Re: What does Data Logger Accuracy = ± 0.25% of Full scale ± 1 Digit Mean?
Accuracy as a Percentage
The accuracy of an instrument is often stated as a percentage. For example, a voltmeter may be specified as 1% accurate. An important question is: 1% of what? Is it 1% of the reading or 1% of the "full-scale"?
Suppose you have a meter which reads voltages in the range of 0 to 100 volts. Then the full-scale value is 100 volts. Now suppose you use that meter to measure an unknown voltage, Vx, and it reads 50 volts.
If the accuracy of your meter is ±2% of the reading, then the actual voltage is somewhere between 49 volts and 51 volts since 2% of 50 volts is 1 volt.
On the other hand, if the accuracy is ±2% of full-scale (or f.s.) then the actual voltage is somewhere between 48 volts and 52 volts since 2% of 100 volts is 2 volts.
Accuracy is often given as percentage of full-scale, which means you should use the lowest scale you can to make the measurement. Suppose a 5% voltmeter has two ranges, 0-10 volts and 0 to 20 volts. If you want to measure a 9-volt battery then you should use the 10-volt scale since 5% of 10 volts is 0.5 volts while 5% of 20 volts is 1.0 volts.
Digital Meters: That Last Digit
Digital meters are often compared by the number of digits they can display. For example, a 2-digit meter can display values from 00 to 99 while a 3-digit meter can display values from 000 to 999.
Suppose you have a 2-digit voltmeter that reads 0 to 99 volts. Effectively it has a full-scale capability of 100 volts. Suppose you use it to measure a voltage with a value of 50.5 volts. What will the meter read? The only choices are 50 or 51, so either way there will be an error. That fact about digital meters is expressed by saying that all readings are plus or minus a count of one.
Digital Meters: Accuracy vs. Resolution
The fact that the reading on a digital meter is always uncertain by a count of 1, either up or down, defines the resolution of the meter. Resolution is the smallest change an instrument can measure (or "resolve"), so in a digital instrument it is the last bit: +/- a count of 1.
Like accuracy, resolution can be expressed as a percentage. A 2-digit meter has 1% resolution (1 count out of 99) while a 3-digit meter has 0.1% resolution (1 count out of 999). However, resolution is not accuracy. A 3-digit meter has 0.1% resolution buy may only have 0.5% f.s. accuracy. Read the specifications of the meter carefully: it is usually the case that the resolution is better than the accuracy.