Harwil 35 mm flow switch with 1/2" thread

This Harwil flow switch has a 35 mm reed, and is typically used to measure the flow in 3/4" pipe. It has a 1/2-inch thread, and should be screwed into a suitable tee fitting. It can handle a maximum current of 1A, so it is designed for pilot duty only (i.e. you cannot use it to directly cut the power to the heater).

It is installed on spas manufactured by Cal Spas, Artesian Spas, Dimension One, LA Spas, Sundance Spas and Jacuzzi with the white PVC tee, among others.

Dimensions (size "Medium", in red on the diagram)
Thread diameter: 21 mm (1/2 inch MPT)
Length of contact pin: 35 mm
Total height (excluding contact pin): 79 mm

Part numbers
Harwil: Q12DS-3-3-4S-1
Sundance Spas and Jacuzzi : 6560-852
Replaces Q-12DS50/3/4/4S/NO/1

Installation instructions