Accuracy BM235 vs BM867 cant seem to get it right. - Page 1
Forum section: electronics / beginners

author: Runenaldo
author_rank: Newbie
timestamp: February 02, 2017, 09:37:21 am
content: Hello everyone!This is my first post so bare with meI just bought my first multimeter (BM867S)and was all happy given the accuracy of the meter 0.3%, then I started looking into a second meter as that is apparently a must in Daves worldand wanted a smaller form factor and ended up liking the BM235 after seeing a few videos Dave made, especially the one were he testet 40 of them side by side.Now after I saw this video I started to look into the tech sheet of the manuals and found the numbers compared to each other where not that far of.. Then I tried to calculate the accuracy of the two meters and found that the BM235 seemed to beat the BM867S and thought to my self I must be doing something wrong..And this is where you come inCan someone please take a moment and explain to me as if I was 5 years old, how to calculate the accuracy of the two meters compared, on a few different perameters?I have tried to grasp this whole concept by watching a few videos on youtube and followed the second post in this threadhttps://www.eevblog.com/forum/beginners/been-looking-to-buy-my-first-multimeter-but-i-need-(a-little)-help/msg587145/#msg587145I think I'm having trouble with the different counts 6.000 vs. 50.000 in respects to what the meters will display fx. 1.99, 1,999 or 1,9999 in a given measurement and the digets ei. 3d vs. 30d and where to put them into the count.BM867:https://www.welectron.com/mediafiles/datasheets/brymen/BM860s-catalog.pdfBM235:https://www.welectron.com/mediafiles/datasheets/brymen/BM230-catalog.pdf

author: EEVblog
author_rank: Unknown
timestamp: February 02, 2017, 11:52:35 am
content: Accuracy specs can be a complicated thing.The specs in the manual tell you nothing about how accurate the meter willactuallybe on any particular range or reading, it's a just a guaranteed maximum limit it met when it last left the calibration lab.The BM860 is a much more nominally accurate meter than the BM235

author: newbrain
author_rank: Super Contributor
timestamp: February 02, 2017, 12:21:04 pm
content: Quote from: Runenaldo on February 02, 2017, 09:37:21 amWelcome, nudist friend!Quote from: Runenaldo on February 02, 2017, 09:37:21 amI'll have a shot at it, let me know if this is not clear!Accuracy is given as a percentage of the reading, so the worst case as absolute value will be at the maximum possible value for the range.To this relative error, an mostly range independent uncertainty is added, expressed as a number of less significant digits (rigthmost on the display).Let's take, as an example, DC volts in the 5.0000 (867) or 6.000 (235):BM867: 0.03% + 2d => 5.0000V * 0.03% = 0.0015V, adding the 2 digits we obtain 0.0017V = 1.7mV as maximum absolute errorBM235: 0.3% + 2d => 6.000V * 0.3% = 0.018V, adding the 2 digits we obtain 0.020 = 20mV as maximum absolute errorSo, as you can see, measuring voltages in that range will be about an order of magnitude more accurate on the 867S than on the 235.The same exercise will yield similar results for other ranges and type of measurements.As Dave says, these accuracy values are the guaranteed ones, it might happen that a 235 is spot on (exact to the last digit) and an 867 has the maximum admissible deviation...

author: Runenaldo
author_rank: Newbie
timestamp: February 02, 2017, 12:21:32 pm
content: Quote from: EEVblog on February 02, 2017, 11:52:35 amJa that is what I also gathered from your 40 BM235 test, they seemed all to be spot on the reading. Hope the same goes for the BM867S.But I also see you and others make the calculation where 5V measured can be 4.945V and 5.045V + 5d and so it becomes 4.950V and 5.050V and still be within a certain spec. The thing is that now I'm trying to compare a meter with 6.000 count to a 50.000 count and cant seem to grasp how many digits I have to account for to compare the two meters and also how to put in 30d into a measurements of say 5.0000V will it then become 5.0030V or 5.00030V, i'm leaning towards the first, but I could be wrong?Also and excuse my noob level, but if we assume that 5.0030V is the correct way to do it then why have that last digit if all your readings accuracy is tied up to a +-x% + 30d then the last digit is anyway invalid or?

author: hayatepilot
author_rank: Frequent Contributor
timestamp: February 02, 2017, 12:36:46 pm
content: Quote from: Runenaldo on February 02, 2017, 12:21:32 pmYou can use the higher resolution to make comparative measurements or see small drifts better.So if you compare two voltages of lets say 1.00000V and 1.00015V: The meter may show you 1.00020V and 1.00035V. This is a offset of 20 digits or 0.02%.You don't know what the real value of your voltage is, but you know that the difference between the two is 0.15mVGreetings

author: Runenaldo
author_rank: Newbie
timestamp: February 02, 2017, 12:44:12 pm
content: Thank you for the explanation, but I think I have more trouble with the AC voltage, so I try to follow your calculation:BM867: 0.8% + 60d => 5.0000VAC * 0.8% = 0.0400V, adding the 60 digits we obtain 0.0460V = 46mV as maximum absolute errorBM235: 0.7% + 3d => 5.000VAC * 0.7% = 0.035V, adding the 3 digits we obtain 0.038V = 38mV as maximum absolute errorI just dont know if the 60d are put in correctly and if they are, then why is the BM867 worse of than the BM235?

author: Runenaldo
author_rank: Newbie
timestamp: February 02, 2017, 12:49:24 pm
content: Quote from: hayatepilot on February 02, 2017, 12:36:46 pmThank you, that makes sense

author: daveshah
author_rank: Unknown
timestamp: February 02, 2017, 01:35:21 pm
content: Quote from: Runenaldo on February 02, 2017, 12:44:12 pmYes in theory (although of course these are worst case limits) the BM867 is worse. My guess is that the BM867 is an older design so perhaps uses an inferior RMS converter.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 02, 2017, 02:49:36 pm
content: Quote from: Runenaldo on February 02, 2017, 12:44:12 pmYou are using the wrong figures:The 235 has an accuracy of 0.7%+3 digits only at a fixed frequency (50Hz or 60 Hz)In the comparable bandwidth of 45Hz to 300Hz, the 235 accuracy is worse, at 2%+ 3 digits or 103mV.The quoted accuracy in the manual is valid for 1 year. So, a brand new meter should have a much tighter accuracy band.Some high-end multimeters sometimes quote accuracy on shorter timespans, like 30 days or 24hrs.

author: newbrain
author_rank: Super Contributor
timestamp: February 02, 2017, 03:40:23 pm
content: Quote from: Wytnucls on February 02, 2017, 02:49:36 pmWhat Wytnucls said.Just a small additional consideration:True RMS converter usually involve several non linear analog operations, and reaching a very high accuracy is not at all easy, especially in a hand held DMM.Even 7.5 and 8.5 digits bench DMMs will have AC accuracy specified as %, while DC is in the ppm (parts per million) single/double digit range.E.g. (over-simplifying the detailed specs):Keithely 2010 DC V: 24-41ppm AC V: 0.06% (10Hz-20kHz)Keysight 3458A DC V: 8-10ppm AC V: 0.007-0.06% (1Hz-20kHz)

author: Lightages
author_rank: Unknown
timestamp: February 02, 2017, 04:38:04 pm
content: This webpage used to work and showed the accuracy of meters over their ranges and allowed you to compare them. It doesn't seem to work anymore bu maybe somebody could make it work.http://www.johannes-bauer.com/electronics/mmcomparison/app/index.html

author: Runenaldo
author_rank: Newbie
timestamp: February 02, 2017, 08:51:00 pm
content: Thanks all of you, it really helped me understandQuote from: Lightages on February 02, 2017, 04:38:04 pmI really hope that someone can fix that, it would be really helpfull