Bench DMMs on the eBay - Page 1
Forum section: electronics / general

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 02:43:50 am
content: Looking at some bench DMMs on the eBAY I noticed that many of them show readings of significant mV or even volts without any leads connected.e.g.http://www.ebay.com/itm/Hewlett-Packard-HP-3478A-MULTIMETER-/400280471286?pt=LH_DefaultDomain_0&hash=item5d329346f6I actually checked on some pictures to see if perhaps rear terminals were used, but that was not the case.That is not normal, is it?Thanks

author: grenert
author_rank: Frequent Contributor
timestamp: March 04, 2012, 03:45:52 am
content: If I hear this question one more time, I think I am going to cry.

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 04:25:15 am
content: Have you any decency?

author: PeterG
author_rank: Frequent Contributor
timestamp: March 04, 2012, 04:50:02 am
content: Quote from: bubucis on March 04, 2012, 02:43:50 amHey Grenert, is this normal for a meter to act in this way?Regards

author: PeterG
author_rank: Frequent Contributor
timestamp: March 04, 2012, 04:51:30 am
content: Hi Bubucis, yes this is normal for some meters. All you need to do is Rel the meter to set it toZero before you use it.Regards

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 05:10:46 am
content: Hi,I am not really sure why is this normal, and why should I have to rel the inputs.I have bunch of handheld DMMs, one of them is 50K count, and they are consistently at ~ 0 without leads or at low mV if in the higher gain mode.My M.Sc. is in EE, but I specialized in fiber optics, but I do know the basics.I was looking on ebay for something better than my handheld fluke 289 and noticed a lot of bogus readings with bench DMMs, thus the question.I also have no experience in eBay, but thought if I was selling a DMM I would show it connected to something and measuring something:)Thanks!

author: vtl
author_rank: Regular Contributor
timestamp: March 04, 2012, 05:11:11 am
content: You do not want to rel the meter. When you short out the leads the meter will indeed read zero. This is because the lowest voltage range on these bench meters is very high impedance and you get a lot of pickup when its not connected to anything.

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 05:19:50 am
content: I read about that when googling, but my example link was showing 1.66V without any connections, do they really build up that much voltage?I was looking at this post:https://www.eevblog.com/forum/general-chat/fluke-8840-behaviour/msg77525/#msg77525and Fluke screenshots do seem to show some significant voltage.What I really would like to know is whether eBay folks are just cheating and selling faulty stuff, or do these more sensitive higher impedance bench meters can pickup such a significant voltage when sitting unattended, and I should pay no heed to this on eBay?Thanks again!

author: EEVblog
author_rank: Unknown
timestamp: March 04, 2012, 05:27:41 am
content: It is not uncommon for bench meters to have high impedance mV ranges (and in some cases, the 2V range too), so they are easily able to "build up a charge" on an open connected input and retain that value for a long time. It's the nature of the beast.Dave.

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 05:38:53 am
content: Thanks!That is reassuring. I was looking at some cheap HP 3456A and noticed that they had some "funny readings"I ended up buying a meter where the seller had actually photographed the 3456A with the Fluke calibrator in the picture (and the corresponding reading), but it was a bit more expensive. No big deal, but made me think that perhaps lot of the eBay is about a perception and what one can read about the item via the description and the picture. While both can be fake of course:(In the last couple of days looking at the eBay I have developed a sort of rule to at least ignore DMMs that are not powered on in the picture, do you guys concur?Thanks.

author: EEVblog
author_rank: Unknown
timestamp: March 04, 2012, 07:51:44 am
content: Quote from: bubucis on March 04, 2012, 05:38:53 amYep, generally good rule to follow. No excuse for the seller to not post a photo powered on.Same with scopes, you want to see at least traces for all channels up, and preferably the probe cal signal.I've broken those rules before, and have both won and lost.Dave.

author: bubucis
author_rank: Contributor
timestamp: March 04, 2012, 08:22:47 am
content: Thanks for the advice!

author: rf-loop
author_rank: Super Contributor
timestamp: March 04, 2012, 08:55:02 am
content: Quote from: bubucis on March 04, 2012, 05:38:53 amExample HP3456A have 10 Giga ohm input with DC 0.1, 1 and 10V range.and it is 1000 times more than 10Mega ohm. Also whole measurement unit is of course constructed fully floating.Open test leads it can show "whatever". Just walk near open leads and it pick up some charge..It is Voltage meter! And good Voltage meter need as low current as possible. 10 Giga ohm is not very good but "so so" ok for 6.5 digit meter. This beats today many cheap meters in accuracy ans stability. It IS good meter. In some things it is better than 3457 ... what ever Agilent is talking. This Agilent talk is more becouse 3457 was new and extremely cheap to do. Of course they then talk that it is "better" becouse they want sell it. (yes there is something more but accuracy... maybe -56A win)

author: saturation
author_rank: Super Contributor
timestamp: March 04, 2012, 02:16:19 pm
content: Most knowledgeable sellers of HP gear will post or make obvious the bare minimum:It passes its self test mode, most units have one or reports status on power on. Photos of the self test are invaluable as it will light up all the annunicators, wherever they are.The serial number. From here you can determine series for that particular model; it may have pros and cons well described by HP enthusiasts: serviceability, and reliability.The above are most important, and a liquidator often will not mention both, but you can request it be done via email.Post photos of, rather than describe the units condition. Photos also should focus on the wear points: knobs and buttons, annunciators, and blemishes.If the first 2 are good, then calibration aside, the unit is working and any readouts are immaterial. You are then concerned with the condition, because you'd like to make a guestimate of whether this device will still work over time for you and its easily serviced by you.A better unit is 'pulled from a working environment' recently and never stored, which suggests climate control adequate for human workers and likely the electrolytic caps in the unit are continuous charge and formed, and less likely to fail. If the unit was stored for X months in unregulated climate conditions [ worse is freezing winter], as it was 'pulled....' years or months ago, it may work briefly, but the unit could fail in your hands in the near future. Obtaining the service manual will describe whether it has too much unobtanium to be serviceable, even if working at the time of the sale.

author: saturation
author_rank: Super Contributor
timestamp: March 04, 2012, 04:11:53 pm
content: Welcome. Lastly, do not buy old HP T&M gear based on availability from eBay or similar; know which item you want before you start browsing so you recognize good from bad. If you are in the market for scope, know which models are worth owning, or restoring, AFAIK sorry to say,noneof older HP DSO are. Agilent brands are more recent vintage, but they were not built like the golden years of HP: tip, if you cannot find a service manual with a schematic, its probably has too much unobtanium.

author: Lawsen
author_rank: Frequent Contributor
timestamp: March 15, 2012, 06:07:32 am
content: The HP/Agilent 34401A with the green vacuum fluorescent display or Dave Jone's HP-3468A with 5.5 digits in liquid crystal display, but one digit less than HP-34401A with 6.5 digits. There are other brands like Keithley and Fluke. All of them have varied condition from parts junk to usable condition.