Author Topic: DT9939 Calibration  (Read 3046 times)

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Offline dseyst

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DT9939 Calibration
« on: January 20, 2017, 12:40:01 pm »
Does anyone know the procedure to calibrate the DC voltage range of this meter?
 

Offline retiredcaps

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Re: DT9939 Calibration
« Reply #1 on: January 20, 2017, 07:23:12 pm »
How much is your meter out by?
 

Offline dseyst

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Re: DT9939 Calibration
« Reply #2 on: January 20, 2017, 08:29:52 pm »
Believe it or not, more than my UT61e. It's not so far out that I would consider it a problem, but if I could calibrate it I would. Otherwise I'll live with it. I got it off eBay for under $100 (new).
 

Offline retiredcaps

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Re: DT9939 Calibration
« Reply #3 on: January 20, 2017, 09:30:55 pm »
What is your voltage reference that you are using?  Is the DT9939 within its specification?
 

Offline dseyst

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Re: DT9939 Calibration
« Reply #4 on: January 20, 2017, 10:34:34 pm »
I'm using one of those cheap AD584-M voltage references.I can list the references and the results fromt the two mentioned meters. I do believe the DT9939 is within specs.

REF 2.49923  DT9939 2.4980  UT61E 2.499
REF 4.99852  DT9939 4.994  UT61E 4.998
REF 7.50045  DT9939 7.494  UT61E 7.500
REF 9.99918  DT9939 9.992  UT61E 9.999
 

Offline retiredcaps

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Re: DT9939 Calibration
« Reply #5 on: January 20, 2017, 11:16:51 pm »
I'm using one of those cheap AD584-M voltage references.I can list the references and the results fromt the two mentioned meters. I do believe the DT9939 is within specs.

REF 2.49923  DT9939 2.4980  UT61E 2.499
REF 4.99852  DT9939 4.994  UT61E 4.998
REF 7.50045  DT9939 7.494  UT61E 7.500
REF 9.99918  DT9939 9.992  UT61E 9.999
I'm not going to do the math above to check, but the AD584-M has potentially dubious readings on the paper.  See

https://www.eevblog.com/forum/metrology/are-cheap-ad584-units-worth-it/

The AD584-M itself might be a decent reference, but if sellers are just cutting and pasting/writing down the same values on each unit, the numbers are worthless.

While I don't have one or need one you will likely find a better and honest reference at

voltagestandard.com

Yes, you pay more, but so far I haven't seen anyone on eevblog complain or say anything negative about it?
 

Offline dseyst

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Re: DT9939 Calibration
« Reply #6 on: January 20, 2017, 11:27:46 pm »
I think you're right. A better voltage reference would be a good idea.
 

Offline retiredcaps

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Re: DT9939 Calibration
« Reply #7 on: January 20, 2017, 11:46:05 pm »
I think you're right. A better voltage reference would be a good idea.
It is up to you, but both DT99399 and UT61E seem to be close enough to each other that I would say you have fairly decent accuracy depending on your requirements.
 

Offline dseyst

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Re: DT9939 Calibration
« Reply #8 on: January 21, 2017, 09:37:33 pm »
A little math shows that the DT9939 is within specs. Nevertheless, my obsessive side wants to be able to calibrate this meter. I don't mind buying a supposedly better reference standard but being able to "calibrate" this new CEM multimeter is something I'm still interested in.
 

Online rstofer

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Re: DT9939 Calibration
« Reply #9 on: January 22, 2017, 12:14:48 am »
Calibration is different than 'adjustment'.  If you send it to a cal lab all you will likely get is the same chart you already produced from your 'standard'.  That chart would be more useful if the error % was included.

It sounds like you want to adjust the meter.  Can you find any information re: the internals?  Does it even have pots to adjust various readings?  Open it up and see what you have.  It my be that there are no adjustments and what you got is what you got.

Given that you have a standard, has it been calibrated traceable to NIST?  If not, it might be very good but you can't prove it.
 

Offline joeqsmith

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Re: DT9939 Calibration
« Reply #10 on: January 22, 2017, 12:25:35 am »
I would imagine CEM or Extech would provide the adjustment procedure.  Someone else had asked me recently about this and I believe Extech supplied it to them.   
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
 

Offline dseyst

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Re: DT9939 Calibration
« Reply #11 on: January 22, 2017, 02:11:43 am »
I emailed CEM a while ago, but in the meantime I took off the back of the meter and used the pot at the bottom left of the metal shield on the board to fine tune the dc voltage. The pot is very sensitive and it probably took me about 20 minutes to adjust the meter.  I would't be surprised if something else is out of whack now, but the dc voltage readings more closely follow the voltage reference that I have.
 


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