Brymen, difference in reading between mA and uA scale

[J-R]

Oh boy, BM869s vs. BM789...

Brymen claims better accuracy from the BM789, 0.075% + 20d from lower ranges, vs. the BM869s at 0.15% + 20d.  In my testing, I found them to be virtually identical.  For example, a quick test at 50mA, the BM789 was 6 counts high, while the BM869s was 4 counts low.

The higher number of nominal counts with the BM789 (approx. 66,000) vs. the BM869s (approx. 53,000) seems like an interesting difference, and the calibration point for the BM789 is mid-scale (so 3V for the 6V range, for example) vs. top of the scale for the BM869s (5V for the 5V range).

In my testing of voltage, I found that the BM869s had some error mid-scale, mostly with 500,000 count mode (for example, around 2.5V), and the BM789 had some error at the top end of the scale (5V).  This is somewhat to be expected as each DMM has to extrapolate perhaps the proper values from a single calibration point (plus zero).  So there is not a clear winner unless you pick a specific calibration point, such as 5V.  In that case, you could adjust the BM869s to be spot-on at 5V, while the BM789 would have a hard time keeping up since the nearest calibration point is 3V.

If you are looking for high accuracy, the solution is probably a bench DMM...

[J-R]

Shunt measurements on my BM789:
A - 0.0212 Ohms
mA - 2.474 Ohms
uA - 102.5 Ohms

And for easy comparisons, my BM869s:
A - 0.0276 Ohms
mA - 1.758 Ohms
uA - 101.543 Ohms

Would be nice if some other forum users could measure their BM789/BM869s and post their results.  I wonder if they changed the specs or if the manual is truly just completely wrong...

[Fungus]

Oh boy, BM869s vs. BM789...

Brymen claims better accuracy from the BM789, 0.075% + 20d from lower ranges, vs. the BM869s at 0.15% + 20d.

The 789 has higher burden voltage so it can give more accurate readings (there's a bigger voltage to measure). 

[EEVblog]

What do you mean by "real current"?
Your meter is reading the actual current through its terminals, which is lower than that which would flow into a short circuit, since your meter is not a short circuit.

Maybe I expressed myself wrong, by real current I meant the current that flows through the circuit when I am not measuring

If it's a large current mA to A then you can use a clamp meter to measure current without disturbing your circuit. But they aren't very accurate like what you can get with a proper current meter.

As I said, adding a resistor (shunt resistor it's called in a current meter) in series with the circuit you are measuring may change the current in that circuit. It's up to the user to not only be aware of that, but also know if it will impact your circuit or not and by how much.
My Silicon Chip article:
https://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf

[J-R]

I don't see enough of a difference between the shunts used in the BM869s and the BM789 to outright state the BM789 is more accurate, and as I mentioned in my testing I did not see a visible indication that the BM789 was any better than the BM869s.  If the uA shunt was truly 200 Ohms like the manual states, then that would be a bigger difference compared to 100 Ohms.

[Kim Christensen]

Many people have already explained it, but this effect is true for measuring anything in the real world. It's called the observer effect.

[EEVblog]

I don't see enough of a difference between the shunts used in the BM869s and the BM789 to outright state the BM789 is more accurate, and as I mentioned in my testing I did not see a visible indication that the BM789 was any better than the BM869s.  If the uA shunt was truly 200 Ohms like the manual states, then that would be a bigger difference compared to 100 Ohms.

Mine are both identical:

[Fungus]

I just realized my Brymen BM857s has better current measurement specs than the BM869s. 

(it's probably no better in practice, but...)

[LazyJack]

Maybe I expressed myself wrong, by real current I meant the current that flows through the circuit when I am not measuring

You still don't understand. The meter is part of the circuit. If you are not measuring and substitute the meter with a short then it is a different circuit with different current. Please draw the circuit for yourself and using Ohm's law calculate the current for the different scenarios.

[sonpul]

I think he understands this. He doesn't understand why you can't make relative measurements like the REL button. Subtracting the resistance of probes, input capacitance, etc. It looks like he would like to automatically offset the display using Ohm's law.

[wasedadoc]

The effect of the shunt is the current equivalent of the Ohms/Volt loading effect that one needed to be mindful of when using an analogue multimeter to measure voltage.  Depending on the resistance of the voltage source the current through the meter (needed to move the needle) causes a voltage drop.  Now that DMMs routinely have 10 MegOhm input resitance the efect is less of an issue but it is still something to be aware of.

[BeBuLamar]

The effect of the shunt is the current equivalent of the Ohms/Volt loading effect that one needed to be mindful of when using an analogue multimeter to measure voltage.  Depending on the resistance of the voltage source the current through the meter (needed to move the needle) causes a voltage drop.  Now that DMMs routinely have 10 MegOhm input resitance the efect is less of an issue but it is still something to be aware of.

The 10 Megaohm input resistance is irrelevance in the case of measuring current. It's the shunt resistor that matters.

[wasedadoc]

The effect of the shunt is the current equivalent of the Ohms/Volt loading effect that one needed to be mindful of when using an analogue multimeter to measure voltage.  Depending on the resistance of the voltage source the current through the meter (needed to move the needle) causes a voltage drop.  Now that DMMs routinely have 10 MegOhm input resitance the efect is less of an issue but it is still something to be aware of.

The 10 Megaohm input resistance is irrelevance in the case of measuring current. It's the shunt resistor that matters.

@BeBuLamar. Try reading what I wrote more carefully.

[MarioBros69]

The effect of the shunt is the current equivalent of the Ohms/Volt loading effect that one needed to be mindful of when using an analogue multimeter to measure voltage.  Depending on the resistance of the voltage source the current through the meter (needed to move the needle) causes a voltage drop.  Now that DMMs routinely have 10 MegOhm input resitance the efect is less of an issue but it is still something to be aware of.

The 10 Megaohm input resistance is irrelevance in the case of measuring current. It's the shunt resistor that matters.

@BeBuLamar. Try reading what I wrote more carefully.

In the video that appears a few comments above, what does the calibration device do to compensate for the internal resistance of the Brymen?
When the Brymen is connected to the device, its internal resistance is also being put in series with the calibration device.

[IanB]

In the video that appears a few comments above, what does the calibration device do to compensate for the internal resistance of the Brymen?
When the Brymen is connected to the device, its internal resistance is also being put in series with the calibration device.

The calibration device produces a known fixed current using a regulator.

[TimFox]

If you don't have a calibrated constant-current source, you can always connect a calibrated ammeter in series with the ammeter being tested and supply DC through a resistor to that series connection.

[EEVblog]

In the video that appears a few comments above, what does the calibration device do to compensate for the internal resistance of the Brymen?
When the Brymen is connected to the device, its internal resistance is also being put in series with the calibration device.

It's called a Contant Current Source. It doesn't matter what the load resisatnce is, it will push a constant current into that load, even a short circuit. The only limit is the maximum compliance voltage.