Model 1800 from Thomas Products Ltd., of Southington, Conn., is a low cost in-line flow switch. All plastic components are made of PVC, the o-ring is viton “A” and the return spring is 316 stainless steel. Process connections are standard with 1″ slip ports and optional adapters are available in 3/4” and 1/2” slip as well as 3/4” and 1/2” NPT. The unit operates by a positive displacement principle. A shuttle housing a magnet is displaced by the liquid’s flow or no flow to actuate a hermetically sealed reed switch encapsulated within the stem.