In a fire sprinkler system, flow sensors are used having strain gauges
mounted on supports which project into the sprinkler pipes. When water
flow occurs, the supports bend, changing the strain gauge resistance and
producing a signal which can be analyzed to indicate which sprinkler
heads are open. Since there are no mechanical switches, the sensor units
are much smaller and cheaper than previously, and provide more
information. A strain gauge can also or alternatively be located outside
the pipe to detect vibration caused by flowing water. As an alternative
to strain gauges, piezo film can be used, with a vibration analyzer to
analyze the signal from the film to determine whether water flow is
occurring and which sprinkler heads are open. A piezo film can also be
placed on the pipe exterior and the signals from the two piezo films can
be applied to differential inputs of a differential amplifier, to cancel
signals caused by external influences such as pipe vibration. A
thermistor can also be used to sense water flow, using temperature
changes when the water begins to flow.