need to measure very low air velocity

I need to measure the velocity of very low airflow.........less than 1/2
foot per second or so. When I say measure, I mean an analog signal or that
I can store in a data logger and calibrate against some known. This is not
a switch application.

I have an Extech "hot wire" anemometer but aside from the fact that it does
not measure airflow at the velocities I need, the sensor "head" is
directional.

I have seen some ideas for using self-heated transistors, but don't know if
this type of idea is sensitive enough.

I think I have seen a design using a piezo sensor of some sort that could
sense the wafting of a hand over the sensor, but I don't remember if it
output an analog (relative) signal or not.

The application for this device is to measure the air movement at a forest
floor to about 6 inches from the ground. The sensor and data logger would
be left in place to collect data, since a persons presence might have a
large effect.

Any ideas on how I might build this?

Thanks
Jim

 

The application for this device is to measure the air movement at a forest
floor to about 6 inches from the ground. The sensor and data logger would
be left in place to collect data, since a persons presence might have a
large effect.

Any ideas on how I might build this?

Thanks
Jim



Here is a link http://www.specialinstruments.com/pdf_e/28-29.pdf that might
prove helpful. I'm not sure that these devices will measure (at least with
any accuracy) 0.5 ft. / sec. air flow rates. You might try experimenting
with a ribbon mic http://www.wesdooley.com/pdf/r84_short_manual_ver1.pdf.
They are extremely sensitive to minute air flow. It has a figure eight polar
pattern so wind direction shouldn't be a consideration; keeping it dry
probably will. You will also require about 60 db. voltage amplification to
get the minute voltage generated by the ribbon up to a useful level.

 

Of course, if you have a steady breeze (no fluctuations) all bets are off

 

I think you might wind up having to construct your own transducer. You could
use a pressure sensitive transistor
http://www.omega.com/Pressure/pdf/PX70.pdf mechanically coupled to a
diaphragm of some sort. This might provide the required sensitivity that you
need but wouldn't address the directionality problem. Integrating the output
from several such units might do the trick. I don't know of any simple
answer. Any transducer engineers out there?

 

So I'm thinking of my relies above and the more I think about them, the less
satisfactory they become and the problem becomes increasingly less trivial
which is making me increasingly manic.

We're starting out with a lousy 0.5 ft/sec wind velocity which needs to be
"captured" by a sail...or diaphragm. We can't put any unnecessary "drag" or
"damping" on the diaphragm as the pressure sensitive transistor would
provide, so why not silver the diaphragm and let it deflect an off axis IR
beam. This essentially frictionless beam displacement could then be detected
with a photo diode array, and then scaled to useable output levels.

Again, this method suffers from directional sensitivity so several units off
axis to each other would have to be used and the outputs integrated.

 

Start with a funnel. At the "focus" of the funnel, the air velocity will be
much higher. Now, in a narrow tube that is driven by this higher velocity air,
you place a thin tissue paper strand and use an optical sensor to see it move.
Alternatively, place your hot wire sensor at the narrow end of the funnel in
a tube.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

Hmmm. I wonder if a small, frictionless anemometer would work.
Build a conventional anemometer spinner, but make it a couple of
cm across, sense it's rotation with LED and phototransistor, then
suspend it with passive maglev like this:

Maglev seismic sensor
http://www.jclahr.com/science/physics/diamag/seismo/index.html

Maglev toy
http://www.teachersource.com/catalog/images/lev100.gif
http://www.teachersource.com/catalog/page/Electricity_Magnetism_Engines/Magnet_Products/

Maglev seismic sensor
http://www.jclahr.com/science/physics/diamag/seismo/index.html

Or perhaps you could make two separate 1-axis sensors as
above, using levitated pencil leads with small vanes
dangling below like a pendulum, one for X and one for Y.

High-diamagnet pyrolytic graphite samples are available from
scitoys for a few bucks:
https://4ua.com/scitoys/cgi-bin/shop.exe?page=magnet_desc.html



Or less exotic: dangle a few inches of lightweight fishing line
downwards, attach an opaque horizontal foil square to the bottom,
then sense X and Y deflection with photosensors and diffused LED
bars. You could even use EM damping to stop any resonant swinging
if the foil was copper, and a supermagnet disk was held just below.
If fishing line doesn't offer enough of a sail, glue on some foil
bits.

 

Try a search for ultrasonic anemometer. These have the sort of precision
you're looking for I believe.

Al

 

Thanks for the info; I didn't know such a beast existed. Sounds like the
best solution to the problem by far. I would always opt for an off the shelf
solution rather than screwing around with custom instrumentation.