One small error:
As external pressure increases the balloon would get
smaller, not larger.

Pressure sensors can be made in the following way:
One fixed plate is separated from one plate that is
mounted to a standard barometric cell (a sealed chamber
that has a corrugated surface that allows it to expand and
contract readily. Looks something like a hockey puck
with circular ripples.) When the chamber expands it
pushes the moveable capacitor plate towards the fixed
plate. The changing capacitance is usually used to
vary the frequency of a high frequency oscillator.
The frequency is then calibrated to the pressure.

Fr. Tom McGahee




----- Original Message -----
From: Sean H. Breheny <shb7@CORNELL.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, March 04, 2001 4:05 PM
Subject: Re: [OT]: Weather Sensors?


> Hi again Herbert,
>
> BTW, what did you end up doing for the wind vane?
>
> I have a crude idea for a pressure sensor. It's mainly just an interesting
> idea to try, and wouldn't be a reliable long term sensor. What about taking
> a small (maybe 1 cm by 4 cm) piece of paper, drawing a heavy pencil line on
> it, with large "dots" at each end. Now, blow up a small rubber balloon. Use
> double-sided tape to attach the paper to the surface of the balloon. Then,
> take two small strips of aluminum foil, and use single sided tape to attach
> them to the dots at the ends of the pencil line, and cover over the whole
> piece of paper with tape to prevent water from getting to it.
>
> If you make electrical contact to the strips, you should have a variable
> resistor whose resistance is proportional to air pressure (less air
> pressure makes the balloon expand, which reduces surface curvature, which
> shortens the pencil line). If you fed the resistor with an accurate
> constant current source, you could use an ADC to just measure the voltage
> across it.
>
> Sean
>
>
>
> At 03:31 PM 3/4/01 -0500, Herbert Graf wrote:
> >         As some of you might remember I recently added a wind vane to my
> > house
> >monitoring system and now I've gotten the weather bug! I have a temperature
> >sensor but now I want to get a humidity and pressure sensor (any other
> >weather variables come to anybody's minds?). Anyone have any ideas on where
> >to get these CHEAP? Or even to build them? I have some ideas on building a
> >humidity sensor but not much on a pressure sensor. It doesn't need to be
> >calibrated, knowing that one day was more humid than another is good enough
> >for me. Thanks on any leads. TTYL
> >
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