The application is to keep the pressure in a plenum constant at some 
user-defined value as outlet valves are opened and closed.  The 
original approach was to monitor the state of the valves and apply 
offsets that approximate the load that the valves caused.

While simple, its a pain to setup.

Its actually kind of funny - that's how the job was originally 
presented: monitor valves and generate a DC signal that changes as 
the number of open valves changes.  That's easy.

Then they wanted to add trims to each input channel so as to affect 
how much change each valve introduces.  This is to compensate for 
different sizes of valves.  Still easy.

Then make it so that the main board handles a small number of valves 
and add expansion boards as needed.  Still easy but the packaging and 
interconnect logistics are starting to take their toll.

And so on.

Finally asked what the actual desired outcome of all this 
was.  Answer: keep plenum pressure constant as number of open valves 
changes.  OK, sez I, lets just monitor plenum pressure.

As I mentioned in my initial post, I've used Motorola and Honeywell 
pressure sensors in past projects.  However, they are a little too 
pricey for this application.  Ideally, I'm looking in the Can $5.00 
range or less.  Anything more than Can $10.00 is just too expensive.

The Smartec sensors mentioned by Vasile look to be exactly what I'm 
looking for.

Thanks again to everyone for their suggestions!

dwayne

At 02:11 AM 4/9/2007, Russell McMahon wrote:
> > Got a new project to work on that requires a low cost air pressure
> > sensor.  Expected pressure is quite low: 0 to 2" water column.
>
>Does "air pressure sensor" mean is/isn't there or you need to measure
>pressure in that range?
>
>For the former (on/off) depending on your tradeoff between mechanical
>and electrical costs you could use some form of bellows/diaphragm
>arrangement and a microswitch or similar.
>
>The truly Heath Robinson could also look at something similar with a
>spring and pot depending on requisite resolution etc.
>
>How many bits resolution do you need?
>What is the order of maximum acceptable cost?


-- 
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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