Why is a pigtail or siphon required in steam pressure gauge lines?

Pressure gauge siphons are used to protect the pressure gauge from the effect of hot pressure media such as steam and also to reduce the effect of rapid pressure surges. The pressure medium forms a condensate and is collected inside the coil or pigtail portion of the siphon. The condensate prevents the hot media from coming in direct contact with the pressure instrument. When the siphon is first installed, it should be filled with water or any other suitable separating liquid.

It can also be used to reduce the potentially damaging effects of rapid pressure changes. These low-cost devices allow systems builders to use a pressure sensor with a much lower temperature range in high-temperature applications.