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A bore gauge is a measuring device for determining the interior size of a hole, cylinder, or pipe. In conjunction with a micrometer or master calibration rings, a bore gauge can give an exact reading of bore size. The measurement is derived by inserting the base of the bore gauge into the opening to be measured and moving the base gently until it exactly fits the diameter of the opening. Instruments available include dial and electronic gauges with different base configurations.
A dial bore gauge has a base with a fixed point on one side and a protruding stud on the opposite side that will retract when it contacts the inside wall of the pipe or cylinder. A spring-loaded centralizing mechanism in combination with the one fixed and one moving point provide very exact measurements. These gauges may have variable sized bases for use with a variety of opening sizes or they may have interchangeable extension studs to adjust the base size.
A user records a measurement of the interior diameter of the end of the pipe and selects a base or extension stud of a suitable size. Using a master calibration ring or a micrometer, the user determines the distance between the fixed point and the stud on the base and notes the reading. The dial readout on the gauge has a bezel that is zeroed to match the size of the base being used.
The base is inserted into the pipe opening at a slight angle. Once the base is inside, the user gently moves it back and forth until the gauge is centered in the pipe. The bore gauge is then removed from the pipe and the measurement is read on the dial. The result, either a positive or negative number, is added to or subtracted from the micrometer or master calibration ring measurement to calculate a final outcome.
Some gauges may replace the traditional dial with a digital or electronic display. An electronic readout is calibrated by measuring the base of the gauge in a master calibration ring. Any subsequent measurements are compared to that calibration point. The base of the gauge is similarly inserted in the bore opening and moved back and forth until the gauge is centered. The readout indicates a "minimum" measurement that is the diameter of the bore at the point where it was measured.
Fully electronic bore gauges are now available that read bore openings through the use of transducers to signal readings. Models include units connected to a readout by cable and others utilizing wireless technology. Electronic gauges can be manufactured with either two or three sensor points for measuring accuracy. Some gauges can be modified with extensions for measuring greater depths and wider size ranges.