Flow Switches

Chapter 18 - Discrete Process Measurement

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A flow switch is one detecting the flow of some fluid through a pipe. Flow switches often use “paddles” as the flow-sensing element, the motion of which actuates one or more switch contacts.

Recall from section 9.1 that the “normal” status of a switch is the resting condition of no stimulation. A flow switch will be in its “normal” status when it senses minimum flow (i.e. no fluid moving through the pipe). For a flow switch, “normal” status is any fluid flow rate below the trip threshold of the switch.

A simple paddle placed in the midst of a fluid stream generates a mechanical force which may be used to actuate a switch mechanism, as shown in the following photograph:

Like all other process switches, flow switches exhibit deadband (also called differential) in their switching action. A flow switch that trips at 15 GPM rising, for example, will not re-set at 15 GPM falling. That switch would more likely reset at some lower flow rate such as 14 GPM. With the “trip” and “reset” points being different, the switch avoids unnecessary cycling if the process flow rate hovers near one critical value.