General Characteristics for Selection of Measuring Instruments

Various specific terms describe the characteristics and quality of measuring instruments, defined as follows:

• Accuracy: The degree of agreement of the measured dimension with its true magnitude.
• Amplification: The ratio of measuring instrument output to the input dimension; it is also called magnification.
• Calibration: The adjustment or setting of a measuring instrument to give readings that are accurate within a reference standard.
• Drift: An instrument’s capability to maintain its calibration over time; also called stability.
• Linearity: The accuracy of the readings of a tool over its full working range.
• Magnification: The ratio of measuring instrument output to the input dimension; also called amplification.
• Precision: Degree to which a measuring instrument gives a repeated measurement of the same standard.
• Repeat accuracy: The same as accuracy, but repeated many times.
• Resolution: the Smallest dimension that can be read on an instrument.
• Rule of 10 (gage maker’s rule): An instrument or gage should be ten times more accurate than the dimensional tolerances of the part being measured. A factor of 4 is known as the mil standard rule.
• Sensitivity: Smallest difference in a dimension that an instrument can distinguish or detect.
• The speed of response: How rapidly a measuring instrument indicates a measurement, particularly when some parts are measured in rapid succession.
• Stability: An instrument’s capability to maintain its calibration over time; also called drift.

The selection of an appropriate measuring instrument for a particular application also depends on four factors, they are

1. The size and type of parts to be measured,
2. The environment (temperature, humidity, dust, pressure, and so on),
3. The skills required by the operator, and
4. The cost of equipment.