Micrometer - Zero Error

Zero error is any nonzero reading when the jaws are closed. It is the calibration error of the device—often caused by knocks or overstrains.

The way to use a micrometer with zero error is to use the formula 'actual reading = main scale + micro scale − (zero error)'. One way to appreciate this concept, and to remember the "formula", is to compare this idea to tare weight. If you are measuring the weight of some water, and you have a cup containing the water on a weighing scale, you must subtract the weight of the cup itself from your measurement, if you want to know the true weight of only the water itself. Similarly, if your length measurement on your micrometer "starts at one" instead of "starting at zero", then you must remember to subtract that one from your total value to yield the true length of the object by itself.

The zero-error compensation formula described above is appropriate for anyone, such as a school student, who doesn't know how, or is not allowed, to adjust (recalibrate) the micrometer to eliminate the error. In contrast, machinists in a commercial manufacturing environment, upon discovering a zero error in their micrometer, would generally not continue using it and applying the "formula". Instead, they would take a moment to adjust the micrometer back to zero (that is, recalibrate it). But everyone, whether student or machinist, should understand the compensation concept, even if they avoid using it.

"Positive zero error" refers to the fact that when the jaws of the micrometer are just closed, the reading is a positive reading away from the actual reading of 0.00mm. If the reading is 0.15mm, the zero error is referred to as +0.15mm.

"Negative zero error" refers to the fact that when the jaws of the micrometer are just closed, the reading is a negative reading away from the actual reading of 0.00mm. If the reading is -0.09mm, the zero error is referred to as -0.09mm.