These little valves save lives! They keep your tank from bursting should your pressure switch stop working. Usually you want a valve rated 30psi higher than your pressure switch cut out pressure. To determine the size of the end that fits into the tank, measure the OD of the threads, then subtract 1/4" because pipe sizes always measure 1/4" over the actual size. (For instance, threads meaure about 1/2", subtract 1/4", you have a 1/4" MPT safety valve)