Different DC value when reversing polarity ?! - Page 1
Forum section: electronics / general

author: JeLo
author_rank: Contributor
timestamp: December 15, 2022, 02:05:32 am
content: I got a great deal on a Brymen BM155 clamp meter that I couldn't pass on. Just been testing it and found something strange. When reading DCV on a new 1.5V battery it reads 1.3~1.2V. Took out my fluke 189 that's been siting idle for a long time and it reads 1.58 (and a lot more significant digits after...but about that.). Tried with an old cheap Mastech MS8260E that is actually amazing for what it does and it's about the same as the Fluke, grossly. In the process I found out the leads are probably not original, which I really don't mind given the deal I got, so I tried various leads, including fluke ones, same result but now I found out that if I reverse the leads it reads -1.8V! WTF ?!Should I send it back ? They even included the PC connection attached to the MM, although they forgot to include the software (which is available for download) and the PC connection cables (maybe it was included as a mistake ?).... Is there any way to compensate the DC offset or calibrate or correct the error ? UPDATE: Thinking about it the error seems to be the same when reversing the leads because if I subtract the difference (1.2-1.5) and add it to the negative value it gives -1.8V. So this is a permanent offset of something.... Some of you guys must be familiar with this...Cheers

author: bdunham7
author_rank: Super Contributor
timestamp: December 15, 2022, 02:08:11 am
content: What does it read if you short the leads?

author: JeLo
author_rank: Contributor
timestamp: December 15, 2022, 02:50:17 am
content: @bdunham7Just tested that.. At first it read 000.0, then after a few seconds -000.1 and then after some more seconds it stabilized at -000.2 and it's staying there.--UPDATE-- I just read the official accuracy specs and it's only got a 600V scale for DC, no other scale. For DCV the accuracy is 0.5% + 5 digits, so that's half a volt right there!So I guess it's within spec after all. This is a clamp meter geared towards power measurements, I bought it because the electricity network is bugging me to upgrade my subscription to 10KVA instead of 6.9KVA. They say my usage excceded my plan, which is a blatant lie, I only have a 4.5KW water pump... I suspect it's the pump motor start surge current that's triggering the smart meter, the old one was fine for 10 years GGRRRRRRr... I'll probably make a post about that later on...Anyway, still interested to know if the error is correctableNot that I'll probably do anything about it....

author: srb1954
author_rank: Super Contributor
timestamp: December 15, 2022, 06:43:39 am
content: Quote from: JeLo on December 15, 2022, 02:50:17 amEven if the pump is rated at 4.5kW the current drawn might be higher than expected due to a poor power factor bought about by a deteriorating power factor correction capacitor. These have a limited live and tend to lose capacity over time.

author: J-R
author_rank: Super Contributor
timestamp: December 15, 2022, 11:23:23 pm
content: It's annoyingly common for clamp meters to have only one or two voltage ranges, or be missing other common measurement features, perhaps to protect their regular DMM sales!There are some that I know do have low V and mV ranges, such as the UEi DL489 and the Uni-T UT210D & E.If the pump supports it, you could look into a soft-starter box. They are commonly used with A/C units on RVs.