Low cost water flow switch?

Is anyone aware of any low cost flow switch for potable water service that could be used to trip a low power (nominal 12VDC at < 0.5 amp) relay at
about 5-10% of typical max residential flow rate (ie about 0.5 litres per minute or 0.1 usgpm) but not unduly restrict max flow. Purpose is to trip an alarm or shutdown in the event of lost water flow on small lab condenser when water pressure and flowrate fluctuates all over the place (20 to 60 psig). non hazardous location, could be plastic.
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Dear Bob:

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Try Compac Engineering. http://www.iprocessmart.com/erecta/erecta_series5.htm
Gems will do it too (not as cheaply), but since aquired by Danaher, I try not to touch their products.
David A. Smith
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If you are willing to screw around with a home brew, some version of a differential thermistor flow sensor might do it: One thermistor in the flow, a second shielded from the flow, but at the same temperature. Both in series from a 12 volt source, and the voltage across the lower one is proportional to flow rate. This *could* be made to fire a solid state DC relay at a suitable set point - but it could be fiddly to set up. Cheap though....
Brian W

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RS sell a flow switch. We bought one a few months back. Can't remember the details but it's in the catalogue and is quite cheap.
John

an
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