Tecmark Tridelta (TDI) ABS plastic Universal Pressure Switch for electric heaters used on spas and hot tubs, 3903. This is a very universal SPST, Normally Open, 1 amp, 1.5 PSI water pressure switch with two 1/4"" standard tab wire terminals (and also screw terminals), Common and NO. 1/8"" NPT plastic water connection threads and case are ABS plastic for corrosion resistance. Tridelta/Tecmark pressure switch 3903 is a very common 1/8"" pressure switch, and fits a variety of spa and pool heaters. Note: Tapered 1/8"" NPT threads to attach the water line or to screw into heater tube or header actually measures 3/8"" OD when measured across the threads.
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.
Adjustable Pressure Switch comes factory set at 1.5 psi but can be adjusted from 1 to 5 psi. Pressure switches do not normally need adjustment after installation, and to avoid heater damage or overheated water, large adjustments should be avoided. 1-2 psi is the correct setting for most spas, tubs and whirlpools. High Altitude locations or spa systems located several feet Below the spa, may need 3-5 psi settings. Refer to your spa heater manual or heater documentation, for any instructions on heater pressure switch settings.