EEVBLOG 121GW 1ms Peak - Page 1
Forum section: electronics / testgear

author: wjsmarine4342
author_rank: Newbie
timestamp: April 12, 2019, 07:09:47 am
content: Hi All,Just joined, apologies if this is in the wrong area. I did a search and couldn't find anything apart from BlueTooth references so here's my post:I recently purchased the 121GW meter (v1.57) after using a couple of BM235 units at work, the trusty Fluke 87 is there also.I tried the 121GW, BM235 and Fluke 87 in series on DC Amp range with all recording/MinMaxAv of a load pulsing every 5 seconds. Surprisingly the 121GW appeared slowest showing a Max of 149mA behind the BM235 of 177mA with the most believable being the Fluke 87 indicating 390mA. Admittedly the current load is a burst of 40khz pulses lasting 150ms (50% duty cycle) but I would have expected better from the 121GW. Mins and Av are almost identical but it's the Max that is of most interest to me at the moment.Hence my interest in seeing if the 1ms Peak feature actually works...I must say the bling of the 121GW is nice although I'm a little confused with the 1ms Peak feature - it doesn't appear to work or I'm not doing something right... The manual doesn't say much at all about this apart from holding the button down to initiate/release BlueTooth operation. Certainly a momentary push doesn't show anything different on the display as far as icons go and there was no difference in the Max reading when MinMaxAv was taking place and the 1ms Peak enabled (or not).Can anyone elaborate on use of the 1ms Peak button or offer some guidance on how it is enabled? If my current setup (no pun intended) is an unsuitable test then I'm open to suggestion on a better method. What are the specs for the 1ms Peak function and what ranges does it cover?Thanks,Bill

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 12, 2019, 10:01:02 am
content: A more careful reading of the manual might have helped you.. page 32, where the upper knobs are explained:'1ms PEAK Button: Used to select the 1ms peak capture mode(VAC only)..'So it's not working in any other mode, especially NOT in DC current mode.Your 150ms long DC pulse @ 50% D.C. will be averaged by any of your DMMs, but the different integration methods of their individual A/D converters will give different results, and their readings will also vary from sample to sample, depending on the timing of conversion vs. your pulse duration.None of them will measure the peaking current, I assume. In fact, you really don't know WHAT the other, supposed 'better' DMMs will measure. It's only random stuff, supposedly.So you have to chose another method, maybe a bench DMM with fast DCI digitizing possibility, where you can scroll through the acquired data, and measuring peak current by means of cursors. KS 34465A can do so, for example.Frank

author: wjsmarine4342
author_rank: Newbie
timestamp: April 13, 2019, 04:44:34 am
content: Thanks Frank, that explains it - although I couldn't find that information in the pdf (24Jan18 version) doing a search.My meter is at work so I can't check its paper manual until Monday, that's probably where you found that info.The fact remains though, the 121GW isn't as good as its cheaper brother (BM235) in this mode regardless of what A2D conversion process it employs. That irks me a bit.As a matter of curiosity are there any specs available indicating speeds of the Min/Max/Av sampling algorithm between these meters? This function I use a lot because most of our deployments run from dry cells and the averaging function is very helpful in estimating capacity required for the job.Kind regards,Bill

author: EEVblog
author_rank: Unknown
timestamp: April 13, 2019, 05:12:47 am
content: Quote from: wjsmarine4342 on April 12, 2019, 07:09:47 am1ms = 1KHzi.e. it will capture the peak value of signals with a minimum length of 1ms.You are trying to measure a 40KHz signal, with positive pulses of only 12.5us and expecting a consistent result, there's your problem.

author: wjsmarine4342
author_rank: Newbie
timestamp: April 13, 2019, 09:56:45 am
content: Sure, no argument there I already did the math before posting... As far as consistency goes the 121GW is just that - consistently worse compared to the other two.Still miffed about the apparent difference in performance though no matter which way you slice it. For my money the BM235 is a better buy.I'll ask again: are there any specs on what the sample rates are for Min/Max/Av function between the two DMM's? Only curious, not looking for a war.Cheers,Bill