Industrial torque meters cost a lot ($4500). I'll show you how to build a low buck version with an Arduino.

Why? Because I want to test power tools to see if the actual specs live up to the marketing specs.

The reason this works is Load Cells are cheap on-line. They work by measuring the deflection in an aluminum beam. Normally that deflection is detected by an electrical device called a Wheatstone Bridge. When the Wheatstone Bridge stretches or contracts it changes the resistance, which is then read by a micro-controller.

The trick here is the Wheatstone Bridge doesn't care if it is stretched by a simple weight or by torque. All we need to do is calibrate the software to change the common, dirt-cheap load cell to a rare and very expensive torque meter.