These little valves save lives! They keep your air compressor tank from bursting should your pressure switch stop working. Usually, you want a valve rated 30 PSI higher than your pressure switch cut-out pressure. To determine the size of the end that fits into the tank, measure the OD (Outside Diameter) of the threads, then subtract 1/4" because pipe sizes always measure 1/4" over the actual size. 

Click here to find out how to troubleshoot a safety relief valve.