New BM786 not reading zero volts when probes are shorted - Page 1
Forum section: electronics / testgear

author: BlueFeather
author_rank: Newbie
timestamp: July 01, 2022, 02:54:04 am
content: I have a new BM786 multimeter that is reading a steady -0.0001 volts when the probes are shorted in DC voltage mode. I tried swapping the leads around to see if I would get +0.0001 (indicating a bad connection and/or bad probes) but still got the same -0.0001 volts when shorted. It's soooo close to being perfect!Is there a way to set the zero volts calibration point? I know there is the delta button, but I don't want to have to use that to zero the meter whenever I want to measure small voltages. Has anyone else had this issue? I have sent out an email to Bryman to see if they could help (don't know how quickly they will respond though).

author: Specmaster
author_rank: Super Contributor
timestamp: July 01, 2022, 08:55:59 am
content: Most meters will do similar, especially in an electrically noisy environment such as a lab and is the result of the leads picking up some microvolts from the airborne magnetic fields radiating from other equipment or even mains cables near by. Its no big deal in the grand scheme of things, I mean 0.0001 is not going to make your readings invalid.My 6.5 digit meter will happily read 0.00003v with no leads being plugged in unless I null the reading, which I don't because it is such a small insignificant number. Even if I do null the reading, it will still flicker between 0.00000 and 0.00001

author: nomead
author_rank: Contributor
timestamp: July 01, 2022, 11:56:31 am
content: My BM786 also reads -0,0001 DCV, 000,01 mVDC and up to 0,0070 VAC depending on how close to EMI sources i get. So nothing unusual here.Zero calibration is dependent on temp-co and component aging so exact zero values are somewhat pointless.Even voltnuts are prepared to drifting with their high end crystal aligned Hi-Fi cables.

author: BlueFeather
author_rank: Newbie
timestamp: July 01, 2022, 09:44:48 pm
content: Quote from: Specmaster on July 01, 2022, 08:55:59 amWith an open circuit, the voltage does move around a bit from noise (that is normal with any meter). What bothers me is that it reads a very consistent -0.0001 volts DC with no flickering when the probes are shorted. The meter is able to read 0.0000 volts in AC mode though (I would think that would be more susceptible to noise). The same -0.0001 VDC result is shown when away from any cellphones and or noise.I will try refrigerating it for a bit to see if it is temperature-dependent.

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2022, 09:50:36 pm
content: It's completely within spec and it may vary a few counts at times. Some meters will lie to you and display a 0.0000 result anytime the actual reading is within 2-3 counts of zero just to avoid this exact complaint. I call this 'OCD friendly'. The fact that yours shows you a one-count error just tells you that it is being honest. If it really, really bothers you, then exchange it and hope for better. AFAIK Brymen won't release calibration information, although they might have something for zeroing.

author: BlueFeather
author_rank: Newbie
timestamp: July 01, 2022, 10:14:27 pm
content: I can confirm that it has a slight temperature dependence. When refrigerated, it then reads +0.0001. As it warmed up to approximately that of an airconditioned room, it reads 0.0000.At least this means that the calibration is reasonable. My OCD is a lot less triggered now.Edit: If anyone *does* know how to adjust the zero volts calibration point, I am still curious about it.

author: Specmaster
author_rank: Super Contributor
timestamp: July 02, 2022, 12:19:24 am
content: The reason why the meter reads a small voltage even when the probes are shorted is precisely because like it or not there is EMI signals in the air all the time, this is after all is said and done just why radios and TV etc work with no direct connection with the signal source. So when you short the probes together, there is still something like 2 metres of cable of the probes acting like an antenna picking up this EMI signal and that is what the meter is indicating.Your meter is perfectly fine and is within specification, and is just in a way demonstrating to you just how sensitive your new meter is, try the same with a 3.5 digit display and you will get the perfect 0.000 display that you are seeking, purely because the meter cannot display the low level of voltage that your BM768 can.This keysight website might help you to understand your meter better.https://blogs.keysight.com/blogs/tech/bench.entry.html/2019/01/10/what_are_digits_acc-lInJ.html