GW Instek GDM-8251A strange behavior. No zero at mV & Ohm. - Page 1
Forum section: electronics / testgear

author: Steps
author_rank: Contributor
timestamp: July 10, 2018, 10:51:07 am
content: Dear colleaguesI have GW Instek GDM-8251A which has strange behavior. Last 3 digits are crazy blinking at lower range of DCV and Ohm. And it not going to zero when short the leads, still blinking. So resistance of 1.5 ohm is not possible to measure. Is it hardware (dried caps, dying op amps) or software problem (calibration)? Can anybody advise or had such experience? Video I'll do later.

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: July 10, 2018, 11:53:12 pm
content: Anything like the user in this threads experience?https://www.eevblog.com/forum/testgear/gdm-8251a-not-stable-measurement-question-for-users/

author: Synthtech
author_rank: Regular Contributor
timestamp: July 11, 2018, 08:35:40 am
content: I have a GDM-8251A also, I regard it as a poor quality instrument, poor accuracy and somewhat flakey. Forget using 2 wire measurement for resistance, forget about measurement repeatability. This should be a really good meter but I have to say that as my first experience with the GW Instek brand it left me cold. It sits on the shelf unused.

author: exe
author_rank: Unknown
timestamp: July 11, 2018, 10:55:29 am
content: Quote from: Synthtech on July 11, 2018, 08:35:40 amDid you try changing the probes?

author: Synthtech
author_rank: Regular Contributor
timestamp: July 11, 2018, 11:32:06 am
content: Quote from: exe on July 11, 2018, 10:55:29 amYes, and the normal probes that I used with it are Fluke probes. I don’t know what it is with that meter, maybe I got a bad one, who knows?

author: exe
author_rank: Unknown
timestamp: July 11, 2018, 02:59:42 pm
content: I'd ask support about this problem. Esp. if you bought it new and it was like that from the beginning.

author: innkeeper
author_rank: Unknown
timestamp: July 11, 2018, 05:53:00 pm
content: Hi Steps,I own the same meter...and Yes, I've experienced this. Yes i think its pretty much normal..but first, lets make sure your meter is ok....you can do this by using a very short piece of wire to short the input. if it becomes relatively stable, then, your meter is not doing anything unusual.i found that some leads, at low resistance, will vary widely with just the slightest movement of the wire, and being high impedance, it picks up nose quite easily. If your making low voltage and low resistance measurements accurately can be a challenge ...voltage measurements that low, consider using leads that are shielded.for low resistance..i've assumed that because the leads are stranded, that simply by moving the leads the overall resistance can change slightly. I've set up testing rigs to make sure noting mvoes when doing low resistance measurements. ... though I find using an old-fashioned whetstone bridge a much more accurate way to measure very low resistances.as far as offsets go, there is a rel function on the meter. that's how i deal with the resistance offsets.unconnected, yea the reading jumps wildly, though this is something i have observed with many meters.i can get my shorted voltage to read 0.0000 when i am using shielded leads with a banana to bnc adapter, and ensuring the bnc shield is on the ground side. pamona makes the banana to BNC adapter i use. This also helps with low resistance stability, however, the resistance of my coax leads is much higher than my standard test leads.also, there is a filter feature, which averages over x number of samples. this is also a good feature to make use of. unless you are running off a lead acid cell, there is noise on most DC lines, and with the fast sampling rate of the meter, its gonna pick up varying voltages. the filter feature smooths those out. it doesn't do that automatically as they have no idea of your need, most of the time though, for normal measurements, you want the filter on.hope that helps some... if you need more details, i can make a video or take some pictures.

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: July 11, 2018, 10:59:23 pm
content: I've also got a GDM-8251A and have not had any problems with it. How exactly are you measuring a short? Touching the probe tips together, or using a proper, quality short wire, directly from one input to the other (or even better a real short)? I can get results similar to what you report using several of my probe sets and just trying to touch their tips together.

author: Steps
author_rank: Contributor
timestamp: July 23, 2018, 12:12:51 pm
content: I really appreciate everybody for the replies!So, yes. My case quite same like in the linked thread above, I guess.I played with settings - changed "filter" from 10 to 03, "delay" from 10 to 150. At Least it gave more "stable" readings.Yes, I believe for low resistance measurements better to use detached device like milliohm meter or bridge.https://youtu.be/TimnOsGB7U4

author: exe
author_rank: Unknown
timestamp: July 23, 2018, 12:46:19 pm
content: Quote from: Steps on July 23, 2018, 12:12:51 pmWhat about 4-wire measurements? (haven't seen the video yet, sorry if this was discussed in it)

author: Steps
author_rank: Contributor
timestamp: July 23, 2018, 01:03:41 pm
content: Quote from: exe on July 23, 2018, 12:46:19 pmI do not have proper 4w probes (calvin crocs with interconnection in the end). I have one, but it's not proper enough. I'll try to do video, anyway.

author: GreyWoolfe
author_rank: Unknown
timestamp: July 24, 2018, 12:25:53 am
content: Quote from: Steps on July 10, 2018, 10:51:07 amI have no trouble measuring a 1R1 resistor with mine. With a shorting bar of unknown quality, the meter shows between .072 and .077\$\Omega\$. Shorting my Kelvin clips in 4W reads between .012-.015\$\Omega\$. Shorting a set of Brymen gold plated leads shows .154-.156\$\Omega\$. Measuring a 1R1 resistor with the Brymen leads reads a solid 1.251\$\Omega\$, no fluctuations. I didn't check DCV.

author: Steps
author_rank: Contributor
timestamp: July 25, 2018, 10:09:43 am
content: I prepared video with measuring of various resistors by the subject.So, probably, my adjustments of delay and filter done the trick or electromagnetic fields had high intensity in first case. But now behavior looks like ok.https://youtu.be/R6PcMUg9mlM

author: exe
author_rank: Unknown
timestamp: July 25, 2018, 10:39:49 am
content: Quote from: Steps on July 25, 2018, 10:09:43 amI just watched first part of the video without sound, but immediately see a major problem: your setup is not a proper Kelvin connection. You can't do sub-ohm measurement with your arrangement.

author: Steps
author_rank: Contributor
timestamp: July 25, 2018, 10:48:59 am
content: Quote from: Steps on July 23, 2018, 01:03:41 pmYes, it's just ordinary probe ends connected by 4 stackable wires. I do not have Kelvin crocodiles yet, I will do it by myself.But it looks like working.

author: CustomEngineerer
author_rank: Frequent Contributor
timestamp: July 25, 2018, 09:15:37 pm
content: Another potential issue I notice is you turn on the meter and immediately start making measurements. It can take 30 minutes to several hours for meters to warm-up and stabilize. I know on my GDM-8251A, all measurements drift some within the first 30 minutes. If you really want to make sure its behaving as it should you should give it a minimum 30 minutes to warm up before you start checking it.

author: Synthtech
author_rank: Regular Contributor
timestamp: July 25, 2018, 09:26:41 pm
content: Quote from: CustomEngineerer on July 25, 2018, 09:15:37 pmI agree, the GW takes an especially long time to stabilise after turn-on, 30 min warm-up is a good idea. Mine is sitting on the shelf above a 34401A so I rarely turn it on, it’s mostly unused, I treat it like the bench version of a $60.00 hand-held meter - I only use it when I have to and I don’t particularly trust it, it disagrees with the Keysight by too great a margin.