Products > Test Equipment
Brymen 869s DC offset issue when reading ACmV
mqsaharan:
--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---I agree a 189 or 187 may be a good choice, but they seem to go for a lot and they are all getting pretty old.
I need something I can reliably get several of for the folks on the manufacturing floor so would rather not putz with old stuff.
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If you are using it on a production floor, like bdunham7 said, I would also suggest you to use a quality instrument like 287. But if budget is the issue then you might have to compromise and use a cheaper quality instrument. But you'll have to keep it in check which sometimes is extra headache.
With AC coupled dB measurement requirement, your choices are limited in handheld DMM catagory. Mostly expensive models cover your requirement. If you glance at dB column of the DMM spreadsheet, you'll find many multimeters supporting dB function. But it is difficult to confirm which ones have AC coupled inputs.
https://www.eevblog.com/forum/testgear/multimeter-spreadsheet/
--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---As to the 61E comment. My issue with Uni-T is not temperature drift. It's time drift. Take a Ut-61E and wait 6 months and it will be way out of calibration, unacceptably so. That doesn't help me trust Uni-T. Even the cheap Anegs and Harbor Freight meters hold calibration. Not so much Uni-T.
--- End quote ---
The video I linked in my last post is a no nonsense video regarding drift. In that video you'll learn about the drift of UT61E, BM869s and UT181A along with a few others. If I remember correctly, a few years back when 61E's drifting problem was kind of a hot topic, 61E's drift boils down to temperature cycling. Then again, I don't remember that discussion. I didn't take notes as I don't own one.
Fungus:
--- Quote from: bdunham7 on January 23, 2022, 04:08:11 am ---
--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.
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There was a long discussion previously on this exact subject, I don't remember which thread. z with old stuff.
--- End quote ---
There was a lso a discussion of how that AC coupling capacitor can be charged up to hundreds of volts then used to zap other electronics. :popcorn:
--- Quote from: bdunham7 on January 23, 2022, 04:08:11 am ---Nobody ever got fired for buying Fluke, right? The 289 can be fairly criticized--it is expensive, big, cumbersome to use according to some and it eats batteries like popcorn. But for some uses it seems to be only current production tool I know o
f that works, other than a bench meter which would have another set of issues. There may be alternatives that will perform to your expectations, but unless you get lucky, they won't be all that cheap anyway.
--- End quote ---
Yep. The Hioki DT4282 can do it, too, but it's not much cheaper than the Fluke. You may just have to bite the bullet.
_Wim_:
--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---The 0.22 cap was derived by testing. Any smaller and the error was too great for a 500Hz test signal and our calibration procedure. Any larger was unneeded. No idea why but that is the reality. I get it shouldn't need to be that big but that's the measurement I got.
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What accuracy where you aiming for in the calibration procedure? Can you describe the test you performed? From your first post I would presume a 500Hz 100mVac signal riding on top of 7.5Vdc and then checking the deviation?
--- Quote from: AnalogEngineer on January 23, 2022, 03:38:13 am ---I still haven't had anyone confirm this is really a 869 design error. It seems strange that it is since other meters handle this fine.
--- End quote ---
I do not think this is a design error they omitted the series cap, but this is related to the fact the BM869s has dual display capability on mVac mode where it can measure Vac and Vdc simultaneously (which many meters can't do). It would have been nice of course to have a dedicated AC-mode (with series cap), as this would allow them to calibrate out the capacitive divider formed by the series and parallel input capacitance.
Related to settling time, I would expect other meters to also to have a relatively low settling time with your test signal, as otherwise they would have too much error when measuring low frequency AC signals. As stated in a previous post, the Tektronix bench DMM also uses a 100nF series cap, which would only half your settling time, and maybe the 289 uses something similar?
_Wim_:
--- Quote from: bdunham7 on January 23, 2022, 04:08:11 am ---The real issue, IMO, are the cases where you overload the mVAC range with DC but you just get a wrong reading rather than an OL indication of some sort. There was no consensus IIRC, but IMO that is not an acceptable result from a professional tool.
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I agree that this could be handled better. I wonder if this could be related to the fact that the OP does not achieve the expected accuracy? Many he could try manual ranging and see if the accuracy improves on a high range for his test signal.
Fungus:
--- Quote from: _Wim_ on January 23, 2022, 07:14:42 am ---I wonder if this could be related to the fact that the OP does not achieve the expected accuracy? Many he could try manual ranging and see if the accuracy improves on a high range for his test signal.
--- End quote ---
A BM869S can easily read a 100mV AC signal with 7.5V DC offset in the normal AC+DC range.
I think the problem is that he wants it in dBm
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