Brymen mV AC range - Page 1
Forum section: electronics / beginners

author: Kirken
author_rank: Newbie
timestamp: May 13, 2020, 04:52:38 pm
content: Hey there,I consider to purchase one of Brymen's midrange DMMs like the BM836 or BM829s for some lower voltage AC applications.Scrolling through the manuals I found a note for the BM836 mV range which says: "Signal peak absolute values, including DCbias,less than 1100mVpeak". Does that mean that everything below 1,1V will be rectified peak readings instead of RMS?The cheaper BM829s does not come with this annotation. Sadly, it is limited to 6000 counts only.Looking forward to your replies.Regards,Kirken

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2020, 09:33:53 pm
content: It is a limit on the crest factor. It means that the peak AC+DC voltage cannot exceed 1100 millivolts for accurate AC measurements.I suspect the extra limitation is because the millivolt AC range is DC coupled so any DC offset counts toward the crest factor.

author: Kirken
author_rank: Newbie
timestamp: May 14, 2020, 01:17:17 pm
content: Thank you for the reply.That shouldn't be an issue then since I only want to measure AC with minimum DC components.

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 03:31:46 pm
content: I just checked and my best meter, an old Tektronix DMM916, only does DC volts on the dedicated millivolt range. So it has AC volts, DC volts, AC+DC volts, and DC millivolts.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 14, 2020, 03:47:29 pm
content: As important as the specification you quoted is, it is not very meaningful without a frequency specification, both in its accuracy and derating.Does the manual mention anything related to that?

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2020, 04:10:41 pm
content: Quote from: schmitt trigger on May 14, 2020, 03:47:29 pmOver the frequency range that a multimeter supports, which will typically be up to 10s of kHz, crest factor and derating do not change significantly.

author: Kirken
author_rank: Newbie
timestamp: May 14, 2020, 04:51:24 pm
content: Yes, I already had that in mind. Here are the specs for theBM836nevertheless:AC Voltage (20.000 count mode):50Hz ~ 60Hz199.99mV³, 1.9999V, 19.999V, 1000.0V0.5% + 30d40Hz ~ 1kHz199.99mV³, 1.9999V, 19.999V1% +30d³)Signal peak absolute values, including DC bias, less than 1100mV peakI considered other models like the BM869s (50k counts, 0.3% + 20d @ 45Hz ~ 5kHz) or the BM859s (50k counts, 0.3% +20d/10d @ 45Hz ~ 5kHz). But unfortunately they do only come with a 500mV range. That would probably introduce larger errors measuring below 200mV. The BM836 is specified with a 200mV range as listed above.Apart from this, what would be your estimation as a technical limit of the lower mV range, looking at the devices mentioned above? How low could you measure mV AC (~50Hz) with reasonable confidence of lets say 1% or 0.5% total?Maybe I can find a cheap HP / Keysight like the 34401A in the next week. If all goes wrong, I will probably compensate with higher amplification to work around the noise and lack of accuracy issues in the mV range.RegardsKirken