This clever device allows you to back fill your tank with an inert gas while cold crashing to avoid the ingress of O2 through the vacuum relief. It is normally used on a tee with a normal PRV/Vacuum Breaker. The NPT port is hooked up to a regulator and set to a low pressure. The exact setting used will depend on the volume of your head space and your regulator. Start at 2 PSI and adjust from there. A pin spanner is required for disassembly.
1/2" female NPT port