Brymen BM869 Resistance Quirk

[EEVblog]

A thread specifically for this quirk.

[Martin72]

Yeah, as known as in my daily test practice, I rub the leads on my shirt or put them "hard" on the mains cable...
In private, I got a BM869s, will take it to work as we got resistance decades there and test it.

[gnavigator1007]

Tried it on the bm235 yet?

[EEVblog]

Tried it on the bm235 yet?

Yep, not a problem.

[gnavigator1007]

Was curious as Joeqsmith noted the original commenter on his video did seem to imply that this was common in Brymen meters, but was vague. Pretty interesting following along as issues like this are just being found on such a mature product that has been popular in the community.

[dcac]

I can confirm this too. With my BM896S I cannot measure 470K - 990K with handheld probes without it starting to alternate between 500K and 5M range. However if I manually select 5M range I can get a perfectly stable reading.

I did not have the test leads near any mains powered equipment - just holding the probes one in each hand.   

I would not be surprised if this problem is (much) more noticeable in 50Hz environment than in 60Hz. I'm in Sweden so here it's 50Hz but Joe is in the US 60Hz so perhaps that's why he hasn't noticed it that much yet.

[coromonadalix]

would be nice to try other meters with the same chipset

[dcac]

I've always thought Brymen did their own chipset - but perhaps they use the same on some models.

[coromonadalix]

oops  yes indeed  they are btc branded if i recall     my bad

I  tought some amprobe meters like the am140 am160  where using the same ic's  since they have 500,000 count mode ?

I have an Amprobe  am140, but no decade resistor box         out of curiosity

[Fungus]

Look at the bar graph, it's obvious where it happens - right around the point where a 50,000 count meter switches between the 500k and 5M ranges.

Seems to me like the threshold where it switches should be just a teeny bit lower.

[floobydust]

What is the test current? Handhelds seem to compromise due to their low supply voltage.

34401A Ohms 10MEG 0.5uA
34401A Ohms 1MEG  5uA
34401A Ohms 100k  10uA

AN8008 Ohms 999.9k 0.2uA
AN8008 Ohms 99.99k 1.2uA

TECH310 Ohms 20MEG 0.3uA
TECH310 Ohms 2MEG  0.5uA

Some of Dave's rubbin' is making large common-mode voltage due to static electricity. You can try it by touching one multimeter lead/banana jack (while on ohms with test resistor connected) while shuffling your feet.

[AVGresponding]

I measured the current at 1uA for the 500k range, and 0.1uA for the 5M.

Used my Keithley 2000 and Fluke 289, they agreed with minor LSD differences.

[radiolistener]

There is also another electrostatic issue with brymen. I have 867S, when I touch left bottom side of the LCD with some electrified stick, it affects measurements. And LCD segments starts to blinking. You can use some polyethylene film to make electrostatic charge on the display for the same effect.

[hammy]

A thread specifically for this quirk.

This only happens in autorange mode.

BTW we all know all devices have their own quirks. This is the reason it is important to KNOW your multimeter.
Induction from a mains wire to your test leads is something you know to avoid during a test.

Using some cheap copper-wire for measurements is allways a bad idea, especially with unknown isolation who is prone for static charge.
This is the reason technicians should do measurements and tests in the real world, and not egg-headed academics. You need to know the real world influences and the limits of your measuring device.

This is the perfect storm in a teacup!
  

[AVGresponding]

Hardly a storm in a teacup. As the thread title states; it's a quirk.

Exploring the conditions where this quirk manifests is important for knowing "the real world influences and the limits of your measuring device" as you put it.

I don't see anyone here as an "egg-headed academic", pretty much everyone I've conversed with on this forum has a great deal of real world, practical, test and measurement experience.

[Psi]

Antistatic mats are pretty good at coupling mains noise into your circuits.
I've seen that on more than one occasion, circuit gets all screwy when sitting on the workbench matt but works fine if lifted up.

It's probably why Dave had it fail once when there was no mains psu/cable nearby, the meter wires were probably resting on the antistatic mat.

[dcac]

Look at the bar graph, it's obvious where it happens - right around the point where a 50,000 count meter switches between the 500k and 5M ranges.

Seems to me like the threshold where it switches should be just a teeny bit lower.

Yeah or a slightly longer delay before the meter decides to switch range - BM896 is quite fast at auto ranging so quirks like this could be seen as a trade off for that speed. Still out of 10 or so different handheld multimeters the BM896 is the only one that has this problem on my work bench.

[Fungus]

Dave,

To test this on the new Brymen 786 you need to use something like 680k Ohms - it's a 60,000 count meter.

[Martin72]

In private, I got a BM869s, will take it to work as we got resistance decades there and test it.

Test it, same here....
But: Regardless if it´s in auto- or manual range.

[Martin72]

Further testing at home, a few minutes ago...

Instead a decade, I took a single metalfim resistor with 560k and put it in the shortest way to the sockets of my 869s, with wago cage-clamps:

https://www.reichelt.de/de/de/bananenstecker-4-mm-cage-clamp-orange-wago-215-211-p100778.html?PROVID=2788&gclid=CjwKCAiA7939BRBMEiwA-hX5J_1QEKpYP8unTK6uJHkBykHVb1sbwAPEngR5MCL3_3C-NwRtVNTPFhoCOmAQAvD_BwE&&r=1

Result: Stable displaying value, you can "knock" on it, nothing happens.
Then I took 1m 4mm cables, value becomes instable when moving the cables.

[EEVblog]

Dave,
To test this on the new Brymen 786 you need to use something like 680k Ohms - it's a 60,000 count meter.

Yep, did that. Ultimate range switching threshold is about 650k, less if you do big jumps. It's as smooth as silk, no sign of this issue at all.

[floobydust]

I would check if it's sensitive to the range switching time-constant, the firmware might not be waiting long enough and then just bounce back and forth between ranges.
Add capacitance (across) the resistor, say 100pF or 220pF and see if changes things for the worse. It should make no difference...

[cluca1969]

I did some testing on my BM869s and it seems that there are still some capacitive and inductive input problems. I used a resistance decades and I varied values between 510K-600K, it seems that these areas are the most affected. I used different cables, some shielded and others I shielded myself, in vain, the defect (anomaly) did not disappear. But to shorten the discussion I will tell you what I managed to get after a few hours of testing. I put a capacitor with a value of 47nF - 100nF at the input of BM869s and the device became very stable regardless of the position of the cables or the touch with my hand of its active parts or of the resistive decade. (First, I made measurements in Manual Mode and here it seems to be fine).

[wraper]

Tried it on the bm235 yet?

Yep, not a problem.

Appears to be autoranging issue. If you use a multimeter with different resolution, of course the magic value will be different even if autoranging algorithm behind it is exactly the same. You should have selected the range manually and see if quirk goes away.

[bdunham7]

I measured the current at 1uA for the 500k range, and 0.1uA for the 5M.

Used my Keithley 2000 and Fluke 289, they agreed with minor LSD differences.

Can you check with a range-appropriate resistor in  the circuit?  I think that the switch from 500K to 5M is going from a constant-current range to a proportional-current range where a 10M or so resistor is switched in across the output and the resistance is calculated.  These proportional resistance ranges are always less accurate and less stable, so I suppose they are more susceptible to interference as well.