Accuracy Specifications for Instruments

[Rainer Dynszis]

Hello all,

my first and probably dumb problem is:

I have trouble understanding the accuracy specifications for instruments, such as multimeters and the like.

For instance, the manual for the Fluke 28 II says that DC current readings in Ampere are accurate to +-(0.2% + 4 digits), and I'm not sure what to make of that. My understanding is that you always have a certain probability of error, but the concept of probability is nowhere to be seen in these specifications.

Is +-(0.2% + 4 digits) the standard deviation of measurements around the true value, or does the reading never deviate more than +-(0.2% + 4 digits) from the true value, or what does it mean, really?

[bob91343]

The confidence level of that specification is very high.  In fact, the manufacturer assumes you will take the specification as a maximum, not a standard deviation.  All units that fall outside this range will be rejected.

So your last sentence with bold face font for the word 'never' is true.  The error will never exceed that amount.

[magic]

Well, there is one exception to the "never": those specifications are usually only guaranteed for some time after last calibration.
Which means that the device can slowly drift and needs to be periodically checked for accuracy and adjusted.

[Rainer Dynszis]

Thanks. From the word "guaranteed" I understand the error is "never" bigger in the sense that otherwise a new unit would be eligible for replacement or repair.

I forgot that there are two error sources, (1) nonperfect repeatability, and (2) systematic deviations due to nonlinearities or miscalibration.

I conjecture that (1) is smaller than (2) by orders of magnitude, so that probabilities don't really come into play here, yes?

[ArthurDent]

Some instruments I've had are more specific and will give those accuracy specs you showed but will say that they only hold for something like 50% to full scale and the error may be greater on the lower part of the range.

[barry14]

One should also note that most accuracy specifications only hold for a limited range of environmental conditions: mostly temperature but humidity, vibration and altitude may also be specified.