You need a mechanical spring calibration device to calibrate this rebound tester.

spring calibration device

The force required to compress the spring 3.302 ± 0.0254 mm (0.130 ± 0.010 inches), while mounted in the receptacle, shall be 44.45 ± 0.4445 N (4532.6 ± 45.33 gf).

After the mechanical spring calibration device is mounted into the instrument, do the 3 sets of 5 readings and average each set. Each set becomes a test determination.

Average the 3 test determinations to the nearest whole number. This whole number is the resilience calibration value of this instrument.