Film-Tech Forum ARCHIVE: Anemometer

This is topic Anemometer in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Chris Hipp (Member # 1788) on 05-08-2004, 12:13 AM:

Anyone have any recommendations on a good one that isn't oo expensive, under $100 would be good. Thanks

Posted by Brad Allen (Member # 470) on 05-08-2004, 12:51 AM:

Chris, I assume you'll be using this for it's normal purpose? Weather?
If so, this is about as cheap as you can go and still have reliable equipment.
Weather station

Posted by Chris Hipp (Member # 1788) on 05-08-2004, 01:19 AM:

umm...I must be confused. I just need one to measure CFM for lamphouse exhaust and air ducts.

Posted by Josh Jones (Member # 447) on 05-08-2004, 02:27 AM:

Those I believe, are quite pricey. You could use the old stick trick. Ask Gord, he'll know....

Josh

Posted by Steve Guttag (Member # 268) on 05-08-2004, 03:05 AM:

I don't know of any in the $100 and below category. Extech does have a selection that are not too pricey but definately over $100. http://www.extech.com/instrument/products/400_450/407114.html

Steve

Posted by Carl Martin (Member # 1146) on 05-08-2004, 06:29 AM:

out of curiosity, what does cfm stand for, besides my name?

carl

Posted by John Pytlak (Member # 331) on 05-08-2004, 07:56 AM:

cfm = cubic feet per minute (a measure of the VOLUME of air being drawn per minute)

Posted by John Anastasio (Member # 607) on 05-08-2004, 07:57 AM:

Cubic feet per minute, and it's not a measure of air speed, which is what an anemometer will tell you. It's a measure of air volume over a given time interval. You also need to know the the diameter of the pipe to get the volume of air moved per minute. You could air moving through a BIG stack at the same speed as the air moving in a small stack, but you'll move more air through the larger one at that speed. There are some simple and cheap airspeed indicators used by ultralight pilots that consist of a pivoted indicator attached to a spring. You could probably build a cheap one from a small piece of sheet metal attached to a protractor and calibrated against a known air flow. Early pilots used a piece of cord tied to a strut. It told you not only speed, but direction of the relative wind.

Posted by John Walsh (Member # 168) on 05-08-2004, 09:23 AM:

Extech makes one for $90: http://www.extech.com/instrument/products/451_499/45118.html
But it doesn't read in cfm.

The better one like Steve showed, is about $250.

It the cheap ones do not measure in cfm, but you can use a converter to figure it out. You can get a cardboard 'slide rule' calculator or get the formula : http://www.nfsrps.com/airflow_formulas.html

Be aware though, that the cheaper meters like the one above can be off by 40-50% if you don't hold it exactly right, and in the right place, etc. With any anemometer, the proper way to use it is to take several readings around the duct and average them out. If I just couldn't afford a better meter, I would try to borrow a good one and use to compare with cheap ones to get initial values.

Posted by John Pytlak (Member # 331) on 05-08-2004, 09:33 AM:

Wouldn't the temperature of the hot air being pulled into the stack also be a good measure of how effective the ventilation was? The designers of the ventilation system need the CFM specification, but in reality, it's how hot the inside of the lamphouse gets that determines the required air flow.

Posted by Steve Kraus (Member # 476) on 05-08-2004, 09:58 AM:

Plus I don't know how accurate an air speed measurement coupled with stack diameter would be as it ignores laminar air flow.

Posted by John Anastasio (Member # 607) on 05-08-2004, 06:25 PM:

I was just making the point that only knowing the speed of the air doesn't give you any indication of the actual flow in CFM, any more than knowing only the voltage alone in a circuit would give you the current...but if you know the speed and you know the area of the duct's cross section, you can certainly calculate the flow. Laminar flow is of course what you want in a duct if you want it to move air smoothly and be quiet. Turbulence only creates unwanted resistance and noise. Why do I have this sudden urge to watch "Brazil" now?