> Air pressure is another thing of interest to me at the moment. Does
> anyone know of a cheap sensor?
>

DIY's are also possible for (static) pressure, but I think they can be a bit
trickier, for two reasons... Linearity and, if you're
doing barometric pressure, the small changes with respect to a "baseline" of
1000 units (millibars?). Often you see barometric
pressure in this scale quoted with one decimal place too, which means 5 orders
of magnitude between your resolution and range.

One interesting side note is the intriguing way that top loading washing machine
water level sensor work.. They essentially trap a
column of air between the water level in the base of the wash drum at the
bottom, and the sensor itself, usually mounted slightly
above the top of the drum level. As the water level rises, the air column is
put under more and more pressure until it finally
trips the switch, turning off the water inlet, and starting the wash cycle.
Amusing messes can be made by forgetting this, and
equalising the pressure when there is already some amount of water in the drum.
I did this once in the process of figuring out why
my washer was overflowing. In the first instance the cold-water solenoid wasn't
closing properly, so the wash cycle would start but
water would continue and eventually overflow the drum. So I swapped the hot and
cold solenoids around, since my missus does
cold-only washes. However in the process of figuring out the solenoid was sus,
I disconnected the pipe from the level sensor with a
half full drum. When I reconnected it, having convinced myself I'd fixed the
problem, it dutifully tried to fully fill an already
half full drum, the extra half being continuously dumped on the floor, and since
the level switch never tripped the solenoid stayed
open and the wash cycle never started. Sigh.

Anyway, back to air pressure measurements.... Essentially you measure the change
in volume of a sealed container of gas, along with
temperature. Then use the gas laws to figure out the change in pressure
associated with your sensor.

Eg. you could have a tin can solder-sealed all around at a given (and measured)
pressure / temperature. From that point on,
assuming no gas leaks out, the tin can will either expand or contract depending
on changes in the surrounding air pressure. One
_could_ attach a steel strain guage to the can and measure the strain on a flat
side-wall, figure out the pressure difference (guage
pressure), and then you know the difference between now, and when you sealed
your tin. You use your base calibration to then
determine absolute external pressure.

However, the can itself will resist pressure changes in a not-necessarily easily
predicted manner. An alternative is to have a
sealed cylinder with a piston, again set up at a known pressure/temperature, and
then measure changes in the height of the piston.
You'd have to take into account friction in the piston seals etc.

If you're sensing line air pressure in high pressure systems, that's all well
and good, but measuring barometric pressure amounts to
measuring maximum changes of under 100 units out of 1000, eg, a 10 percent
change. Probably the easiest way to do that would be to
get a resonable quality mechanical barometric pressure guage and instrument the
indicator dial! (eg. with a shaft angle sensor or
something).

Essentially most mechanical barometric devices convert small pressure changes
around some "average" into a coiling or uncoiling
motion of a long tube wound in an open spiral, which then rotates a needle with
it.

Dynamic pressure is another thing altogether. This will relate to your
windspeed measurement too. Maybe Andrew's platinum balls
can do double duty!

Actually speaking of balls and electronics and the interesting things some
clever people can do with them,.. check out
http://www.daves-domain.com/movies/Thisguyisamazing.wmv (Sound required).

Cheers,
David.

----- Original Message -----
From: "Chris" <cholme@...>
To: <mugwa@yahoogroups.com>
Sent: Tuesday, October 04, 2005 6:25 AM
Subject: RE: [mugwa] Sensing wind speed
>
> Regards
>
> Chris Holme
>
> -----Original Message-----
> From: mugwa@yahoogroups.com [mailto:mugwa@yahoogroups.com] On Behalf Of
> ron.casotti@...
> Sent: Monday, 3 October 2005 11:53 PM
> To: mugwa@yahoogroups.com
> Subject: RE: [mugwa] Sensing wind speed
>
>
>
> Another way which I don't think anyones mentioned, is using differential
> pressure sensors and a venturie . 2 or 3 sensors
>
> could be used to extend the range.
>
>
>
> Ron.
>
>
>
>
>
> -----Original Message-----
> From: mugwa@yahoogroups.com [mailto:mugwa@yahoogroups.com] On Behalf Of
> Chris
> Sent: Monday, 3 October 2005 9:37 PM
> To: mugwa@yahoogroups.com
> Subject: RE: [mugwa] Sensing wind speed
>
>
>
> Thanks fellows, I am most interested in the linear thing Dave mentioned
> concerning thermal gradients. Being in possession of nifty things like
> difference amplifiers and high resolution ADCs (all grafted through
> various sample programs) I think that I will experiment with heated temp
> sensors.
>
> Although Andrews plutonium balls sound like fun too.
>
> Regards
> Chris Holme
>
> -----Original Message-----
> From: mugwa@yahoogroups.com [mailto:mugwa@yahoogroups.com] On Behalf Of
> David Emrich
> Sent: Monday, 3 October 2005 8:50 PM
> To: mugwa@yahoogroups.com
> Subject: Re: [mugwa] Sensing wind speed
>
> > Hello Guys
> > I've been looking around and trying to learn something about sensing
> > wind speed. Apart from anemometers with three cups it appears that my
> > options are ultrasonic transducers or using a pair of LM335's, one of
> > which is self heated.
> > Does anyone have any practical experience with wind speed? Or, has
> > anyone heard of another way to measure wind speed?
>
> I've done the anemometer type thingy, and you're not limited to three
> cups... Four is just as workable, and slightly easier to make
> (90 degree angles being what they are, versus 120 degree ones). I used
> bits of 2mm galvanised fencing wire and table-tennis balls
> cut in half. Mounted them onto a small 12V motor from an eftpos journal
> printer which I mounted vertically. I reduced the bearing
> friction in the motor by supporting the "short" end of the shaft on a
> hardened steel ball and only using the inbuilt bronze bushes
> for side-thrust. I reduced the brush/commutator friction by removing
> the brushes and commutator! I reduced the rotating intertia
> by removing the coils and armature. Oh, and I took out the magnets too
> :-S Not much of the original motor left except the shaft and
> bronze bushes. Oh, and the photo-interruptor disc on the back which
> gave me something like 20 slots per rev. BUT, it did
> accurately read windspeeds down below 1 kmh, and over 80kmh (half a knot
> to 40 knots), compared to a commercial unit rated well
> beyond my meagre usage requirements (and even further beyond my price
> tag!). It could probably have gone higher, maybe even to
> 150kmh, I just didn't test it that hard.
>
> I've also used the two-temperature-sensors and (in my case independent)
> heater element, that works quite well, and is valid for just
> about ANY type of fluid, not just gases.
>
> One simple way to measure windspeed that sometimes is good enough is a
> large vane which is suspended vertically by a thinish (but
> rigid) wire, from a hinge. The more the wind blows, the more the vane
> deflects,... if it's horizontal, time to batten down the
> hatches! I don't remember how the relationship between angle and
> windspeed goes, but it's probably NOT linear, whereas within
> reasonable limits, both the cups-anemometer and thermal gradient methods
> ARE linear with speed.
>
> With any wind sensor, there are some issues to think about (you can
> decide to ignore them, but you need to know what they are to
> decide if you CAN ignore them!).
>
> 1. Turbulence. This can be from nearby objects, or your sensor device
> itself. Pretty obviously, you can't tolerate much of this
> from either source, unless you're purely interested in a "wind /
> no-wind" sensor.
>
> 2. Direction. Wind-sensors for (eg.) sailboating might require some
> degree of dependence on all three directions, since the boat
> can be keeled over quite sharply, affecting the apparent direction of
> the wind on the sails. Therefore some kind of "3D windspeed"
> (more probably wind-velocity) sensor might be required. Meteorological
> wind sensors should be designed to read wind speed
> independent of horizontal direction (that is, they're essentially two-D
> devices). Assuming one has handled point 1 appropriately,
> and is on level ground, one doesn't normally expect much wind to be
> travelling in anything other than a horizontal direction. :-)
> Lastly, windspeed in a pipe is obvioulsy constrained to flow directly
> along the pipe, sufficiently far away from corners and
> fittings, and is pretty much one-dimensional.
>
> 3. "Dynamic range". Just how much (and how little) wind speed do you
> need to accurately read?
>
> 4. Linearity. Do you need it? Can you work with a simple fudge table?
> Can you measure wind speed in your own not-necessarily-linear
> units and be happy with that? (A related problem occurs with thermistor
> based temperature measurement,... one of our projects at
> work measured temperature in what became known as Leotherms... the guy
> writing the micro-code was Leo!).
>
> I think that about covers it.
>
> Hope you can get something useful out of all that.
>
> Cheers,
> David.
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
> _____
>
> YAHOO! GROUPS LINKS
>
>
>
> * Visit your group "mugwa <http://groups.yahoo.com/group/mugwa>
> " on the web.
>
>
> * To unsubscribe from this group, send an email to:
> mugwa-unsubscribe@yahoogroups.com
> <mailto:mugwa-unsubscribe@yahoogroups.com?subject=Unsubscribe>
>
>
> * Your use of Yahoo! Groups is subject to the Yahoo!
> <http://docs.yahoo.com/info/terms/> Terms of Service.
>
>
>
> _____
>
>