Multimeter probe strangeness - Page 1
Forum section: electronics / general

author: rkoval
author_rank: Contributor
timestamp: February 29, 2012, 11:02:01 pm
content: I just bought an Extech 570 Multimeter. The test probes act very funny. On the Ohms mode, touching the probe tips together make the reading jump all over the place. Depending on where I touch the tips, gives me a different response. Some readings are close to zero, other spots are just jumpy. It appears the coating on the tips may not be very uniform in their resistance.I checked a set of fluke meter probes and they seem better, but still not rock solid. Same with an old RadioShack meter, better but not rock solid.I can touch the Extech tips together and just move them back and forth along the lengths and I get different response. Very annoying.I purchased a second set of probes and they also show this same type of behavior. Not as bad as the Extech ones, but yeah, annoying.I was taking readings of Caps and resistors, and depending on how I held the probes on the leads, gave me different readings, some being jumpy.Has anyone had this same experience?Rich

author: retiredcaps
author_rank: Super Contributor
timestamp: March 01, 2012, 02:44:34 am
content: If you have tried multiple probes and they all react the same way, then I would suspect your meter. Try taking your Extech probes and putting them into another multimeter doing the same ohms test. If the readings are steady, then would definitely point at your multimeter.

author: rkoval
author_rank: Contributor
timestamp: March 01, 2012, 03:15:57 am
content: yes, did that already. Used my old RadioShack probes in the new meter, works fine.Must be something with the coating on the tips. Just strange that a nice meter would ship with poor quality probes.

author: rkoval
author_rank: Contributor
timestamp: March 01, 2012, 03:23:47 am
content: Oh yes, the new probes I bought came with some lead clips that attached to the probe tips. The clips give nice steady readings. I suspect the attachments give better readings due to the multiple contact points they make with the probe.I would just think that the metal tips would provide uniform conductance anywhere you touch on them.

author: IanB
author_rank: Super Contributor
timestamp: March 01, 2012, 05:06:19 am
content: Quote from: rkoval on March 01, 2012, 03:23:47 amHave you ever done the experiment where you put a loudspeaker in the circuit with a bare metal contact and scrape the contact? All sorts of contact noise results.A plain metal to metal contact is anything but a clean low resistance connection. There are surface dirt, oxide layers, finger grease films, all sorts of things to get in the way. Meter test probes have a pointed end for good reason and the correct way to use the probes for sensitive measurements is to dig the point into the contact surface. If that is not possible, use clip probes with positive mechanical contact. If you just touch two metal surfaces together you are going to get variable contact resistance anywhere between 5 and 50 milliohms.In short I do not find your experimental results surprising. You must take steps to ensure a more positive electrical contact if you care about good connections.

author: rkoval
author_rank: Contributor
timestamp: March 01, 2012, 05:56:45 am
content: The speaker test seems to be and interesting thing to try.I have cleaned with IPA and no change.The resistances I see are up to a few hundred Ohms. I would not fret with milliOhm values.I think the coating is to resist oxidation and wear. Don't know, just have to live with it.Curious to see how some higher price ones respond.

author: robrenz
author_rank: Super Contributor
timestamp: March 01, 2012, 01:39:27 pm
content: I have had a similar situation with lower end meter probes. In all cases I have found it to be a virtualy invisible layer of who knows what on the probe tips. I take some MEK and steel wool to the tips and whatever it is, is gone. I use 0000 ( very fine) steel wool because it can scrape off whatever the coating is without removing the plating. IPA would probably work instead of MEK. In all cases the variable conductivity like you described disappears. I found this out on a $25.00 meter that Circuit specialists gives for free if you buy over $50.00 of product.http://www.circuitspecialists.com/low-cost-multimeter.htmlIt has a continuity buzzer and when I first tried it it was very sketchy and rarely beeped. For some reason I thought to do the steel wool treatment and it is the fastest responding beeper I have seen (including my Fluke 87). It gives the speaker response as Ian described below.

author: G7PSK
author_rank: Super Contributor
timestamp: March 01, 2012, 03:41:10 pm
content: Have you tried rubbing the probes with a piece of scotchbright, It sounds to me like there could be a lacquer of some type on the probe tips that needs removing.

author: robrenz
author_rank: Super Contributor
timestamp: March 01, 2012, 04:55:22 pm
content: G7PSK, I agree, it seems like a lacquer. I am not trying to be argumentivebut anything with abrasives content is more likely to remove whatever plating is on the probes compared to fine steel wool. It may come off with just an agressive solvent like MEK or acetone or fingernail polish remover.

author: G7PSK
author_rank: Super Contributor
timestamp: March 01, 2012, 06:11:13 pm
content: Scotchbright is much kinder than steel wool and far less likely to remove any plating, it is a plastic material available in many grades. It is used for such things as polishing stainless steel etc. the white one is the finest.

author: robrenz
author_rank: Super Contributor
timestamp: March 01, 2012, 06:34:14 pm
content: Scotch brite pads toward the bottom of this link show they all have abrasive content even though the white pad is talc which is barely abrasive. Only the Bear tex (Norton) white pad has no abrasives.http://www.evenfallstudios.com/metrology/grit_size_comparisons.htmlBut I googled "0000 steel wool removing plating" and see that it can remove soft platings even though it is often used to remove superficial rust from chrome.With all that said it seems better to try just a solvent first.

author: G7PSK
author_rank: Super Contributor
timestamp: March 01, 2012, 08:39:38 pm
content: Yes try nail varnish remover on some cotton wool will remove most lacquer.

author: rkoval
author_rank: Contributor
timestamp: March 02, 2012, 12:33:03 am
content: I like the suggestions. Thank you.The meter was $250. I did not think I should get shitty probes. I shouldn't assume.I did contact Extech when I thought the problem may have been a bad probe wire. They were going to send me new ones, but haven't gotten them yet.Ill try your suggestions.Thanks

author: robrenz
author_rank: Super Contributor
timestamp: March 02, 2012, 01:07:09 am
content: I just happened to get another one of those Circuit Specialist free meters today with my order. Same situation with the probes barely if any continuity. I had to use a very fine 800 grit sanding sponge on the probes to get good contact. I tried acetone and mek first but they did nothing. Tried the steel wool and only slight imporvement. Finaly did the super fine sanding sponge ( from the beauty shop for nails) and got great contact. The white scotch brite would have worked, I just dont have any that fine.

author: rkoval
author_rank: Contributor
timestamp: March 02, 2012, 01:11:22 am
content: I just tried acetone. That is all I have. It seemed to help. I have some scotch bright around somewhere. I'll try that as well.Thanks,Rich

author: vk6zgo
author_rank: Super Contributor
timestamp: March 02, 2012, 03:17:56 am
content: Quote from: G7PSK on March 01, 2012, 08:39:38 pmYes,it's incredibly savage!At my last work,we had a switchmode power supply meltdown.The factory back in the dear old People's Republic sent us a new one mounted on a heatsink that was too big to fit the equipment,so we had to clean the melted switchmode guts off the old one & fit the new supply to it.We tried everything we had to get it off,with no progress.I borrowed my wife's nail varnish remover,& a small amount on a cloth (about as much as she uses for one hands worth of nails),removed every last bit of the crud.Ladies must have impervious fingers!

author: Unknown
author_rank: Unknown
timestamp: March 03, 2012, 11:39:58 pm
content: The active component in nail polish remover is most likely the acetone that Rich ended up using.

author: IanB
author_rank: Super Contributor
timestamp: March 04, 2012, 01:32:06 am
content: Acetone is funny stuff. Some things it will dissolve readily, but other things it won't touch. The same is true of other solvents, but with different materials each time. So really, you need an arsenal of solvents at your disposal. There isn't really a single solvent that will clean everything--but that "Goo Gone" stuff you can get from the hardware store comes close...

author: SeanB
author_rank: Super Contributor
timestamp: March 04, 2012, 02:39:41 pm
content: Best probes i have came with a Major Tech megger. Gold plated, good quality and ultra flexible. Probe clips as well. Only reason I think these are included is because they have a 2kV rating, really needed on a 1000v plus megger.