Learning Library · Multimeters, explained · 2
Reading an accuracy spec: ±(% + counts)
A multimeter's display tells you what it measured. Its accuracy specification tells you how far the true value could be from that. The spec has two parts, and the second one matters far more than most people expect.
Top: the reading and the band the true value must lie in, split into the % of reading part (cyan) and the fixed part (amber). Bottom: the same spec as a percentage of the reading, everywhere on the range. The dot is your reading.
Two terms, added together
A spec such as ±(0.5% + 3) means: take 0.5% of the reading, then add 3 counts, where one count is one step of the last digit on the range in use. The result is the most the reading can be off in either direction.
The first term scales with the reading. The second does not: 3 counts is 3 mV on a 6 V range whether you are measuring 5 V or 50 mV. Near the bottom of a range that fixed part dominates, and the error, as a percentage of what you measured, climbs steeply.
Bench meters often state the fixed part as a percentage of the range instead of counts, as in ±(0.004% of reading + 0.0004% of range). It works the same way: 0.0004% of a 20 V range is a fixed 80 µV.
Try this
- With the handheld example, ±(0.5% + 3) on a 6 V range, enter 5.000 V. The uncertainty is ±28 mV, so the true value lies between 4.972 V and 5.028 V.
- Now enter 0.100 V on the same range. The 3 counts are still 3 mV, and the uncertainty becomes 3.5% of the reading. On the 600 mV range those 3 counts would be 0.3 mV. Use the lowest range that fits.
- Pick UT8804E. Its AC voltage spec on the 60 V range, 45 Hz to 1 kHz, is ±(0.3% + 30). At 12.000 V that is ±66 mV, or 0.55% of the reading: 30 counts sounds like a lot, but each is only 1 mV.
- Pick UT8806E and enter 10 V. On the 20 V range, ±(0.0040% of reading + 0.0004% of range) is ±480 µV, or 0.0048% of the reading.
Where the numbers come from
| Preset | Function and range | Spec | One count |
|---|---|---|---|
| Handheld example | DC V, 6 V range (6,000 counts) | ±(0.5% + 3) | 1 mV |
| UT8802E | DC V, 20 V range | ±(0.1% + 3) | 1 mV |
| UT8804E | AC V, 60 V range, 45 Hz to 1 kHz | ±(0.3% + 30) | 1 mV |
| UT8806E | DC V, 20 V range, 1 year | ±(0.0040% of reading + 0.0004% of range) | 10 µV |
The handheld example is illustrative. The UT8000E figures are from each model's data sheet.
An accuracy spec only holds inside its conditions: the function, the range, the frequency band for AC, the temperature, and the time since calibration. The same meter can have a different spec on every range and every function, so look up the line that matches your measurement.
Resolution shows the step. Accuracy shows the truth. UT8000E bench multimeters with published specs for every range.
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