Thanks all for you kind help on this matter.

/Ake

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Peter L. Peres
Sent: Friday, July 03, 1998 1:39 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: Preassure


  There are two main types of pressure gauges, atmospheric (aka
altimetric, barometric)  and industrial.  The industrial ones you do not
want to use for atmospheric readings. Atmospheric gauges are special in
that they need to read between a small pressure (a 'high') and a partial
vacuum (a 'low', or 50.000 ft ;). This is usually solved by the
manufacturer with a reference cell that contains vacuum.

  Someone makes integrated pressure sensors for atmospheric (and
altimetric) pressure measurements. I don't remember who, but try searching
the web. I have used one once.

  This is a common silicon chip that is etched very thin in the middle and
has a deformation sensor designed into it over the thin area (I think that
it is a form of strain gauge). The chip also contains a calibrated
amplifier. The chip wafer is glued over a vacuumed cell with atmospheric
pressure pushing on the free surface. This deforms the chip and the strain
gauge measures this deformation. The built-in amplifier is calibrated by
the factory to give something like 1 mV / milibar. The case was TO-18
(transistor case with a hole on the top for the atmospheric pressure
line). The range was from 0 (?) to 1500 milibar or so, and for power I can
say that the thing worked from a 9 Volt battery... The power requirements
were not exactly modest (more than 10 mA for the chip alone for sure). The
instrument had an analog 3 1/2 digit read-out built with an ICL 7106 and a
LCD. This measured millivolts at the sensor output and was calibrated to
read millibars or meters alitude (a switch changed between the 2 modes).

  These sensors are not cheap at all but for a one-off project the
straightforward calibration pays off and a rather accurate and stable
instrument results.  The vacuum cell in the gauge will degrade over years,
but buying two and keeping one in store as replacement won't help, unless
the spare is packed in a vacuum too. Even then it may not help, depending
on what is degrading the vacuum. If you plan on long-term use, obtain the
data sheet from the manufacturer and ask about aging. Use in a house
weather monitoring system is long term use in any case (with 5 years
useful lifespan or more to count in from the beginning).

  Nearly all the other pressure gauges that I know of require very
elaborate calibration procedures that are almost impossible to do without
appropriate vacuum equipment (and gauges). I know of people who have
played with transparent garden hoses bent into a 'U' shape and partially
filled with water to calibrate a gauge and other weird experiments
involving inverted bicycle pumps, laboratory rubber tubing and mercury
barometers, but the ready-calibrated chip version is by far the best buy
for a 'sure works' project.

hope this helps,

        Peter