The band wheel on the drive end is in a fixed location. The idler band wheel is in a slide and moves toward and away from the drive wheel. It is "jacked" away with a large diameter threaded rod with a T bar handle to turn it. It is loaded with a heavy spring on the rod. Between the springs collar and the at the frame it jacks against is about a 2" diameter round thing that has a liquid filled pressure guage that directly reads out in pounds. You could probably build one, but I bet they are not too expensive from sawmill manufactures. Prior to using these guages, Timberking used a ruler and calculated the force by knowing the spring constant and setting the force by the compression of the spring. Either way is accurate. If you know the springs K factor accurately the measurement method woudl probably be more accurate. The guage is just quicker. When I get near the mill again I try and post a picture as ut is easier to understand that way.
Andy