Torque wrenches go out of calibration with use. Verifying the accuracy of a torque wrench is a key quality measurement that must be taken for a successful outcome. Two accepted methods may be used. Verification is a process step to validate if the wrench is in or out of calibration. This test can be conducted with a suitable torque tester/checker. Any operator or inspector may verify a wrench by connecting it to a suitable torque tester and pulling the wrench. The torque tester will display a result that must be compared to the allowable tolerance of the wrench. Some torque testers offer a tolerance-setting feature that provides a GO/NG result (RED or GREEN) when the wrench is pulled. If the verification result is OK then the wrench is deemed in tolerance. If it is NG then calibration or adjustment of the wrench is required.
Unless broken, calibrating a torque wrench is a fine-tuning of the wrench to bring it back within tolerance. A competently trained technician with equipment that is certified to the National Institute of Standards and Technology (NIST) must perform the work if a calibration certificate is required. The international standard for torque wrench calibration (ISO6789) mandates all conforming wrenches meet or exceed 5000 cycles before requiring calibration. It also defines the methodology for calibration. Most competent wrench manufacturers use ISO 6789 as the basis for establishing performance criteria or standards for their tools.
A regular scheduled verification and calibration program helps to eliminate errors based on a wrench being out of tolerance. Competent calibration labs in conformance with ISO17025 or best practices will issue “as found” readings for a calibration, allowing examination of the wrench condition at time of calibration. It is an industry practice to reduce the calibration interval by one half if a wrench is found to be out of tolerance at time of calibration. Failure to calibrate a torque wrench may lead to failure of a joined connection.