How often do you calibrate your DMM? - Page 1
Forum section: electronics / testgear

author: JoeB83
author_rank: Regular Contributor
timestamp: July 13, 2015, 12:25:40 am
content: I've been trouble-shooting a power supply over the last two days, and I noticed that all four of my DMMs don't agree on voltage. I have a Fluke 189, 179, Agilent U1241A, and an Agilent 34410A. I'm using the 34410 as my reference, as it was calibrated in January & the fact that it is my bench meter. All of my DMMs are off by 500-800mV. The 189 & 179 were calibrated about 18 months ago, and the U1241A about 2 years ago. I guess it confuses me why they'd go out of spec so quickly. I know this may seem like a small problem to some, but tracking precise voltages is important for the task I'm working on at this moment.Any ideas?Thanks!

author: retiredcaps
author_rank: Super Contributor
timestamp: July 13, 2015, 12:33:37 am
content: Silly question, are you measuring the voltage source with all 4 meters simultaneously?The reputation of those 4 meters suggest it should only be out be 1 or 2 mV not 500 to 800mV.

author: JoeB83
author_rank: Regular Contributor
timestamp: July 13, 2015, 12:40:52 am
content: Quote from: retiredcaps on July 13, 2015, 12:33:37 amYep. When I noticed it, I tried all four at the same time.

author: Fsck
author_rank: Super Contributor
timestamp: July 13, 2015, 12:44:05 am
content: 500 to 800mV of what? if it was 1V, I'd suggest that you have a bunch of faulty meters.I have an 8842a, 3457a, BM869, fluke 87V and they all tend to agree pretty closelythose meters really shouldn't go out of spec quickly. historically, good quality meters don't really drift out of their 1 year specs

author: Lightages
author_rank: Unknown
timestamp: July 13, 2015, 12:44:46 am
content: The other meters are out compared to the Agilent 34410A? Or every meter including Agilent 34410A are out? What is the voltage source?

author: retrolefty
author_rank: Super Contributor
timestamp: July 13, 2015, 12:54:50 am
content: Ah grasshopper, person with one voltmeter always knows the voltage. Person with two or more never quite sure.

author: jwm_
author_rank: Frequent Contributor
timestamp: July 13, 2015, 12:56:59 am
content: Is there a non trivial AC component to the voltage? Maybe they have different low pass cutoffs for DC measurement.

author: tautech
author_rank: Super Contributor
timestamp: July 13, 2015, 01:04:23 am
content: Quote from: jwm_ on July 13, 2015, 12:56:59 amYep, that might do it.Check against a pure DC source like a LA battery.

author: JoeB83
author_rank: Regular Contributor
timestamp: July 13, 2015, 01:40:05 am
content: Thanks for the replies. I've been testing the stability of a Power Designs 0-36V, 0-5A power supply. I've been keeping track of voltage stability at 5, 9, 12, 15, 24, & 36V without load- I was going to start testing with loads of 1-5A until I found the problem with my DMMs.All my measurements are based on the 34410-the 189 & U1241 are closest, 100-150mV, while the 179 was off -800mVGood point with AC component. I'm actually out of the house at the moment, I'll check on my scope if there's any AC.Thanks!

author: JoeB83
author_rank: Regular Contributor
timestamp: July 13, 2015, 01:42:27 am
content: Quote from: retrolefty on July 13, 2015, 12:54:50 amToo true.

author: Muxr
author_rank: Super Contributor
timestamp: July 13, 2015, 02:21:48 am
content: That's sounds like quite a lot of a disparity. I suspect something else might be at play. Noise in the rail you're measuring?

author: Theboel
author_rank: Frequent Contributor
timestamp: July 13, 2015, 03:47:15 am
content: I have 34410A and fluke 189 they tend agreed 10mV (under 4 volt DC) in "quite area" but when its use in my Transmitter (1kW to 35kW FM transmitter) site the result is really really different.

author: Fungus
author_rank: Super Contributor
timestamp: July 13, 2015, 08:06:45 am
content: Quote from: JoeB83 on July 13, 2015, 12:25:40 amNot possible unless it's a power supply in the multi-kV range. Suspect procedural error.Quick check: Measure something else.PS: Maybe the output is unstable, have a look with a 'scope.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 13, 2015, 08:38:05 am
content: My 34410A is probably my most stable DMM and every year for calibration it is only tested and never calibrated, because it is always totally in specs. It seems that something else is disturbing your meters. Take them away from your test setup and compare them to something else, even a cheap voltage reference will do, just for comparing.

author: EEVblog
author_rank: Unknown
timestamp: July 13, 2015, 09:47:46 am
content: Quote from: Muxr on July 13, 2015, 02:21:48 amYep, I would be staggered if those meters were up to 800mV off even at 36V.Something else is wrong.

author: EEVblog
author_rank: Unknown
timestamp: July 13, 2015, 09:48:48 am
content: Stick all the probes into the top of a 9V battery at once and tell us what you get.

author: Fungus
author_rank: Super Contributor
timestamp: July 13, 2015, 09:53:12 am
content: Quote from: JoeB83 on July 13, 2015, 12:40:52 amSimultaneously? In that case you've got a very unstable voltage (massive ripple).

author: JoeB83
author_rank: Regular Contributor
timestamp: July 13, 2015, 06:54:12 pm
content: I measured a 9V battery, all seems to be well. I got:34410A: 9.1223189: 9.123179: 9.14U1241A: 9.13I did look at the output ripple of the PS, it was about 70 mV p-p. Not sure what is up, but I'll check some of my other power supplies in a bit.Thanks for the help!

author: commie
author_rank: Frequent Contributor
timestamp: July 13, 2015, 07:10:35 pm
content: Quote from: JoeB83 on July 13, 2015, 06:54:12 pmWell that seems better!, one thing to remember when comparing voltmeters against each other, make sure that the measuring probe ends(crocs) are on exactly the same spot, particularly if you have quite modest currents active.

author: eas
author_rank: Frequent Contributor
timestamp: July 13, 2015, 10:36:43 pm
content: For what it is worth, I have a Power Designs TP340A, which is a triple output bench supply, not one of their precision PSUs. Source C can deliver up to 5A, though at a lower voltage than your supply. When I measure it at 10v, delivering ~0.5A to a static resistive load I see ~78uV of AC voltage. I get this measurement using the (true RMS) ACV mode of my Keithley 2700 in slow mode, which has an AC bandwidth of 3-300Hz. With no load, that number only changes by 1-2uV.I doubt your PSU should be significantly worse, certainly not three orders of magnitude worse.