Multimeter tolerance question...

[DavidR357]

I have a cheap UNIT-T UT30B and was looking at the tolerances at http://www.uni-trend.com/en/product/2014_0801_919.html

Under the parameter tab I'm looking at the DC current row. It has 'Best Accuracy' at '+- 1% +2'. What does the +2 mean?
Also, does 'Best Accuracy' mean that it is likely that my meter is worse than 1%? If that's the case, why state a tolerance if your readings fall outside of the tolerance?

Also, the meter has different ranges labeled '200u, 2000u, 20m, 200m, 10' and the website says these refer to 200mA / 2000mA / 20mA / 200mA / 10A. I thought uA referred to microamps? Is there a mistake somewhere, or does u and m represent the same unit with amperage?


I'm trying to make an LM317T constant current circuit as a laser driver. I need around 300mA, and I want to make sure that when my meter measures this the true value is not too large for the diode. I could measure the resistance between the adjust pin and output pin, but the resistance is so low (~3ohms) that I don't think the meter would give an accurate reading.

Thanks

[TimFox]

The "accuracy" spec is typical of digital-readout instruments.  It means that the reading should be correct to within the sum of +/-1% of reading (or, sometimes, full-scale) plus two digits of the readout.  For a 1.000 V reading on a 3.5 digit meter with 2 V full-scale, that means that the worst-case error should be +/- (0.01 V + 0.002 V) for percentage and digits, respectively.

[Rick Law]

TimFox is exactly right.  Just to expand on that a little.

Example of 1%+ 2 is one percent of reading or of the full scale and 2 counts.  You do have to check your manual carefully on if whether it is %reading or %full scale.

Full scale means maximum reading for that range.  So, if you are set to 100.0ohm max (range) and the reading is 50.0ohms 1% of range is 1 ohm and 1% of reading is 0.5ohm.

With  this example again; the reading is 50.0ohm.  The least significant digit there is the 0 on the right.  The '2' part of '1%+2' means the least significant digit can be off by 2 counts.  So, that 2 translates to +- 00.2 ohm off.

So, total error could for 1% of full range is 1ohm+0.2ohm = +- 1.2ohm total
total error for  1% of reading is 0.5+0.2 = +- 0.7 ohm total.

You have to check the manual on is it full range or reading.

On one of my old Radio Shack DMM, it has +- % of reading & +- % of full scale & N-counts.  So, it is not always %scale or %reading; it could be AND instead of OR.

EDIT: eliminated a couple of extra words that I failed to remove the first time around.

[rsjsouza]

All others have said enough about it, but sometimes I like to have a visual feel of what all this is about. In this case, check this app note from Keithley that has a nice graph.

[Muxr]

Dave covered this in one of his early videos: