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[DPRG] Water pressure sensors recomendations?

Subject: [DPRG] Water pressure sensors recomendations?
From: Robert Singleton sluggy7584 at gmail.com
Date: Mon Apr 9 15:39:21 CDT 2012

I like pressure sensors, but I have a bias from one of my other control
hobbies.

For electronic fuel injection, manifold pressure is one of the best ways to
derive the mass air flow into the engine, a critical piece of the fueling
equation. Arguably the most popular is a Freescale device, the MPX4250. It
comes in several flavors, but the most used is like this one:

http://www.diyautotune.com/catalog/mpx4250-25-bar-map-sensor-p-106.html

This sensor, with the AP suffix, reads absolute pressure. The analog
voltage from it while sitting with nothing on the inlet will be the local
barmetric pressure. The sensor can read pressure down to 0 or up to a
maximum of 36 psi, about 2.5 atmospheres at most inhabited regions of the
planet :)

For your rain barrel level, I think I would plumb in a fitting near the
bottom of the barrel and connect a well sealed semi-rigid tubing, something
like ice maker line, to an MPX4250AP positioned anywhere above the top of
the barrel. It could even go through a wall to electronics mounted inside.

As water fills the barrel, it will exert pressure on the column of air in
the tube. Pressure will go up in a fairly linear fashion as the barrel
fills. The only tricky bit will be calibrating it. An empty barrel will be
at local barometric pressure and a full barrel will be at whatever pressure
the rising column of water in the sensor tube can exert above that. I
presume that can be calculated or measured at deployment.

Some variation of this technique has been used to sense water level in
washing machines for decades.

I think I'd mount the barrel end of the tube up a bit from the bottom, an
inch or so, to keep muck, sand, leaves, etc that will inevitably end up in
the bottom of the barrel out of the sensor tubing. The interior of the
sensor inlet is impervious to water, but by letting water into the bottom
of the tube and measuring the pressure of the air column above it, you
prevent mud and critters from getting to the inside of sensor. You can
further protect the tube from anything that might try to crawl up it by
fitting an aquarium air stone to the underwater end of the tubing.

The most critical piece of this scheme is that the sensor tubing must not
leak or the pressure will slowly equalize to atmospheric. The relative
pressures are pretty low, so this should not be a big problem.

 Lesser issues could be the effect of barometric pressure (probably
minimal, though one could use a second sensor to read the barometric
pressure and compensate mathematically) and the temperature of air in the
sensor tube.

I think the best argument for this kind of monitoring method is that,
except for the insertion and sealing of the tubing, there are essentially
no mechanical pieces to deal with.

On the other hand, the actual level in the barrel is derived rather than
measured and thus accuracy is only as good as the calibration and dynamic
range of the sensor.

Sluggy!

P.S. for my next trick, I will determine the wind speed by measuring how
quickly a bit of wire cools after I heat it with an electric current....

http://www.instrumart.com/products/25511/testo-425-thermal-anemometer

-- 

"Just because you do not take an interest in politics doesn't
mean politics won't take an interest in you." - Pericles
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