500mA range
50mA/50Hz
500mA/50Hz

5A range
500mA/50Hz
5A/50Hz

10A range
1A/50Hz
10A/50Hz


Notes:
Interesting that the 10kHz calibration points are only done for certain ranges.
When performing the offset and gain steps at the 50Hz frequency you do get the countdown timer and etc., but not for 10kHz.
I sanity checked one 121GW and the only range I found that needed major help was 50mA @ 10kHz.  It was 50.7mA before and 50.0mA afterwards.
Given the frequencies involved, this calibration procedure is doable for a typical hobbyist with just a function generator, an audio amp and another DMM with more digits and calibration data...