The leakdown tester that I have works between 80 and 100 PSI. The guage works in the following manner:

Set up regulator at the compressor to between 80 and 100 PSI.

Adjust regulator on leak down tester to set the needle to zero.

Attach outlet hose to injector or glow plug hole (whichever adapter is supplied). Attach outlet hose to tester. Where the needle moves to, indicates percentage of blowby. The limit for most engines is 20%