# Precise to accurate

## Main Question or Discussion Point

How can anybody convert precise readings from any measuring instruments to the accurate value without calibration?

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berkeman
Mentor
How can anybody convert precise readings from any measuring instruments to the accurate value without calibration?
Beats me. Is it a trick question?

Surely you can't. Your readings are only as good as the instrument. I don't see how you could convert that to the actual reading.

If my desk is 40.5 inches and I measure it at 40 +-1 then there's no way to get to the real value. You can only improve the accuracy of your reading by using more precise instruments.

Accuracy and precision are two different things. A measurement system can be accurate but not precise, precise but not accurate, neither, or both.

See Accuracy versus precision; the target analogy here:

http://en.wikipedia.org/wiki/Accuracy_and_precision

russ_watters
Mentor
Right, and the implication that has for the OP's question is that you can turn a precise set of readings into an accurate and precise set of readings if you know the offset.....which is, of course, the definition of "calibration". So the question begs the answer it rejects. It's like asking "how can you calibrate without calibrating?" It's nonsensical.

Maybe i was not clear enough, i got this assignment on how to have accurate readings from a measuring cylinder. Now when you are taking readings, the scales in the cylinder are not accurate. Of course it gives precise measurements but accuracy is important. That's why i was wondering if there is anyway to have accuracy from any measuring device.
BTW, i just wanted to try my luck to see from previous question if there was just any conversion equations, which sounds stupid to me now. Thanks.

If it has a fixed scale, you won't get more accurate than is already there.

To improve it, you would need remove the old scale and put a new, more precisely calibrated one on.

You can try to account for this bias, i.e. apply a correction. But of course you would need to figure out what this correction needs to be.

Take any measurement, you could think of a ruler as an example. Maybe the measurements you make need to be corrected by an offset. Or maybe you need to multiply the measurements by some factor. Or maybe you need to do both of the previous steps, say measurement corrected=measurement*a+b. Or it might be more complicated than this. So really this correction depends and can only be figured out using another device that is more accurate than the one you have.