Friday, February 17, 2012

Torque Wrench - Accuracy and Precision

Accuracy and Precision are terms that have very different meanings in the measurement industry whereas in common language there are often used interchangeably with the same meaning. This leads to confusion. Accuracy describes the average whereas precision describes the standard deviation of the average. Precision is often contained by the tolerance. Tolerance is used to define the allowable deviation from average (the amount of acceptable variance, or precision). Confused? An example might help.

To be precise is to hit the same spot every time. Notice on the target the shooter is precise but not accurate.

Now look at the next target. The shooter is accurate (when you average the numbers) but not precise.

The next target is both precise and accurate. Precise is repeatability and accurate hits the spot.

These examples show that we need to know both: accuracy and precision. One or the other gives us little useful information.

This has special meaning for torque wrenches. When the gears wear in a snap style torque wrench the precision gets awful. Lets say we are calibrating your torque wrench and set it to 40 lb-ft. We will click it 10 times and average the 10 readings to arrive at the number we put on your calibration report we send back with the wrench. The first pull  and your wrench clicks at 50, next click at 30 and so on until we reach our 10 pulls. We average the values and it comes out to 40. Perfect accuracy!

Your wrench may never have clicked at 40. When you use this wrench you think --aww it's perfectly accurate at 40 which means that if it clicks at 40 it is at 40. But you are wrong because your wrench is not precise. When you use the wrench you are not averaging over 10 pulls -- you get what you get-- so you want your wrench to click at the same value each time (precise) and you want it to click at the correct value(accurate).

So now you know that your torque wrench that advertises itself as accurate to within 5% really means: "On the average I click to within 5% of the set point; however, any one discrete click may occur at any value." Rather worthless information isn't it.

But if the wrench manufacturer also states that are statistically certain that 95% of the time the wrench clicks to within 1 lb-ft. of the set point, then we are more confident that when we use the wrench we will establish the amount of tightening to within the manufacturer's torque tolerance.

To be a little more precise in my writing, calibration shops now use the term uncertainty rather than certainty to define the degree of statistic confidence they have in their reported value. Any modern calibration statement must include an uncertainty value.

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