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.