Hello
I need help to understand specifications of measuring instruments. I understand what is accuracy but I don't understand what is precision
Example : voltmeter with 3% accuracy
Voltmeter has accuracy 3% if the measured value is 10v that means voltmeter might read between 7 or 13 volt
What is meaning of precision for measuring instrument?
Can some one help with example: voltmeter with 2% precision?
Hi,
Usually the accuracy is a simple statement of how close the instrument can get to showing the actual quantity being measured.
For example, you have a calibrated voltage reference standard of 4.000 volts DC that is accurate to plus or minus 100uv. That means it can acatully be 3.9999v to 4.0001v. You read this voltage with a DC volt meter with 1 percent accuracy. 1 percent of 4 is 0.04 volts. so your reading will be between 3.96v and 4.04v. That is simply the actual voltage plus and minus the percent of the voltage is can be off by.
4.000*0.01=0.040, and 0.040 subtracted from 4.000 is 3.960 and added to 4.000 is 4.040 so the range is 3.960 to 4.040 volts.
Because of digital meters they usually also specify a 'count' that goes with that percentage. The 'count' may be "plus 2 counts" for example.
With the previous example and a meter that could read 4.000 volts with four digits like that, the range was 3.960 to 4.040 but because of this 'count' we have to add that to the upper range so we get a grand total of 3.960 to 4.042, because '2' is the count that must be added to 4.040 to get the final upper range figure.
Resolution is a different spec. That is the lowest quantity that the meter can 'resolve".
For example, if you have a meter that reads this small set of voltages over time:
1.000, 1.001, 1.002, 1.003
then it looks like the resolution is 0.001 because that's the smallest difference it can read.
Readings like this:
1.0001, 1.0002, 1.0003, 1.0004
would mean that the meter has 100 microvolt resolution because it can tell the difference between one voltage and another voltage that is 100 microvolts above that one.