Accuracy, Resolution and other matters…

June 20, 2011

Have you ever been forced to check for soft foot when your brackets were less than 10 inches apart and the MTBM’s supports were several feet distant? If you answer yes to that question or if you have ever made foot corrections under similar circumstances, then resolution matters to you.

Resolution: The smallest detectable increment of measurement.

Synonyms: sensitivity, fineness

For laser shaft alignment systems mounted on a pair of shafts, resolution is the smallest movement between the two shafts that the electronics can detect. On all systems LUDECA sells, this value is 1 micron (.00004″) or better. Typically, for any measurement system to be accurate, it must have a resolution at least four times better than the minimum increment of value it is designed to display, and for a “precision” measurement instrument, this ratio should be ten times or better.

Linearity: The closeness of a calibration curve to a straight line. Having output directly proportional to input.

Synonyms: Straightness, direct proportion.

For a laser shaft alignment system, linearity is easy to evaluate. A graph of the known position versus the displayed value should be a straight line. If it is, we say the system is linear. Our products are linear to one percent.

Repeatability: The ability of an instrument to reproduce displayed values when the same input is applied to it consecutively under the same conditions. Repeatability is expressed as the difference between two or more sets of measured values when given identical input positions of the sensor(s).

Reproducibility: The ability of the experimenter (user) to reproduce or duplicate the conditions of an experiment or measurement.  Reproducibility and Repeatability are not the same, but Repeatability (of measurement results) depends on Reproducibility. For instance, if the anchor bolts are loose and the machine moves on you as you are turning the shafts to take alignment readings, your readings will not be repeatable, although they may be accurate for the conditions extant at each reading.

Accuracy: The ratio of the error in a measurement to the ideal, or expected, value. How close a result is to the true value of the parameter being measured.

Synonyms: deviation, error, how close a value is to being correct or true.

Accuracy is how close a measuring system comes to measuring the truth. For specific measurements or specifications, the term error is used and is expressed either as a percent of full scale or as an absolute value. Accuracy is adversely affected by non-linearity, non-repeatability, poor reproducibility and poor resolution.

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