Our precision diameter tapes are manufactured to exacting standards in temperature controlled facilities at 68F (20C). The lines and numerals on our standard tapes are engraved and acid etched on a ground surface. The tapes have fixed readings that do not require periodic adjustments. Our standard Imperial tapes (inches) read to 0.001" diameter with an accuracy of 0.001" up to 144 inches. Our standard metric tapes (millimeters) read to 0.01mm diameter with an accuracy of 0.03mm up to 3, 600 millimeters. Each Pi Tape brand precision measuring tape is delivered with a Calibration Report traceable to the National Institute of Standards and Technology and all calibrations are performed in accordance with ANSINCSL Z540-1-1994, ISOIEC 17025, ISO 10012-1, MIL-STD 45662A and 10CFR, part 21. Pi Tape - the original precision diameter measuring tape.