Shorted probes on multimeter,give random results - Page 1
Forum section: electronics / beginners

author: laptopmd
author_rank: Newbie
timestamp: August 27, 2014, 11:17:18 pm
content: Hello Everybody, hello Dave.This is my noob question : i own a Vicky VC8145 bench multimeter, i know it is crap, it is from china, it is an emty box, bla bla bla... but.. this question is about a strange behavior that this instrument have, and i don't understand if it is because of the low quality of my instrument, or the quality of the cable probes, or the probes itself, or if it is just... "normal".The problem is : when i short the probes together when measuring ohm, i don't get a steady 0.0000 ohm , or at least a constant value, but the values change continuosly. Instead, when i measure a resistor or a track, i get a constant measure.My probes are not the original ones, i have bought "quality" probes from a well know electronic seller, they are rated "1000V CAT III 15A CE" (if this mean quality)Is this a "feature" of the multimeter, or caused by some internal analog to digital conversion.. or one of the possibilities listed above ?Thank you all !Dave (another one)

author: wraper
author_rank: Unknown
timestamp: August 27, 2014, 11:39:36 pm
content: First of all it is likely 000.00 ohm. Second you won't get steady figure as in the lowest range contact resistance matters a lot. You cannot connect two probes by hand stable enough. If you want to see "stable" figure, just connect two terminals directly with cable which have banana plugs on both ends.

author: Neilm
author_rank: Super Contributor
timestamp: August 28, 2014, 06:24:28 pm
content: What is the magnitude of the variation? Test leads are usually about 10 to 50 milli ohms, so if your readings are in that sort of magnitude there is no issue.Also, if your meter can measure 0.1 m ohms you could have your measurements being affected by the Seebeck affect caused by different metals touching (the way a thermocouple works). Cheap meters often "cost save" on terminals and you can see this if you look. Set your meter to volts and short the test leads together.

author: wiss
author_rank: Frequent Contributor
timestamp: August 29, 2014, 09:13:03 am
content: Do you press the probe-tips togeather using your fingers? In that case both Seebeck and 50/60 Hz line pickup.

author: robrenz
author_rank: Super Contributor
timestamp: August 29, 2014, 12:04:36 pm
content: In my experience it is a clear lacquer like coating common on the cheap Chinese probe tips. Scrub the probe tip with acetone or MEK and steel wool or something like Scotch Brite. Then the probes will behave as expected.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: August 29, 2014, 01:35:49 pm
content: Depending on the resolution (Count) on your meter, you should never really see a true zero ohms unless you Null out the test lead resistance. All test leads and wire have some resistance (In the mOhms). I agree with a couple of the posts on this topic about cleaning the test leads and holding them together by hand especially if your using your fingers. You can try a good quality short jumper made with 4mm banana jacks to see if the meter is more stable. If you see a .01 - .03 value you can use your "Zero" or "Rel" function to null out the lead resistance and you should see a stable 0 Ohm reading. I usually make my own test jumper leads using an HQ silicon test lead cable and HQ banana jacks, then test them with a precision uOhm meter. If you still have a reading that is jumping around then you might have an issue with the meter itself of your getting some external interference that is causing the fluctuations. During standard calibration checks of all my multimeters (And I have about 20 of them), I have a very short know resistance value jumper which I use for testing and all of them will give me a stable reading using this method.