Air is added or released from the expansion tank with a Schraider valve just like on you car tires. Check the air pressure with a regular tire pressure gauge. The volume of air in the tank is quite small, so if adding air, it is recommended that you use a hand pump to avoid accidently applying too much pressure and blowing out the tank bladder. I plumbed a gauge into my water supply line between the PRV and expansion tank which really made it simple to adjust the PRV and I can monitor the pressure although it never changes.