# Hot wire anemometer



## DuMass

Do you prefer a hotwire anemometer or vane type?
I currently have the hot wire type, but can see the advantage of the direct CFM readout of the vane type.


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## hvaclover

All the stuff they use to measure airflow with makes my head swim.

But the vane type with cfm reading, you would need to know the register or grill ka factor to calculate air flow, NO?


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## mechanicalDvr

Never used the hotwire type, always vane.


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## DuMass

hvaclover said:


> All the stuff they use to measure airflow with makes my head swim.
> 
> But the vane type with cfm reading, you would need to know the register or grill ka factor to calculate air flow, NO?


Since I don’t own one of the vane type units yet, I couldn't say for sure, but can only surmise that as with anything else, a single CFM outlet reading alone won’t give you the full picture without taking into account other data like blower speed and duct volume capacity, etc.. to correlate.


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## DuMass

mechanicalDvr said:


> Never used the hotwire type, always vane.


Do these meters allow for input of system variables, or simply provide a CFM reading at the register?


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## beenthere

If you use a vane type.

You can make a cardboard box to cover the register. Then cut a hole in it for your vane(same size). Then you can ccalculate CFM based on the FPM and free area of the hole for your vanemanometer.

Works good for CFMs under 200.

For large return grilles, need to take multiple readings and average them. And then useAK of grille.


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## nicktech

this is the way to use a vane anemometer or velometer;

this instrument measures velocity FPM. 
CFM= Area (sq ft) x velocity (fpm)

either remove the register or cross reference the manufacter's data. a register's free area is usually 65% the end boot size.
hartandcooley.com is a good reference site.

many readings are needed to get an accurate fpm, technically one needs to traverse the duct in different spots. 12 area needed, but practically i use 3. 
average out the readings to get FPM. convert sq in to sqft by dividing the sq in by 144. this is you area in sqft. 
multiple the area and velocity together to get the cfm. 
some anemometers have a keypad to enter the area to put out cfm, but initially all the readings are FPM. cfms cant be determined w/o knowing free area.


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## nicktech

beenthere said:


> If you use a vane type.
> 
> You can make a cardboard box to cover the register. Then cut a hole in it for your vane(same size). Then you can ccalculate CFM based on the FPM and free area of the hole for your vanemanometer.


resticting the airflow to fit the vane will increase velocity and give a false reading.


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