UT39e+ DC precision

The UT39E+ manual declares it’s precision as:

For temperatures between 18 - 28 C.

This means that for the 200.00mV range the maximal error is stated as +-0.13mV.

The measurements were done with a 0.1% resistor decade and 0.1% 100uA current source that was connected in parallel with the decade to generate precise voltage. There were three multimeters connected to the voltage source, UT39E+, AN8008 and GDM-8145. The results are summarized in the graph below. The temperature during the measurement was 19.5 C which is well within the precision range.

I did two measurements, one that spans 29mV-32mV and second that spans 39mV-42mV, in both cases the voltage was increased in 0.1mV steps and the values were written down.

It’s clear from the measurements that everything looks nice in the 20mV-32mV range, the GDM-8145 has 0.03mV offset that is still there even when leads are shorted, the AN8008 follows it rather closely (minus the offset) and the UT39E+ is somewhere between, but overall the meters are in +-0.04mV range which is rather nice.

But in the 39mV-42mV UT39E+ shows non-linearity and maximal difference from the other two meters and even it’s own measurement is +-0.20mV even after discarding 0.04mV from the difference to account for the differencies between the three meters when they were in agreement. Clearly either I got a broken unit or the specs are fake.

Either way the UT39E+ canot be trusted within +-0.20mV which means that the due to the non-linearity the actual resolution is 0.40mV not the 0.01mV as stated and the precision is 0.2% not the advertised 0.05%.