Industrial factories utilize steam as a pressurized fuel for energy consumption. This steam needs to be regulated by a gauge, but the gauge must have a way of being safe from the powerful heat and pressure of the steam itself. This is where pigtail siphons (or syphons) come in.
Pigtail siphons are looped pipes filled with water that are placed in piping configurations directly before an instrument gauge, normally to measure steam pressure, in order to shield steam and possible surges from the gauge's sensitive mechanics.
The trapped water within the loop of the pigtail siphon stops super-heated steam vapor, and allows the heat to disperse through the water, rather than letting it pass and damage or destroy the gauge on the other end.
Pigtail siphons are meant for vertical applications when installed in a piping configuration. If it is in a horizontal application, it is referred to as a coil siphon and has a slight dip in its loop design to provide for the trapped water in a horizontal position.