Pres-Air-Trol 21 amp pressure switch TNP411R-1.8PR is a miniature pressure switch, set at 1.8 psi and rising. Connects with a 1/8"" slip barb hose CENTER connection. Switch is SPST with two wire terminals for common and NO wires. Threads shown in picture are used to mount the pressure switch to chassis or panel. Hose is pushed onto 1/8"" slip spigot inside the threaded mounting nipple. Non-Adjustable pressure switch is Normally Open and comes factory set at 1.8 psi. Made in the USA by PresAir.
Pressure switches are used on spa, tub and bath heaters to measure the amount of water pressure inside of the heater, as an approximation of water flow. Pressure switches are safety devices intended to prevent heater damage and overheated water hazards. If your pressure switch is causing the heater to shut down, look for causes of low flow in and out of the heater, and remove the spa filter temporarily to improve flow. Pressure switches do fail eventually (which is why sell replacements), but first assume a flow problem, and not a switch problem.