ASTM D668-99 pdf free.Standard Test Methods of Measuring Dimensions of Rigid Rods and Tubes Used forElectrical Insulation.
Determine the average inside diameter bysubtracting twice the average wall thickness from the averageoutside diameter. Determine the average wall thickness froman average of at least six measurements in accordance withSection 13.
Apparatus.A ball foot micrometer or suitable verniercaliper reading to 0.02 mm or 0.001 in. shall be used for themeasurement of tubes 8 mm (/oin.) and over in insidediameter. A dial micrometer reading to 0.02 mm or 0.001 in.shall be used for the measurement of tubes under 8 mm (/16in.) in inside diameter.Test Specimens.Specimens shall consist of tubes ofany length or diameter.
ASTM D668 pdf download.Measure the wall thickness to the nearest0.02 mm or 0.001 in. For tubes 8 mm and over ininside diameter,take four measurements at points 90° apartaround the circumference at each end of the specimen. Fortubes under 8 mm  in inside diameter, measure the wallthickness by placing the specimen over a mandrel of slightlysmaller diameter which is fixed in position at right anglesrelative to the spindle of the dial micrometer. Note a zerosetting with the spindle against the pin. Then place thespecimen over the pin and take readings at various pointsaround the circumference of the specimen. Take at least fourreadings.Report the average of the measurementstaken as the wall thickness of the tube.
Variation in Wall Thickness of Tubes.Specimens shall consist of tubes ofany length or diameter.Measure the wall thickness to the nearest0.02 mm or 0.001 in. For tubes 8 mm  and over ininside diameter，make a series of measurements around eachend of the specimen. For tubes under 8 mm  in insidediameter, measure the variation in wall thickness by rotatingthe specimen around the mandrel and following the generalprocedure in accordance with 13.3 for tubes of this size.Report the difference between the maximumand minimum values as the variation in wall thickness.Also,report all measurements taken.
These tests have been in use for many years， but noinformation has been presented to ASTM upon which to baseastatement of precision. No activity has been planned todevelop such information.
These test methods have no bias because the values forthese measurements are determined solely in terms of these testmethods themselves.ASTM D668 pdf download.