Type 311 Midi-Diaphragm Seal has an all welded metal construction, with no gaskets or bolts, that prevents process media leakage. These are designed to be used on pressure gauges up to 3.5" from 60-1,000 psi, and 4.5" gauges 100-1,000 psi. The seal uses 0.25 NPT or 0.50 NPT instrument connections.