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Brymen Crest and Min/Max Function VDC Capability

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splat2030:

--- Quote from: Fungus on September 19, 2022, 01:22:11 am ---
--- Quote from: splat2030 on September 19, 2022, 12:54:26 am ---I had to pull the catalog brochure which states +100ms for changes greater than .8ms

--- End quote ---

It's the "datasheet", not the "brochure", and it's +/-100 digits.  :)


--- Quote from: splat2030 on September 19, 2022, 12:54:26 am ---but does not specify which scale the meter uses. Can you confirm with your meter that when you put it in crest mode you have a 5 digit display rather than 4 digit?

--- End quote ---

If I manual range to the 50V mode then press "crest" I get two digits before and two digits after the decimal point.

I find it very hard to believe the accuracy is 1V though, that would be completely useless.

I think we're not fully understanding the phrase "+100 digits for changes greater than .8ms".

What's a "change"?

--- End quote ---

My understanding of the accuracy statement is that the crest function is capable of measuring changes in a measurement parameter that last for longer than .8ms, which is why they only assign a tolerance to values greater than this threshold. Any change in the parameter less than .8 ms does not get an assigned tolerance because the meter is not capable of registering it. I could be wrong, but this is how I interpreted it.

I looked at the specs for all of the 5+ digit display meters that Brymen lists on their website (860 series, 830 series, 780 series, and 850 series), and all but the 850 series explicitly state in the spec that they use a 4 digit resolution in crest mode, which would give a tolerance of +/- 1 for the best performing meters with +/- 100 digit accuracy in the 50.00 range (see attachments). A meter with a 4 digit display such as the BM829 has a statement similar to the 850 series catalog where it just lists the accuracy, I assume because the meter is only capable of displaying 4 digits. It would seem to me that perhaps this was a mistake with the 850 series documentation, and it should perform like the BM869 and BM789 in crest mode considering yours switches to 4 digit display just like the others. I have a hard time believing that Brymen's newest, most advanced, and more expensive meters like the BM789 and BM869 would be so much less capable than one of their oldest meters.

This video shows that the BM869s switches to 4 digit display in crest mode, but the measurement he takes is low enough to be within the 5.000V range. Low voltages like this should result in +/- .1 (.001*100) tolerance since there would be 3 decimal places of resolution.




radiolistener:

--- Quote from: splat2030 on September 17, 2022, 12:59:02 pm ---Does the Brymen 1ms crest function work with DC voltage, and would it be fast enough to capture the max. voltage drop in this application?

Does anyone have any real world experience using either the Bryman crest or min/max in this application that can provide any further input as to its suitability?
 
--- End quote ---

yes, CREST works with DC and works with AC. In both cases it has the same meaning - it captures min and max amplitude Voltage. For example if I measure mains in AC mode with CREST I see:
max=328.2 V
min=-328.3 V
AC = 240.3 Vrms

But you're needs to note that precision is significantly worse in CREST mode. Also this mode has some noise, so you will not be able to capture exact value, just min/max values of ADC noise around signal.

Also note that there is bandwidth limitation, so the error will be higher for shorter pulse.

Here is a short test, how CREST function of Brymen BM867S measures pulse with different length.
Actual max Voltage is 1.0142 V.
Actual min Voltage is -0.0130 V.

pulse duration = max Voltage / min Voltage
1000 ms = 1.031 / -0.029
100 ms = 1.026 / -0.029
10 ms = 1.028 / -0.027
5 ms = 1.021 / -0.029
4 ms = 1.016 / -0.029
3 ms = 1.006 / -0.029
2 ms = 0.899 / -0.029
1 ms = 0.566 / -0.029

Noise during measurement was about ±0.025 V, it means that it can show 0.566 V and after a second seconds can jump to 0.591 V.

As you can see measurement error starts to grow very significantly at about 2-3 ms pulse. And at 1 ms pulse measurement error is up to 50%

radiolistener:

--- Quote from: splat2030 on September 19, 2022, 02:04:17 am ---It would seem to me that perhaps this was a mistake with the 850 series documentation, and it should perform like the BM869 and BM789 in crest mode considering yours switches to 4 digit display just like the others. I have a hard time believing that Brymen's newest, most advanced, and more expensive meters like the BM789 and BM869 would be so much less capable than one of their oldest meters.

--- End quote ---

You're needs to understand that CREST mode shows raw ADC value at about 1000 times per second rate. There is no way to keep 5-6 digits precision at so fast measurement. This mode is not intended for precise measurement, it is intended to catch short Voltage pulses or to estimate amplitude Voltage of AC waveform. Other DMM cannot do that at all.

Fungus:

--- Quote from: radiolistener on September 19, 2022, 03:17:28 am ---There is no way to keep 5-6 digits precision at so fast measurement.

--- End quote ---

Obviously, but people are suggesting it drops down to less then 2 digits.


--- Quote from: splat2030 on September 19, 2022, 02:04:17 am ---My understanding of the accuracy statement is that the crest function is capable of measuring changes in a measurement parameter that last for longer than .8ms, which is why they only assign a tolerance to values greater than this threshold. Any change in the parameter less than .8 ms does not get an assigned tolerance because the meter is not capable of registering it. I could be wrong, but this is how I interpreted it.

--- End quote ---

OK, so a change that lasts for at least .8ms.

That makes sense.


--- Quote from: splat2030 on September 19, 2022, 02:04:17 am ---+/- 100 digit accuracy

--- End quote ---

But this makes no sense. My Brymen drops down to 4 digits in crest mode so if I put it in the 50V range and tap a 9V battery with the probes, you're suggesting it can read anywhere between 8 and 11 volts?

It's an easy test to do...

I've got a 9V battery here and if I set it to the 50V range and measure the voltage I get "09.465V".

I pressed the crest button and tapped the battery terminal with the probe and I got "09.59V" on screen.
I did it again and got "09.62V".
Third time gave "09.60V".
Fourth time:  "09.60V"..
Fifth time:  "09.62V".

OK, it's a tiny bit high (1.4%) but the error is consistent with "10 displayed digits", ie. 0.1V.


The exact phrase use in the datasheet is: "Specified accuracy +/1 100 digits for changes > 0.8ms in duration"


I think that "specified accuracy" refers to the meter's accuracy in 50,000 count mode, not the "visible accuracy" of the number of digits displayed in crest mode.

It's consistent with my quick test and much more believable than the meter only having +/-1V accuracy (which would be completely useless!)

splat2030:

--- Quote from: Fungus on September 19, 2022, 10:20:43 am ---
--- Quote from: radiolistener on September 19, 2022, 03:17:28 am ---There is no way to keep 5-6 digits precision at so fast measurement.

--- End quote ---

Obviously, but people are suggesting it drops down to less then 2 digits.


--- Quote from: splat2030 on September 19, 2022, 02:04:17 am ---My understanding of the accuracy statement is that the crest function is capable of measuring changes in a measurement parameter that last for longer than .8ms, which is why they only assign a tolerance to values greater than this threshold. Any change in the parameter less than .8 ms does not get an assigned tolerance because the meter is not capable of registering it. I could be wrong, but this is how I interpreted it.

--- End quote ---

OK, so a change that lasts for at least .8ms.

That makes sense.


--- Quote from: splat2030 on September 19, 2022, 02:04:17 am ---+/- 100 digit accuracy

--- End quote ---

But this makes no sense. My Brymen drops down to 4 digits in crest mode so if I put it in the 50V range and tap a 9V battery with the probes, you're suggesting it can read anywhere between 8 and 11 volts?

It's an easy test to do...

I've got a 9V battery here and if I set it to the 50V range and measure the voltage I get "09.465V".

I pressed the crest button and tapped the battery terminal with the probe and I got "09.59V" on screen.
I did it again and got "09.62V".
Third time gave "09.60V".
Fourth time:  "09.60V"..
Fifth time:  "09.62V".

OK, it's a tiny bit high (1.4%) but the error is consistent with "10 displayed digits", ie. 0.1V.


The exact phrase use in the datasheet is: "Specified accuracy +/1 100 digits for changes > 0.8ms in duration"


I think that "specified accuracy" refers to the meter's accuracy in 50,000 count mode, not the "visible accuracy" of the number of digits displayed in crest mode.

It's consistent with my quick test and much more believable than the meter only having +/-1V accuracy (which would be completely useless!)

--- End quote ---

I agree with you that this accuracy level would be practically useless for my measurement, so I hope someone can point out where I have gone wrong in my understanding.

I am not suggesting that every meter will necessarily read to the limits of the tolerance range when taking a measurement, but any random meter could read at the max. or min. published tolerance and still be considered to be within spec. I assume that if these meters are tested for quality at some point during the manufacturing process, then one that reads say 10 volts in crest mode would pass and be sold to a customer assuming my interpretation of this accuracy statement is correct. Every meter is going to read a little differently due to manufacturing variances.

I'm not sure using a 9 volt battery like you are describing would adequately test the function of crest mode, because you are simply taking multiple measurements of a constant 9 volt source that doesn't see any rapid changes once you have the meter connected to the battery. I think a more accurate test would be to simulate a short duration voltage spike on a constant voltage source at varying time intervals.

The exact statement for the BM857s is "Specified accuracy +/- 100 digits for changes > 0.8ms in duration", but this is the only meter with a greater than 4 digit display in Brymen's lineup that doesn't also include an additional caveat statement "6000 count mode only", or "Resolution 5000 counts" when describing crest mode. I would be surprised if this meter is capable of .001 resolution in crest mode when the flagship BM869s is not.

My understanding is that the number of additional counts has a different effect on accuracy based on the resolution of the range you are measuring in. The resolution is .001 in 6.000V range, and the resolution is .01 in 60.00V range, so the effect on accuracy of +/- 100 counts is the difference between .1 and 1 in those two ranges. Since your BM857 meter switches to a resolution of .01 when you enter crest mode measuring 9V (09.00V), then +/- 100 digits should result in +/- 1V of tolerance, regardless of the normal screen resolution.

As an example calculation, if we assume that we are limited to the 6000 count mode for the crest function as is stated in the BM-839 manual, then the only ranges available in DCV are 6.000, 60.00, 600.0, 1000. To measure 9 V, the meter would have to be in the 60.00 V range, otherwise it should show an overload in 6.000 V range. The "specified accuracy" in the 60.00 V range for the BM-839 is +/-(.045% + 1D), so a reading of 9 V would give +/- .014 V (.00045*9 + .01*1), then because we are also in crest mode we need to add the additional +/-100 digits to this (.01*100) = 1 V. The total tolerance per spec for reading for 9 volts in crest mode using the 6000 count range would be +/- 1.014 V.

If we were reading say 5 V in the same 6000 count crest mode, then we would have a resolution of .001 (5.000V) because we are in the 6.000 V range, and the +/- 100 counts would only amount to +/-.1 V (100*.001). The specified accuracy would also be less at +/-.005V (.00045*9 + .001*1), resulting in a total tolerance of +/- .105 V.

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