[sci.electronics] barometric pressure transducer ideas needed

chuckb@tc.fluke.COM (Chuck Bowden) (09/17/87)

Can anyone suggest a way to sense barometric pressure electronically? I have
a 0-5V a/d converter for my computer, and would like to write a barometer
chart recorder program. The output level of the transducer is unimportant,
since it can be amplified so that it falls appropriately into the 0-5V range.

Thanks!

Chuck Bowden, WB7R, chuckb@tc.fluke.COM
John Fluke Mfg. Co., Inc.
M/S 232-E, P.O.Box C-9090, Everett, WA 98206

dmt@ptsfa.UUCP (09/17/87)

In article <1790@sputnik.COM> chuckb@tc.fluke.COM (Chuck Bowden) writes:
>
>Can anyone suggest a way to sense barometric pressure electronically? I have

Motorola makes a pressure transducer (MPX 100) for automotive fuel injection
systems that reads over the range 0 to 15 psi.

The "Electronics & Wireless World" May 1985 issue has an article on how
to build an electronic altimeter that may be used as a barometer.

The June 1986 issue of E&WW adds a digital readout.

E&WW is a British magazine that currently sells at a San Francisco
bookstore for $6.50 a copy.

The articles say the MPX 100 sells for &11.50. I don't know a US
distributor or the price in $$.

-- 
Dave Turner	415/542-1299	{ihnp4,lll-crg,qantel,pyramid}!ptsfa!dmt

russ@crlt.UUCP (Russ Cage) (09/17/87)

In article <1790@sputnik.COM>, chuckb@tc.fluke.COM (Chuck Bowden) writes:
>
>Can anyone suggest a way to sense barometric pressure electronically? I have
>a 0-5V a/d converter for my computer, and would like to write a barometer
>chart recorder program. The output level of the transducer is unimportant,
>since it can be amplified so that it falls appropriately into the 0-5V range.

Here's one idea, and it doesn't even need a A/D:

Take or build a mercury barometer.  Put a contact in the bottom of the
"pot" of mercury, and put a plate above the mercury such that as the
pressure changes and the mercury level in the "pot" goes up and down,
the spacing between the plate and the top of the mercury changes as
well.  Use the resulting variable capacitor as part of the tank circuit
of an oscillator, and connect a frequency counter to the output; read
the count into your computer, and convert to barometric pressure by
interpolating from a table of known values.

Temperature control of the barometer is important for this one.

Here's a second idea, which would use your A/D:

Build the same mercury barometer, but fuse a fine nichrome wire into
the top of the vertical tube and run it down to the bottom end.  Connect
a wire to the nichrome when it comes out of the glass, and another to
the mercury pool at the bottom.  The resistance between the two contacts
is the resistance of the nichrome above the level of the mercury in the
tube, plus the (very small) resistance of the mercury column below that.
Thus, the net resistance is something like C - x*P, where C is the
resistance of the entire nichrome wire, and x is the resistance of the
wire per millimeter.  Feed a current through it, get a voltage out, put
it through your A/D.  Or, use it as the resistance part of a 555 timer
circuit, clock the oscillations, and interpolate again (getting rid of
the need for the A/D).

If you don't like the idea of mercury in the house, you can probably
do the variable capacitor trick using an aneroid barometer, with
slightly more difficulty.

>Thanks!

You're welcome.  (Hey, I enjoy brainstorming, it's fun.)

>Chuck Bowden, WB7R, chuckb@tc.fluke.COM

-- 
  The above are the official opinions and figures of Robust Software, Inc.
HASA, "A" division.                      Go ahead, flame.  I bought Dow stock!
Russ Cage, Robust Software Inc.		    ihnp4!itivax![m-net!rsi,crlt!russ]

rep@genrad.UUCP (Pete Peterson) (09/17/87)

In article <1790@sputnik.COM> chuckb@tc.fluke.COM (Chuck Bowden) writes:
>
>Can anyone suggest a way to sense barometric pressure electronically? I have
>a 0-5V a/d converter for my computer, and would like to write a barometer
>chart recorder program. The output level of the transducer is unimportant,
>since it can be amplified so that it falls appropriately into the 0-5V range.
>
National Semiconductor makes, or at least used to make, a series of
pressure tranducers which came in gage, differential, and absolute
pressure versions.  The latter should be applicable to a barometric
pressure measurement and, in fact, barometry was mentioned as one of
the applications.

Part numbers were LX05xxA, LX06xxA, LX14xxA, LX16xxA, and LX18xxA for the
absolute pressure versions (where xx corresponds to code numbers for
various pressure ranges).  I have no idea what they cost, but would be
interested in finding out if you look into it.

I found these devices in National's "Data Conversion/Acquisition Handbook".

	pete peterson
	{decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep

ron@topaz.rutgers.edu (Ron Natalie) (09/17/87)

You ought to be able to fabricate one out of some mercury filled
speaker cables.

-Ron

larry@kitty.UUCP (Larry Lippman) (09/18/87)

In article <14816@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes:
> You ought to be able to fabricate one out of some mercury filled
> speaker cables.

	Haha!  That was a cheap shot, but was quite amusing.

	Actually, there is a grain of truth in what you said.  If you take
a glass tube about 34 inches long and sealed at one end, fill it with mercury
and carefully invert it into a small cup of mercury (say, 25 cc), you can
create a "traditional" mercury barometer.
	It is rather easy to measure the height of the mercury column using
capacitance measurement (I'd consider using a 1 MHz measurement frequency).
To create such a variable capacitor, carefully cover about 6 inches of the
outside of the glass tube with tightly wrapped aluminum foil.  The aluminum
foil should be tightly wrapped around the glass tube to assure uniform
contact, and then fixed in position with cement or varnish at its seam and
at the ends.  The aluminum foil should be centered around the "measurement
range" (say, +/- from 28 inches).  The other end of the capacitor is formed
by placing a wire in the mercury reservoir.  It would be a good idea to shield
the completed mercury barometer using metal screening (keep out RFI, and
prevent the radiation of the capacitance bridge excitation frequency).
	For practical purposes, barometric pressure is a linear function
of measured capacitance using _this_ technique (there are other ways to
create a capacitance barometer using mercury, but this is the simplest).
	There are a number of bridge circuits available to measure capacitance
which will give you a voltage output; there are also circuits which operate
on other principles, such as changing the frequency of an oscillator which
in turn feeds a frequency-voltage converter, etc.

	Concerning the hazards of mercury: Mercury is a toxic substance if
ingested or inhaled as vapor.  However, there is nothing inherently unsafe
about building such a mercury barometer, PROVIDED:

1.	You work with the mercury within a large, shallow pan to entrain
	any accidental spillage.

2.	You are careful not to spill any mercury, and you thoroughly wash
	your hands after handling any items which may have been in contact
	with mercury.

3.	You keep the mercury barometer in a ventilated, but secure location
	where children or pets have no direct access.

4.	You place the mercury barometer within another vessel of sufficient
	capacity to contain the mercury should the glass column or reservoir
	be broken.

	Since you are measuring barometric pressure, the safest place to use
such a barometer is to place it outdoors in a locked container attached to
the ground or to a building.  The container should be ventilated, but not
enough allow precipitation to enter the container and create capacitance
surement artifacts.

	Of course, if you feel that mercury is too hazardous, you can always
use water instead.  But then you'd need a tube about 35 feet high... :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
<>  UUCP:  {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
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<>  FAX:   716/741-9635 {G1,G2,G3 modes}   "Have you hugged your cat today?" 

georgep@vice.TEK.COM (George Pell) (09/18/87)

In article <1790@sputnik.COM>, chuckb@tc.fluke.COM (Chuck Bowden) writes:
> 
> Can anyone suggest a way to sense barometric pressure electronically? I have
> a 0-5V a/d converter for my computer, and would like to write a barometer
> chart recorder program. The output level of the transducer is unimportant,
> since it can be amplified so that it falls appropriately into the 0-5V range.
> 
> Thanks!
> 
> Chuck Bowden, WB7R, chuckb@tc.fluke.COM
> John Fluke Mfg. Co., Inc.
> M/S 232-E, P.O.Box C-9090, Everett, WA 98206

I have a copy of the SenSym 1984 Pressure Sensor Handbook, 1255 Reamwood Avenue
Sunnyvale, Ca 94089  (408) 744-1500.

On page 9-35 is applications note ssan-12 which describes a solid-state
altimeter for Transponder Applications.  

This is basically a barometer using their LX1802AN absolute pressure 
transducer.

Hope this helps.

geo

georgep@vice.TEK.COM (George Pell) (09/21/87)

In article <14816@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes:
> You ought to be able to fabricate one out of some mercury filled
> speaker cables.
> 
> -Ron

This should be cross posted to rec.humor.funny!

geo

carl@aoa.UUCP (09/22/87)

In article <14816@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes:
>You ought to be able to fabricate one out of some mercury filled
>speaker cables.
>
>-Ron
You creep! I was going to suggest this. Now you'll get the patent
( :==> ) .


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