While the words "pipe" and "tube" can be used interchangeably in conversation and literature, each term has a specific meaning and terminology in engineering. Whereas the measurement of a pipe's wall thickness is called its "schedule," the wall thickness of a tube is known as its "gauge." Because tubes with identical outside diameters can have up to five different wall thicknesses, the best way to measure gauge is to measure the tubes outside and inside diameters and calculate the difference.
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Measuring The Inside Diameter
Bring the "inside jaws" of the vernier caliper together until their distance is about 0.25 inches less than the internal diameter of the tube. Note: for a diagram of the various parts of this instrument, visit "http://www.rit.edu/cos/uphysics/VernierCaliper/caliper.html".
Slide the jaws inside the opening of the tube.
Rest the bottom jaw on the internal surface of the tube.
Open the caliper until the top inside jaw touches the roof of the tube.
Tighten the caliper into place with the coarse locking screw.
Gently tighten the caliper using the fine adjust thumb screw.
Tighten the final locking screw and remove the caliper.
Record the diameter in decimal inches and loosen the coarse and fine locking screws.
Measuring The Outside Diameter
Open the caliper so that the outside jaws are spaced about 0.5 inches wider than the outer diameter of the tube.
Slide the jaws around the outside of the tube, keeping the caliper's handle perpendicular to the tube's shaft.
Rest the upper jaw against the surface of the tube and slide the lower jaw up until it reaches the bottom surface.
Tighten the coarse locking screw.
Adjust the jaws with the fine adjust thumbscrew and tighten the fine locking screw.
Record the outside diameter measurement in decimal inches.
Subtract the inside diameter (from Section 1, Step 8) from the outside diameter to calculate the thickness of the tube, i.e. the gauge.