Originally Posted by
bradtx
This. Dial and beam style tq. wrenches can last decades if not abused. The pointer can be rezeroed as needed, but can indicate that the wrench exceeded it's capacity at some point and may no longer be accurate in either direction. A testing lab can certify accuracy (semi costly).
Brad
You don't need a lab to certify accuracy for a torque wrench in most cases.
All you need is a vise or some other way to support a bolt projecting horizontally a few feet off the ground, the wrench, a ruler some string and a 10# bag potatoes or other known weight.
Simply arrange the setup so the torque wrench handle extends horizontally. It'll probably be necessary to support the wrench in line with the bolt so it doesn't simply flop sideways, but that can be a friend lending his hand. Hang the potatoes a measured distance from the fulcrum to establish the desired torque, ie. at 6" for 60 inch pounds or 5 foot pounds.
Click type wrenches can be calibrated the same way, by dialing in the test torque and gently lowering the weight looking for the click to happen just as you are about to release it. (the actual calibration method varioes with the tool).
This method may not meet Mil Spec, but is good enough for bike related applications. Note that it depends on the spring constant of the tool being unchanged from new, so may be less accurate on tools that were severely abused, but in most cases, the spring constant stays true even when the tool it off. If concerned and wanting greatest accuracy, calibrate at a torque closer to those you actually use.