Author Topic: GDM-8251a not stable measurement, question for users  (Read 552 times)

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Offline Pablo20

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GDM-8251a not stable measurement, question for users
« on: February 06, 2018, 10:23:01 am »
Hi,
On my GDM-8251a is not stable resistance measurement (when leads are shorted the result is not constant). AC and DC are not stable too and without any source connected on display is few hundred mV (like on this film). My question is it normally on this unit or something wrong, how is on your units like this one? Thanks for reply...


« Last Edit: February 22, 2018, 07:00:50 am by Pablo20 »
 

Offline Jester

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Re: GDM-8251a not stable measurement, question for users
« Reply #1 on: February 06, 2018, 11:55:04 am »
The 8251A I have fluctuates between 0.097 and 0.098  \$\Omega\$ with a short applied
 

Offline CustomEngineerer

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Re: GDM-8251a not stable measurement, question for users
« Reply #2 on: February 06, 2018, 01:11:33 pm »
Looks pretty much identical to my GDM-8251a as well. The only one that's different on mine is ohms measurement with leads shorted, mine is more stable. I'm guessing your leads just aren't making a good connection. They could be dirty, oxidized or maybe going bad. You should try using a real shorting connection (shortest piece of copper wire you can find or real shorting bar) to know for sure. The important part is that it makes a good connection, just trying to hold your long test leads together at the end is leaving too many possible variables to know for sure if its the meter or the leads. You could also try the same leads in another meter.

If I uses some of my crappier leads (pretty sure they are the ones that came with my cheapie $5 meter), shorted together at the end the reading bounces around some 200 - 400 m?, however if I use a proper shorting bar it only fluctuates 3 - 4 m?. Though if I move my hand close to the meter it can still jump around 20 - 30 m?.

Edit: Really you should use a proper short to see how much the other modes fluctuate as well. Open circuit fluctuations are expected due to the high sensitivity of the inputs.
« Last Edit: February 06, 2018, 01:13:07 pm by CustomEngineerer »
 
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Offline cdev

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Re: GDM-8251a not stable measurement, question for users
« Reply #3 on: February 06, 2018, 01:36:48 pm »
Mine is very sensitive and takes some time to warm up to be reasonably accurate in the very low ranges.

If you just have leads on the inputs and no short, it shows completely random numbers. When it is shorted with a wire, it still shows random readings but the values are lower. Thats because its picking up local electrical charges in the air, and RF.

 In some places, with them open, it shows pretty high readings. My other meters do that too.

Thats just the electricity thats there, they are not making it up.

For the terminals to be shorted, the shorting conductor needs to be short and ideally, also flat. The best short is a piece of flat copper sheet with holes at the appropriate places. Some Deoxit also helps.

A piece of wire of any real length isn't a short, its a loop antenna.

You know what I found to be interesting, put an LED on there while its set to the low voltage range, and expose it to light. You'll see the tiny amount of current generated by the device.
« Last Edit: February 06, 2018, 01:44:35 pm by cdev »
"What the large print giveth, the small print taketh away."
 
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Offline Pablo20

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Re: GDM-8251a not stable measurement, question for users
« Reply #4 on: February 07, 2018, 12:15:12 am »
Thanks for all for answears.
@Jester: your is very stable I think, very well. Looks that mine is something wrong, any fault?
@CustomEngineerer: I've tried with others cables and piece of wire too, everytime the same.
 

Offline Pablo20

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Re: GDM-8251a not stable measurement, question for users
« Reply #5 on: February 07, 2018, 06:31:07 am »
Hi,
additionally I have observed that when "auto range" is off measurement is stable (on higher range), when auto is on, measurement is unstable (on lower range). Maybe on this meter it is normally, maybe firmware is not perfect...

« Last Edit: February 07, 2018, 06:36:03 am by Pablo20 »
 

Offline CustomEngineerer

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Re: GDM-8251a not stable measurement, question for users
« Reply #6 on: February 07, 2018, 03:13:35 pm »
Lower range is more sensitive as well as shows lower (go figure) resistance levels. Of course you see all 0's when on the higher ranges. For example, on the 10 mega ohms range, the lowest digit displayed is 1 kilo ohm, on the 1 mega ohms range the lowest digit displayed is 100 ohms, so unless something is completely broken (cables or meter) you should see all 0's because the resistance you are measuring is much lower than that. However on auto range, it settles down to the lowest range of 100 ohms, where the display shows down to 10 milli ohms range (aka a tenth of an ohm), which is lower than the resistance of your cables, which combined with other factors such as less than perfect connection and high sensitivity of the meter you are going to see fluctuations. I still haven't seen anything to be worried about, this is all normal behavior.

Instead of holding the tips of those 2 cables together, just use one cable and plug both ends into the sockets on the meter. You are still going to see some fluctuations because you are still make essentially an antenna, but you will at least be making a better connection.
« Last Edit: February 07, 2018, 03:17:17 pm by CustomEngineerer »
 

Offline ChuckDarwin

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Re: GDM-8251a not stable measurement, question for users
« Reply #7 on: February 07, 2018, 03:32:01 pm »
Looking at one of my GDM-8251A (firmware: 2.30), short warm up, filter on.
With decent, not great, quality 50cm banana-to-banana leads shorting, I read 0.057-0.059 ohms with one cable and 0.092-0.097 with another in auto 2 wire mode.
Manual mode 100 ohm reads the same as auto.  Reading is steady.
With 1 meter test leads hand shorted I get about 0.150 ohms once it settles.
With VDC, my supply at hand only does three decimal places, so I see some minor +/- wander past that.  VDC up to 32.000 is solid.
Dug out the meter's twin (Or Jan Dohner)--reads very close, but not exact--older calibration.
When left unconnected, one fluctuates between 600-800mV and the other unit between 400-600mV--ymmv, RF, etc.


 
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Offline Pablo20

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Re: GDM-8251a not stable measurement, question for users
« Reply #8 on: February 22, 2018, 06:59:02 am »
Hi,
thanks all for reply. ChuckDarwin - useful post.
I asked GWInstek about this situation, and I got answer that meter is probably defective.
Repair can be difficult because schematic is not available.
« Last Edit: February 22, 2018, 07:06:28 am by Pablo20 »
 


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