Those DMMs give me the creeps. - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd December 2011, 12:40 PM   #1
Atilla is offline Atilla  Norway
diyAudio Member
 
Join Date: Jul 2008
Default Those DMMs give me the creeps.

So I've got 3 DMMs around, which complately disagree with the voltage output of a transformer I've got. They're creeping me out a bit.

The transformer's rate output is 12VAC. I measure it unloaded and:

Multimeter 1 says 10V
Multimeter 2 says 12.2V
Multimeter 3 says 14.6V

I know DMM 1 is old and its calibration is off a bit, but the other two are just a few years old and fairly good.

Scumbag DMMs.....
  Reply With Quote
Old 2nd December 2011, 02:02 PM   #2
diyAudio Member
 
Forman313's Avatar
 
Join Date: Jan 2010
Do you know how they measure AC? RMS, peak ... might be intentional?

New batteries in all three?
  Reply With Quote
Old 2nd December 2011, 03:04 PM   #3
Atilla is offline Atilla  Norway
diyAudio Member
 
Join Date: Jul 2008
Hmmm, I didn't think of that I need to look at their specs and figure out what they actually measure.

Otherwise, yes the batteries in all are relatively new.
  Reply With Quote
Old 2nd December 2011, 03:38 PM   #4
diyAudio Member
 
Forman313's Avatar
 
Join Date: Jan 2010
My experience with DMMs are quite different. I have a $12-13 one (Biltema.no .. hehe), a Gold tool ($25), a UNI-T ($50) and a Fluke ($250). There is no difference in readings. At least not AC, DC and resistance.

Kinda funny... A DMM that costs the same as a pack of siggarettes is just as accurate as the Fluke.
However, after a few months the probes came apart on the cheap one. With cheap stiff leads, it goes with out saying it wont last.
  Reply With Quote
Old 2nd December 2011, 03:53 PM   #5
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
It's normally down to batteries.
  Reply With Quote
Old 2nd December 2011, 03:58 PM   #6
diyAudio Member
 
noSmoking's Avatar
 
Join Date: May 2010
Location: Texas
hi Atilla,
Any meter unless calibrated recently can be off some meters read AC as a RMS value witch is a average of the voltages,some read peak that will throw you off too.
Because that can vary with your house main load. If you have another exact trans former compare it ,most of the time if you get a reading it;s good and if you don't it's open ,and one that gets extremely hot is a bad too.
Leads can be bought anywhere ,get a set made by fluke they last years.
I hope this has helped you some ,you can measure (with good leads) your house voltage and each meter should be close to all the same reading,also a battery 1.5 v for DC all should read the same , more cost, better parts= better meter.
  Reply With Quote
Old 2nd December 2011, 09:17 PM   #7
diyAudio Member
 
thaumaturge's Avatar
 
Join Date: Aug 2011
Location: Utah
Things are getting better as True RMS converters find their way into more and more meters. But at one time an AC measurement was best viewed with more skepticism than confidence. There were three different schemes: Peak, Average and RMS. I recall one incident where a panel meter on a test fixture wouldn't calibrate correctly when used to monitor the output of a variac. I was just a young buck and was saved by a wise old sage who looked at the output using differential scope probes. The waveform was fine until the variac passed the input winding into auto transformer mode; then it visible distorted from a more perfect sine wave, which is where the average responding meter circuit lost linear tracking. Sine, Triangle and square waves all read different depending on the type of rectifier circuit used. True RMS (the equivalent DC heating value ) converters are now about $4.50 a pop[. So even now you wont find them in a $2.99 cheapo DMM. The very best I know of is the Agilent 3458A, which has an unimaginable AC accuracy of 100 parts per million from 10Hz to 10Mhz. Of course they sell for $7,000 used. But it is easy to understand why many of us old coggers have old HP 3400C's on our benches, or even older HP 400's.
Doc
__________________
Ne timeas a facie mulierum ea ignorare
  Reply With Quote
Old 2nd December 2011, 09:55 PM   #8
diyAudio Member
 
Forman313's Avatar
 
Join Date: Jan 2010
hmm...
I dont know how you did it... but you completely lost me and made perfect sense at the same time.

Talking about rectifier circuit .. IE. half wave and full wave ?
Wouldnt a large enough cap even out the difference?

EDIT: That is, a cap integrated in the DMM? For RMS measurements, a cap is needed, right? With out, the value is either peak or average?

Last edited by Pano; 2nd December 2011 at 11:26 PM.
  Reply With Quote
Old 2nd December 2011, 11:06 PM   #9
diyAudio Member
 
thaumaturge's Avatar
 
Join Date: Aug 2011
Location: Utah
Every value cap has its own time constant that will effect the final result. AC measurement conversion is a real evolving science unto itself. The old simpson and triplett analog meters used just simple diode and cap rectifiers. The smaller the cap the more peak responding the result. Early true RMS conversion was performed using resultant heat measurement "Bolometer" systems, as many RF power measurement systems still do (Agilent/HP RF power sensors such as 8481 & 8484) The Agilent 3458A DMM uses a unique high speed phase lock to determin A/D sample rates then superimposes a high speed jitter clock that essentially moves the sample points around for a closer approximation of the final result. The HP 3400 used a Bolometer type system. Other various schemes have used double balanced demodulators before rectification. But I fear I've just muddied the waters instead of providing clarity...
Doc

Maybe this will help: Measurements of AC magnitude : BASIC AC THEORY
__________________
Ne timeas a facie mulierum ea ignorare

Last edited by thaumaturge; 2nd December 2011 at 11:19 PM.
  Reply With Quote
Old 2nd December 2011, 11:23 PM   #10
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
Meters have impedance, but not low enough to upset the AC voltage leaving a power transformer.

A fair test would be to connect all three meters to the transformer at the same time - in parallel. That way you can watch the readings simultaneously. And that will eliminate any error from changing mains voltages. and differential loading. They still may give wide ranging readings, but at least certain errors would be eliminated.


We used to have some cheap meters that worked well enough for what we used them for, but the AC function basically just put a rectifier in front of the DC input, and put a correction factor on the display reading. Worked OK for just reading the mains voltage or something. Unfortunately it also could read DC voltage that way, of course giving the wrong result. SO one could be distracted and not set the meter correctly and get a funny voltage reading. Also couldn;t measure ripple on a DC line.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Parts give away MikeW Pass Labs 21 12th November 2009 06:53 PM
Any decent, used DMMs (bench/hand) <70? Triophile Parts 10 7th April 2006 06:03 AM
i give up theChris Everything Else 4 5th June 2003 07:01 AM


New To Site? Need Help?

All times are GMT. The time now is 09:44 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2