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Characteristics of A Measurement

In making a measurement, one of the first considerations is the resolutin of the measuring instrument. Resolution refers to the minimum change in value that the instrument can reliably indicate. For instance, many dial gages are graduated in 0.001’?0.1mm) division, but can be read to 0.0005’?0.005mm). If it is necessary to measure a dimension to only 0.005’?0,15mm0, in fact it is likely to be a disadvanage.
There is considerable confusion between the terms repeatability and accuracy as they relate to measurement. Often, too, repeatability and resolution are equated.
Accuracy means closeness to truth. In discussing the accuracy of a micrometer or an electronic gage, reference is to the degree to which it can measure the true size a part.
The first equirement of any measuring system is that it have adequate repeatability. For instance, if widely varying results are obtained with each measure of the length of a gage block, these results are meaningless, and the measuring system is ineffective.
Various sources of error can affect the accuracy of a measurement, and various influences can affect the repeatability. There are two types of errors: fixed or systematic errors and random errors. Fixed errors, as the name implies, do not vary between one measurement and the next; they remain constant during a set of measurements. One example might be a zero error on an electronic meter. Another error of this type would be the error on an electronic meter. Another error of this type would be the error in a master block used for calibrating another gage block. The difference of a few degrees in the mean temperature of the two bolcks during the course of the measurements is another fixed error that could affect results.
Fixed errors affect the accuracy of a measurement, so every effort should be made to isolate and quantify these errors. Once their magnitudes are known, the results can be corrected to allow for them.
Random errors, on the other hand, result from changing conditions during a set of measurement. The conditions under which a measurement is made, such as temperature and gaging force, should be clearly defined, and erevy effort should be made to maintain these conditions constant or at least reduce fluctuation to an insignificant level.
Two basic types of measurement exist: absolute or direct measurement and comparative measurements . Although all measurements are comparative in nature, the term is normally reserved for situations where like or essentially like items are compared. The comparision of two 3’?75mm) gage block is a good example of comparative measurement. If the length of a gage block is measured with a micrometer, or in terms of the wavelength of light, the measurement is classified as direct. Comparative measurements have the advantage of being considerably more accurate than direct measurements. The relative size of two silimmar gage blocks is known with much greater accuracy than in comparative measurement many systemtic errors are eliminated.