No torque wrench needed, air pressure will not mess with them, and it doesnt matter how long you pound on them with an impact. They are designed to stop at the value stamped on the side. Anything more than that and the shaft will just twist. My most commonly used one is the 19mm 100 ft lb and I usually test it every couple months with a torque wrench to make sure it is still doing its job. They are accurate if you buy a good set. I cant say for certain that you will get the same results from a cheap set.