# Is the measurement accuracy of a test instrument consistently the same?

Let's say, for example, an oscilloscope (could be a multimeter, power supply, or other type of equipment) has a voltage tolerance of +/- 3%. I input a signal with a known voltage and determine this oscilloscope is measuring and displaying the signal's voltage at 2% above its true value. This is within spec so I can't complain (but I'm a little annoyed).

Now, assuming environmental conditions are the same (e.g., temperature), will this oscilloscope consistently report ALL voltage measurements at 102% of their true value? And if this is the case, can I add a 2% offset factor to the values displayed to achieve better accuracy?

• Yes and No. I am learning about this at the moment so it's interesting to see this question. There are two components to the error. The first is systematic error which is the difference in the mean of the read values and the actual value (this should be consistent), the second is your random error. The 2% refers to systematic error, but there is also random error in your system. There are ways to account for these things and adjust to improve readings. Before doing this, I would suggest analyzing more samples (a lot more) and trying to better understand the system. jp314 has some good points. – Klik Mar 24 '16 at 4:03

Accuracy of an instrument depends on many factors. While some may remain (relatively) constant, others can drift in unexpected ways.

For a DMM, the greatest source of error is likely the voltage reference -- this basically is the value against which most measurements are compared. When measuring voltages on a single range, this may vary slightly with temperature, but in a good quality ('name brand') DMM, the variation will be small. If you switch ranges (e.g. from the 10 V to the 100 V range), variations in the resistor divider will be different, and so your error will not be the same.

In current measurement, AC voltage measurement and resistance measurement ranges, there are other elements contributing to the errors, and so you cannot compare errors on the DCV range with errors on those ranges. Usually AC range errors are non-linear (significantly greater at low input levels), and so even the linearity isn't necessarily constant.

In fact for low inputs, DC offset voltages also contribute to errors -- and this means that results are not always off by a fixed percentage, but also by a constant value.

In general, for a fixed DCV range, if you verify that the meter reads '0.00' with the leads shorted, the gain error (e.g. 102 % of the true reading) will be quite constant. Same for oscilloscopes inn unchanging scale.

• Consider a DMM that's already been calibrated for offset and gain errors. There will still be some higher-order systematic errors uncorrected, and those would be what you're measuring with your single point test. – The Photon Mar 24 '16 at 4:47

You cannot rely on the systematic error remaining constant. It probably will from day to day (almost certainly, absent high power accidents) but not from year to year.

If accurate measurements are important, you can have your equipment calibrated (usually annually) by a laboratory, against standards traceable to national laboratories. The price for this service is a fraction of the cost of a good multimeter, and several times the cost of a cheap one...