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Brymen Crest and Min/Max Function VDC Capability

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bdunham7:

--- Quote from: splat2030 on September 17, 2022, 12:59:02 pm ---I have been looking to purchase a multimeter for automotive/general usage, but I would like something with a very good min/max function that is capable of measuring the instantaneous max. voltage drop at my 12VDC car battery when under the load of cranking the starter. The overall application of the meter is personal use, so unfortunately the higher end meters like the Fluke in the video below are out of the budget. The Brymen BM-829s has caught my interest because the manual states that their crest function is capable of a 1ms (1000X/s) sampling rate, and the normal min/max function is capable of a 50ms (20X/s) sampling rate. This meter has signifiantly better specs than other similar meters I have looked at such as the UNI-T UT161D, which has a min/max function with a sampling rate of only 333-500ms (2-3X/s). 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 ---

I don't think you need a very fast MIN/MAX function to do this test and it actually might be confusing or yield wrong results if it were too fast.  The 13.5ms figure mentioned is from a specification regarding the ability of automotive electronics  to operate properly in a worst-case momentary voltage drop during starting and isn't intended as a battery test--and if it is used as a battery test, you need different numbers than given in your video. 

Your youtuber is giving you a rough rule-of-thumb method that may work on many modern cars with PM starters and modern electronic engine controls.  I don't know where the 9.6 volts comes from, but from experience I can tell you that very early electronic fuel injection cars would sometimes have long crank times because the controllers went offline at ~9.6V and would only come back to life when the engine was spinning fast enough so that the starter load was reduced enough that the battery voltage would come back up.  So a 'crank, no-start' complaint would be caused by a weak battery, something unthinkable on older models.

The Fluke 98 Scopemeter had a built in battery test function that would graph out the battery voltage during the startup cycle.  I just went out and tried my car, a 2010 Honda Accord I-4 that has been sitting for about a week.  The car starts promptly and the battery is known to be good.  The initial OCV was 12.41V, which dropped to 11.9 when the ignition was turned on.  Ambient temp is 64F after an overnight low of ~56F.  I'm sorry the picture isn't better, but the backlight is weak on the 30-year old Scopemeter and although I have the FlukeView software and cable, it hasn't worked since Windows 95.  As you can see, the voltage briefly drops way below 9.6V, but then pops back up to 10V+ right away. 

A Fluke 27 using MIN/MAX gives a MIN of 10.19 volts and a MAX of 14.05 which indicates a good battery and a good charging system according to the test mentioned--both of which are correct results.  That might be a good meter to start with for your usage, or if you are really interested in automotive, start scouring eBay looking for a Fluke 98. 



joeqsmith:

--- Quote from: splat2030 on September 19, 2022, 01:29:25 pm ---
--- Quote from: joeqsmith on September 19, 2022, 12:43:43 pm ---For any questions regarding the specifications, you could always contact Brymen. 

It seems there is something in the manual for the BM869s where you can place the meter into the 500k mode prior to selecting crest.  This will enable an extra digit but it seems it requires the signal to be stable for a longer period.   I have not checked to see if Brymen has a newer version of the manual and it seems mine was pretty sparse on details.

--- End quote ---

The manual for the BM869s states that it has a 5000 count resolution in crest mode.

--- End quote ---

Could have been Rec mode or that I am confusing it with a different meter.   Easy enough to download the manual and look.   Sure enough, min/max mentions it.

armandine2:
Fluke 98 was re-badged as the Bosch PMS 100

Looking through my copy of Bosch's Engine Test: Fault detection with the oscilloscope they don't mention the crank test - which might indicate that there is some difficulty in the interpretation of trace?

Fungus:

--- Quote from: splat2030 on September 19, 2022, 12:57:14 pm ---It would make it useless for any measurement above 6 V (Assuming 6000 count mode).

--- End quote ---

Which is why I don't think they'd ever make a single meter that works that way, never mind a while range of them.

It would be like making a car whose wheels turn square if you go past 50mph.


--- Quote from: splat2030 on September 19, 2022, 12:57:14 pm ---Measurements under 6 V would have an accuracy of +/- .105 V like I showed.

--- End quote ---

Or ... they might have an accuracy of +/- 0.01V, as shown in joe's video.



I just tried it with some 1ms, 2ms, 3ms... pulses and my meter seems to be about 0.1V high on the 50V range, ie. Specified DCV accuracy+100 counts.

I can do more tests but practical testing only tells me about my meter. My meter might be the golden meter in a million that just happens to work well.

The only way to be 100% sure is to write to Brymen and ask them to clarify the statement in the manual.

(Which they probably will - they respond to email).

Please let us know what they say.

joeqsmith:
If my cars go "click",  I know the battery is dead.   No need for anything too fancy.  If it dies often, I would be looking for the current draw.  I seem to get about six years out of them. 

bdunham7's scope shot is nice.  With a better scope, you should be able to see the compression stroke of each cylinder.   We would expect them to be balanced.    Maybe some monkey math applied to determine a weak cylinder.   Certainly we could see if one was totally gone with a hole in it.   :-DD

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