How can I assess my torque wrench? I have owned a dynamometric wrench for two years now, and I got it second-hand in the first place. I fear that relying on its indication could be even less accurate than tightening by intuition. For example, I know that oscilloscopes need to be tuned once per year.

How can I measure the accuracy and precision of my wrench? I have a tape measure and a low-end electronic scale (rated 45kg+-10g, but I have noticed deviations as much as 50g between measurements of 2kg parts), plus a wide range of inserts for the wrench. This is a 20Nm-100Nm piece of equipment.
How about this:


attach an extension that can be clamped in a vise such that the handle of the torque wrench is horizontal, and its own weight is applying torque that attempts to rotate the extension around its axis
using some wire or string, attach an empty 1 liter plastic bottle to the handle of the torque wrench
adjust the setting of the wrench such that it's own weight (+ the empty bottle) is just enough to cause it to "click"
increase the setting by an amount such as 3Nm (on a wrench that is 30 cm long, this would correspond to a weight of 9N which is slightly under 1kg)
add water to the bottle until the wrench clicks again
measure the mass of the water added and compare to the expected amount based on the torque setting increase and the length of the handle of the wrench


The only missing part is that we don't know if the initial setting (corresponding to the torque of the wrench's own weight) is accurate.  You could find the center of the mass and measure its distance from the point where sockets are attached to the wrench, then calculate the torque from the weight and distance.