The Syphon Tube is connected between the pressure gauge & process in applications where high temperature vapours or fluids are present or temperature above 65 Deg C for eg. steam. It acts as a cooling coil and protects the gauge from high temperature vapours, by dissipating heat. A gauge cock is generally connected between the pressure gauge and syphon for isolation.
These are normally made from Sch. 80, Sch. 160 and Sch.XXS seamless pipes in carbon steel, alloy steel and stainless steel.