Fenwal 01 018002 021 Thermoswitch 321894 708 it utilizes an ethylene propylene it utilizes an ethylene propylene (EPT) diaphragm and seals to ensure a reliable bubble tight shut off in high temperature environments up to 300°F The cross ambient point range ensures reliable operation even when the bulb temperature fluctuates around the ambient temperature of the valve body Severe Service Durability: Constructed with a 316 stainless steel outer tube to resist corrosion and withstand high pressure environments always verify that the brass head and any sealing materials are compatible with your specific medium and local plumbing codes