BM235 variability - Page 1
Forum section: electronics / testgear

author: IanB
author_rank: Super Contributor
timestamp: November 16, 2016, 12:28:56 am
content: Just a quick poll around anyone who has Dave's BM 235 and has tested the DC volt readings against a reference.I have seen a few online reviews and on nearly all of them the meter when tested has indicated 5.000 V when tested against a 5 V reference. However my sample of the meter reads 4.997 V (about 3 counts low). I am fairly sure the reference is accurate from other tests so the meter seems to be off. Oddly the meter at 10 V seems to read spot on, so there is maybe a non-linearity at lower voltages.What do other people see on their samples of the meter (if you have a known reference to compare against)?I know 4.997 is within tolerance, but I am wondering how much variation there is between samples of the meter. Did I get an odd one out? Or did I break it by trying to power it with Energizer Lithium batteries?

author: joeqsmith
author_rank: Super Contributor
timestamp: November 16, 2016, 01:12:14 am
content: Quote from: IanB on November 16, 2016, 12:28:56 amDid you watch Dave's video of them?

author: joeqsmith
author_rank: Super Contributor
timestamp: November 16, 2016, 02:00:10 am
content: Here's my BM235 compared with a few others including my old Fluke 97. Keep in mind, the UT181 has been modified a fair amount after I damaged it. The UT61E has also been modified. The EX540, well it too was modified to the point where I had last changed out the RMS converter. Then there is the UT90A which, while not modified, has been damaged more times than I care to count. Then there's the BM869s which I also damaged and repaired. The only untouched meter is the BM235!

author: IanB
author_rank: Super Contributor
timestamp: November 16, 2016, 02:52:08 am
content: Seems like Dave had a few that read 4.997 V, but none lower than that. I guess I got an outlier. Bummer!

author: joeqsmith
author_rank: Super Contributor
timestamp: November 16, 2016, 03:56:14 am
content: What's the uncertainty of your reference? Is it NIST traceable and did you get the report with it?My old Fluke standard gets checked once in a great while against another meter which is kept in cal. For home use, I never cal anything. I did have my HP34401 cal'ed once with the report but that's been maybe 10 years ago. Now days I check that box of resistors as well and keep records on them. For time, I use the GPS. Just wanting to be clear about the error in the pictures I show.I bought the HP34401 brand new from HP when it first came out. It has never been aligned. Here it is on the Fluke standard. Of all the meters I showed earlier, this HP and my Fluke reference are what I trust.Also shown is my 5V home made reference. This thing is not in an oven and drifts a bit. I use this as a quick check when looking at the meters I test but that's about all it's good for.

author: IanB
author_rank: Super Contributor
timestamp: November 16, 2016, 05:30:46 am
content: Quote from: joeqsmith on November 16, 2016, 03:56:14 amI'd guess about 0.01%QuoteNo, nothing so fancyHowever, as you can see from the attached picture, I do have reasonable agreement between three different meters.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 17, 2016, 03:07:26 am
content: Yea, odd man out I would say.A little strange result but I let the old HP and Fluke sit for about a half hour and then recheck them together. I then adjusted my test box against the HP. The test box was then attached to the BM235A.Looks like they spec it to 0.3%+2d in the 6V range. I have no idea if that's of reading or full scale, +/-0.3% of just +0.3%, Maybe +/-0.15%, +2d or really +/-2d. Some of these manuals leave a lot to the imagination. Assuming +/-0.3% of 6V plus +/-2mV. That's a pretty wide window and we are both well within that.