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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Cheap water flow sensor?
Message-ID: <13321@milton.u.washington.edu>
Date: 20 Dec 90 10:22:45 GMT
References: <4390@alliant.Alliant.COM> <6229@videovax.tv.tek.com> <4258@kitty.UUCP>
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Organization: University of Washington, Seattle
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In article <4258@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
	(original query)
>> >I need a device to give me an binary electrical indication that water
>> >is flowing/not flowing through a standard half-inch domestic water
>> >pipe.  
>
>2.	Create an internal orifice plate as above, and install tees with
>	sidearms as small as possible before and after the orifice plate.
>	Differential pressure in the low in-H2O range will be created by
>	flow fluid across the orifice plate.  Differential pressure switches
>	that are adjustable may readily be found 

	and for that matter, differential pressure sensors with bridge
outputs can be conditioned for any given flowrate threshold.  The main
problem with this approach is: it will howl like a banshee if
the associated piping is tuned for any audible wavelength (which
is rather likely in a domestic installation.)  The orifice/pipe
combination is a water-powered whistle.
	You might want to use a loop of smaller-diameter (copper?)
pipe instead, so simple laminar flow (with drag on the sidewalls of the
pipe) is created.  The differential pressure will go as the square
of the fluid velocity, roughly.  This is the fluid equivalent of a
resistive element, and the differential pressure gauge is the
equivalent of a voltmeter, so is equivalent to a VOM in current 
measurement mode.

	John Whitmore