After replacing the alternator, its belt and the A/C idler+tensioner pulley on my car I used the old parts plus a digital crane scale and turnbuckle to set up a variable belt tension fixture to check my Gates Krikit 91107 and
91132 tension gauges.They are easy to misuse. I found that if the round end doesn't sit flat on the belt it will read low, and if I push the round end down with a second finger it may read high. A little drag toward the round end kept it in contact without excess pressure and gave a reading of 1/2 the pulley axle pull on the crane scale within half a mark, or 5 pounds.
The Krikit is compact enough to fit the very cramped space of the belt cluster between the engine and inner wheel well, where I can't see to measure belt deflection with a straightedge. I had been using the quarter twist tension check and setting them below spec.
The alternator pivot corroded very solidly from road salt so that I couldn't increase tension and I thought the occasional squealing was from that, rather than the harmonic balancer slipping. The first indication I noticed that it was bad was a wobble in the belts that ran on the outer ring, while the center hub belt ran true.