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'[PIC]: data logger'
2002\10\23@023431 by Tony Nixon

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Hi all,

A while back I made a small data logger using Dallas temperature chips
and a humidity sensor controlled by a 16F873.

Now the person wants to measure "wind" velocity as well.

However, the catch is that the wind, if you can call it that, is
actually the small variations in air movement in an average ventilated
room such as in an office.

I haven't got a clue how to measure something like that.

Any ideas :-)

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Tony

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2002\10\23@024100 by Russell McMahon

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Now the person wants to measure "wind" velocity as well.
>
> However, the catch is that the wind, if you can call it that, is
> actually the small variations in air movement in an average ventilated
> room such as in an office.

Consider hot filament resistance change with air flow (filament cools,
resistance drops)
Also can do this as a bridge with one filament shielded and one exposed to
airflow.


       RM

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2002\10\23@032101 by Russell McMahon

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More:
Google on "hotwire anemometer" for too many answers

_____________


Good introduction to hot-wire anemometers


www.efunda.com/designstandards/sensors/hot_wires/hot_wires_intro.cfm
    & theory

http://www.efunda.com/designstandards/sensors/hot_wires/hot_wires_theory.cfm


MAF sensors in cars (hot wire)

       http://www.kemparts.com/TechTalk/tt06.asp


Oz thesis discussing some problems encountered

       http://www.mecheng.adelaide.edu.au/staff/pvl/thesis_summary.html


People saying they don't work well :-)


http://www.superflow.com/support/support-engdyno-anemometer-airflow.htm




{Original Message removed}

2002\10\23@051711 by s Lehmann

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>However, the catch is that the wind, if you can call it that, is
>actually the small variations in air movement in an average ventilated
>room such as in an office.

Hi,

to measure wind movement I used in the past two NPN-transistors
in a TO92 shape, glued together (thermaly conduced) face to face.

One transistor is used as a resistor and heats the second transistor.

In the second transisor the pn- junction (diode) is used for temperatur
measurement.

There are two possibilities: measure temperature direct with the diode
(about 2mV/K but
not linear), or make a feedback loop (for constant temperature) and measure
the
"heating current".

When wind blows to this configuration thermal energy is moved with it, and
changes
temperature or the "heating current".

With one of the above, only wind / air movement could be detected.

Wind speed could be measured with two devices; one is covered and the other
is in
the moved air. Air temperature is compensated with this configuration.
The difference of both measured values relates to wind speed.

Hope this helps

Thomas

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2002\10\23@084206 by Larry Bradley

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I ran across a reference to using standard thermistors in a "self heating"
mode - you put enough current into the thermistor to cause it to heat up
some, then measure the resistance in the air flow. You need two, of course,
one in the air flow and one out of the moving air flow but in the same air
as a reference. Cheaper than the special platinum wires. I also say a
circuit using a pair of transistors, using the PN junction as the
temperature sensor.

Larry

At 08:10 PM 10/23/2002 +1300, you wrote:
{Quote hidden}

>{Original Message removed}

2002\10\23@085829 by Sean Wright

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Hmmm.... sounds like someone wants to build a device to monitor indoor
air quality Tony.  Next, they'll be asking to measure CO2 levels...
Anyways, I think that ultimately if you're measuring air flow in an
office you're looking at reading cfm (cubic feet per minute) or ach (air
changes per hour) and for this you'll need an anemometer type
arrangement or a balometer.  These measurements will be made at each
diffuser in the area to calculate the total flow.  Also, you'll have to
contend with NIST traceability if the results are to be used in any
official capacity, i.e. in court cases, occupational health claims, etc.
Now, lemme tell you; to calibrate these things you need a wind tunnel.
Got one handy?  If you don't then maybe you have a local government
agency or a lab that could do it for you.  Expect to pay a lot though.
You really can calibrate these things in your garage.

Sean Wright
Technician, Environmental Health & Safety
Concordia University
1455 de Maisonneuve Ouest
GM-1100-57
848-4878
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{Original Message removed}

2002\10\23@090450 by Sean Wright

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Can't not can... eek, you can't calibrate them (reliably) in your
garage.

Sean Wright
Technician, EH&S
Concordia University
1455 de Maisonneuve Ouest
GM-1100-57
848-4878
srwrightspamspam_OUTvax2.concordia.ca


{Original Message removed}

2002\10\23@093203 by Reelf Monsees

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Well, you could build an anemometer yourself. The simplest way is
to use two transistors, both heated by current flowing through them.
One is shielded against the 'wind', the other isn't. So, one has a
constant temperature as a reference (this also compensates for changes
in the room temp.), the other changes with the room-temp. and airflow.
Search the web for schematics, there are several solutions with voltage,
current or frequency-output-signals.
If you want to calibrate them, well, depending on the accuracy that's needed, you can do that yourself. First, you could get an anemometer
that is calibrated (watersport-shops usually have them). Second,
mount it on your car and drive along at constant speed(s). Third,
if you know someone who's a private pilot or someone from a private
airfield, they all have anemometers somewhere....


regards,
  Reelf


Sean Wright schrieb:
{Quote hidden}

> {Original Message removed}

2002\10\23@095246 by James Paul

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Tony,

How about building a wheatstone bridge with two of the legs being
thermistors.   One of the thermistors will be exposed to the open air.
The other one will be exposed to ambient temperature, but would be
shielded from the ambient air movements.   This way, the room temperature
will not affect the measurement.  The way this would work is that at any
given temperature the thermistors are exposed to the same temperature, so
the bridge is balanced, and the wind velocity will be zero.  However,
since one of the thermistors is exposed to air movements, it will be
cooled by air moving across it, but the shielded one won't.   Therefore a
difference will ensue and unbalance the bridge.  This unbalance will be
dependent on the airflow.  ie more airflow, more cooling to the exposed
thermistor, and a greater difference in the balance of the bridge. This
unbalance can be calibrated to give a wind velocity proportionate to the
unbalance.   Hope this helps you out.  Or at least gives you food for
thought.

                                              Regards,

                                                Jim

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2002\10\23@111343 by PicDude

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This is pretty much how a mass-air sensor in a car works.
Within the air tube, there is a smaller tube that samples
a part of the air using the hot-wire method that Russell
describes.  See...
  http://www.allfordmustangs.com/Detailed/583.shtml

The tricky thing will be positioning the sensor to measure
the airflow changes at the locations you need.

Cheers,
-Neil.




Russell McMahon scribbled:
{Quote hidden}

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2002\10\23@112343 by Bourdon, Bruce

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Tony:

I remember an article a friend once showed me where Bob Pease (author of
Electronic Design magazines "Pease Porridge" column) made an anemometer by
breaking the glass envelope of a small bulb and monitoring the current
through it - which provides a means of indirectly measuring airspeed...

I did a quick search but failed to find the exact article, but these may be
of interest:
www.elecdesign.com/1998/may2598/ifd/2IFD.pdf
www.elecdesign.com/Articles/Index.cfm?ArticleID=15177&Extension=pdf
www.elecdesign.com/Articles/Index.cfm?ArticleID=15175&Extension=html
http://www.elecdesign.com/Articles/Index.cfm?ArticleID=17245&Extension=pdf

After reviewing the responses so far, it appears that most are thinking
along the same lines. I've tried it acoustically in the past (with
ultrasonics) but ran into issues such as cavity resonances due to nearby &
surrounding enclosures and turbulence/flutter causing difficulties in
resolution... my guess is that the majority trend of heat loss due to air
movement is the more practical approach.
Bruce.

{Original Message removed}

2002\10\23@112758 by Jim

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 "The tricky thing will be positioning the
  sensor to measure the airflow changes at
  the locations you need."

How about a half-dozen units all wirelessly
linked - one could develop a GOOD idea of
the drafts in a room that way! (Hmmm ... this
is possibly patentable ... hmmm ...)

RF Jim

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{Original Message removed}

2002\10\23@144122 by Wouter van Ooijen

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> Now the person wants to measure "wind" velocity as well.
>
> However, the catch is that the wind, if you can call it that, is
> actually the small variations in air movement in an average ventilated
> room such as in an office.


Two diode (BE?) junctions, same current, one exposed to air, one not,
measure difference in voltage due to temperature difference?

Wouter van Ooijen

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2002\10\23@152826 by PicDude

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Did you see the movie "Twister"?  (Huge bunch o' sensors
floating in the air).

For accuracy though, you would need the sensors to be as
small and aerodynamic as possible, so as not to disturb
the airflow in the room.



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2002\10\24@010555 by Robert Rolf

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WHy not use Schillern? optics? You can easily see airflow over
a large cross section and with strobed laser have a depth axis
as well.

This is another way and sounds like just what you want, if you can
scale it up.
http://www.opticsexpress.org/abstract.cfm?URI=OPEX-1-11-302

Abstract
We describe the design and operation of a high speed optical tomography system
for
measuring two-dimensional images of a dynamic phase object at a rate of 5 kHz.
Data
from a set of eight Hartmann wavefront sensors is back-projected to produce
phase
images showing the details of the inner structure of a heated air flow. The
tomographic
reconstructions have a spatial resolution of approximately 2.0 mm and can
measure
temperature variations across the flow with an accuracy of about 0.7 C. Series
of animated
reconstructions at different downstream locations illustrate the development of
flow
structure and the effect of acoustic flow forcing.

PicDude wrote:
{Quote hidden}

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2002\10\24@085232 by Marco Genovesi

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Tony,  if you have a stand-alone datalogger (battery powered) a big problem
is that most of these prjects have necessity of a lot of power
(also I imagine that they need the adding of some seconds of start-up time):
the more interesting of them, for me, is
http://www.elecdesign.com/1998/may2598/ifd/2IFD.pdf     (thanks Bruce..).

I also had these problems to measure airspeed in a Cave environment:
fortunately, in many caves airspeed is often > 1mt/sec.,  then a simple
mechanical anemometer (with a simple photocoupler) is enough.

Self-heating a thermistor is a good idea: another (strange?) solution may be
a modifyed layout of "AN512 - Implementing Ohmmeter Temperature Sensor.pdf",
with an heated thermistor (but with an A/D on chip a voltage output circuit
is prefereable, i think).

regards
Marco


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