Our standard Outlet Check Valve adds back-flow protection to any 1/4-28 flat-bottom port. Made from PEEK, PCTFE, perfluoroelastomer, PTFE and gold-plated stainless steel it offers excellent chemical resistance. It's spring actuated sealing system eliminates back-flow, helping prevents upstream contamination or damage. It has a cracking pressure of 3psi and a 1/4-28 Flat-Bottom Male to 1/4-28 Flat-Bottom Female thread port configuration.