Brymen BM257s: MIN/MAX vs 5ms crest capture mode - Page 1
Forum section: electronics / testgear

author: jancelot
author_rank: Regular Contributor
timestamp: July 17, 2015, 10:17:29 pm
content: Have the doubt what's the difference between those two modes: the MIN/MAX and the 5 ms crest capture mode. The readings also differ. Examples reading 220 VAC:MIN: 242.8 VMAX: 243.5 VCrest MIN: -332.5 VCrest MAX: 332.9In the user's manual is not explained. Anybody can point out how this works?

author: Lightages
author_rank: Unknown
timestamp: July 17, 2015, 10:35:18 pm
content: Min/Max measures the minimum and maximum of the normal display in the current meter setting. Crest mode measure the absolute values as seen by the ADC. So your first measurement in Min/Max was display the minimum of 242.8VAC and a maximum of 243.5VAC. The Crest measurement was actually measuring the negative and positive peak values of the sine wave within the window of 5ms.

author: jancelot
author_rank: Regular Contributor
timestamp: July 18, 2015, 09:00:36 am
content: I don't understand exactly what's happening here. Why the 332V don't show up in the MAX/MIN? Does it mean that I don't have really 220V but 330 or something like that?

author: cs.dk
author_rank: Unknown
timestamp: July 18, 2015, 09:16:55 am
content: 230Vrms is what gives the same power as a DC source at same voltage level. The peakvoltage is actual +/-325Vpphttp://blewis.co.uk/Physics/A2/Physics_A2.html#toc-Subsubsection-2.4.5

author: helius
author_rank: Super Contributor
timestamp: July 18, 2015, 10:28:44 am
content: Vrmsis also a function of the waveform V(t):Integral sin(t)2dt = x/2 - sin(2x)/4 + CIntegral(0,2pi) sin(t)2dt = [2pi/2 - sin(4pi)/4] - [0/2 - sin(0)/4] = (pi)So the mean of sin(t)2is pi/2pi = 1/2, and the root would make it 1/sqrt(2).This means that the voltage peak of a sinusoid is sqrt(2)*VrmsThe rms of the instantaneous voltage gives the same power because P = V2/R

author: jancelot
author_rank: Regular Contributor
timestamp: July 18, 2015, 01:40:25 pm
content: I see. When I set this multimeter to AC measurement, it's showing merely the True rms value of whatever signal is measuring. It makes sense since both MAX and MIN are always positive values. With crest capture you can see the real maximum and minimum of the input signed values. Comparing both values you can guess if it's a sine wave or any other, provided that there isn't any disturbance which may lead to false deduction. Of course to know exactly what the signal looks like you need an oscilloscope (dedicated or multimeter with oscilloscope functions).The manual says that the Brymen BM257s is a true rms multimeter but doesn't mention that the shown values in AC mode are indeed the true rms. For multimeters that have rms only it will show a different thing. That's just common knowledge.

author: EEVblog
author_rank: Unknown
timestamp: July 18, 2015, 01:47:01 pm
content: Quote from: jancelot on July 18, 2015, 01:40:25 pmThe definition of a True RMS multimeter is that it displays the True TMS value of the AC voltage.

author: helius
author_rank: Super Contributor
timestamp: July 18, 2015, 02:12:24 pm
content: When reading a sinusoid like 220V line, there is no difference between a True RMS meter and any other meter.All meters report RMS values, not peak values. The difference is only when the wave is not a sinusoid, but something else like a square or triangle wave. In that case True RMS will report the RMS of the waveform, using a special chip designed to measure this. An "average reading" DMM will instead report a wrong value, because it is only designed to measure sinusoids.Among True RMS meters, there is always a limit to how distorted an AC waveform can be, to still be read correctly. This is called the crest factor, and higher crest factors (like noise) will not report the correct RMS measurement.

author: jancelot
author_rank: Regular Contributor
timestamp: July 18, 2015, 02:31:32 pm
content: Quote from: EEVblog on July 18, 2015, 01:47:01 pmWhat about AC current measurement, it's also true rms or only applies to voltage?

author: Wytnucls
author_rank: Super Contributor
timestamp: July 18, 2015, 05:35:47 pm
content: It does apply to the AC current mode in the same fashion. However, the TRMS bandwidth is generally less than for AC voltage.

author: Electro Fan
author_rank: Super Contributor
timestamp: July 24, 2015, 11:34:14 pm
content: Quote from: jancelot on July 18, 2015, 02:31:32 pmHi jancelot,Just checking to see if you are happy with your Brymen 257s? I saw that the DC Voltage accuracy is specified at 0.2% + 3d and DC Current is 0.5% +3d; are these consistent with what you are seeing? How do you feel about the accuracy for other measurements/measurements in general on the 257s? Anything that concerns you about the 257s - would you buy it again? Thanks, EF

author: Redeeman
author_rank: Contributor
timestamp: July 25, 2015, 01:44:23 pm
content: Quote from: Electro Fan on July 24, 2015, 11:34:14 pmI have both the BM869s and BM257s, and while i havent used them much yet, i can give you my input. DC voltage is well within 0.2% +3d. (compared to fluke 87V and BM869s)concerns? well.. not really, the screen has somewhat poor viewing angles in some circumstances, "from above" its not very good, "from below" like how it would be when on the tilt stand, very easy to read. nice backlight too. the tiltstand tilts somewhat far back, but not too bad. the EF(NCV) is very aggressive. the continuity tester seems to be latched, it is a little bit slow, its not very annoying, but it can certainly miss if you go very fast. would i buy again? yes. the size is very nice, it feels solid. im very pleased.

author: jancelot
author_rank: Regular Contributor
timestamp: July 25, 2015, 02:49:26 pm
content: Quote from: Electro Fan on July 24, 2015, 11:34:14 pmI can't talk about specifications since don't have precision sources, but the measurements made seem to be right so far, and I'm very happy with it. In the beginning, I was looking for a pocket DMM or the highest range BM869s, but none of them has both NCV and Lo-Z auto-check (essential for all terrain measurements), so I went with this Brymen. It's perfect as an introductory multimeter when you have none or you don't know what the next upgrade will be. It has a lot of features for the quality and price. The latching works pretty well. It has a little bit of delay though but it's ok, on some metals like stainless steel or uncommon metals takes more time the make a perfect contact and you need to slide the probe around, but very good overall. The display is very nice with backlight, the probes are very nice too and also the construction quality is outstanding. There are 4 things I complain about:- The LCD screen plastic cover gets scratched with just looking at it. I stored the brymen with the temperature probe and just by touch of the flying around connectors the lcd got scratched a lot. When you are used to an iPhone or other quality smartphones you notice such things easily.- The lack of an official case, bag, cover or pouch to carry the multimeter with the all the probes, the usb cable and some minor accessories. Tried the Lowepro Dashpoint AVC 1 case but didn't like the outer texture feel, I find it a little bit creepy to the touch. Bough finally the double layer carrying case from 99centHobbies on ebay.- The temperature probe I wish it was of contact type, instead of aerial one. There's a gap between the two pieces where dirt gets trapped if you put the probe on some material.- You need to remove screws to replace battery. Would prefer a quick removal battery cover.- To put the probes in the back holder it's quite difficult because the probes are just a little bit short to leave them resting there, so they get a little bit of strain. The cable remain bent a lot even trying to put it as good as possible.- It's unable to detect red led diodes because they require higher voltage. In the independent test I did, a red led diode draws 16.7 mA at 2.28 volts. Probably starts drawing at just a little less of voltage. The BM257 has a test current of 0.56 mA and up to 1.8 volts. The BM869 has a test current of 0.4 mA and a voltage up to 3.5 volts.