There are two methods for performing dimensional measurements: direct measurement and indirect measurement. With direct measurements, measuring instruments such as Vernier calipers, micrometers, and coordinate measuring machines are used to measure the dimensions of the target directly. These measurements are also known as absolute measurements. Measurements can be performed over a wide range specified by the scale of the measuring instrument, but there is also the chance that the measurement will be wrong due to erroneous readings of the scale.
With indirect measurements, the dimensions are measured using measuring instruments such as dial gauges that look at the difference between targets and reference devices such as gauge blocks and ring gauges. These are also known as comparative measurements due to the fact that a comparison is performed using an object with standard dimensions. The more predetermined that the shape and dimensions of a reference device are, the easier the measurement becomes. However, this method also has the disadvantage of the measurement range being limited.
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