Innovative design that provides consistently accurate readings and rugged, troublefree performance. Certificate of Calibration; Actual certification readings and individual instrument serial number are included. Each torque instrument, as calibrated at the factory, is certified to meet the accuracy specified in ANSI B107.14M-1994 and was calibrated on a torque standard traceable to the National Institute of Standards Technology (NIST). Accuracy; accurate to 4% clockwise and 6% counterclockwise from 20% of full scale to full scale; feature the Snap-on High Strength Sealed Ratchet Head. Sealed ratchet heads keep out dirt and moisture while being virtually maintenance free. U.S. Patent 4,934,220; U.S. Patent 6,125,722 applies to 1/4", 3/8" and 1/2" drive Ratchet Head Models only. Minimal Friction "Hour Glass" Cam. Retains and releases lubricant where needed to reduce friction. Laser Marked Scale. Easier reading with varying light sources. Positive Stop. Instrument can not be accidentally disassembled if wound down past scale.