# Thread: FPM converted to CFM

1. ## FPM converted to CFM

Does anybody know the correct equation to convert FPM to CFM . I just bought my new anometer and was checking the fpm of some registers upstairs in my house but I don't know the equation to convert. I would appreciate a lesson. Thanks

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## attached file

the file attached should help

3. Thanks TLJ ! Good info.

4. You would need to use a hood or something to measure cfm out of a register like that with out a bunch of calculations.
GF recommended once to make up a small version with 12"x12" discharge then you would be reading calculated cfm directly on your fpm gauge

5. So I could just make a 12x12 out of sheet metal with some foam tape on one side for the seal, take the current register off , then take my readings.

fps X .5 = CFM

6. From the attached file tlj000 posted

Air Flow in CFM (Q) = Flow Velocity in Feet Per Minute (V) x Duct Cross Sectional Area (A)

Your area would be 1 not .5
and be reading fpm not fps and I think you got it

IOW
if you make up a hood like that
FMP=CFM

7. Thanks Ice.

8. Just curious, what type of meter did you get?

I've been looking into a hot wire anemometer. Just makes the pressure to fpm conversion much easier.

I still don't understand why, with today's tech tools, they can't make a capacity meter, that's reasonably priced.

A meter I would like to see, would read:

"average" fpm per 10sec or whatever traverse time
capable of input for duct size
dry bulb, wet bulb, calculated enthalpy
calculated capacity per total heat formula QT= 4.5 x cfm x Δh

For higher end models, barometric pressure compensation.
Why would this be so difficult to make???

BTW here's another good airflow read... pg 2-5

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They do make that meter.
http://www.trutechtools.com/Testo-43...ent_p_596.html

The user inputs the duct size in inches, the meter uses 2 wireless probes for the change in enthalpy. The calculation is a bit more complicated than the standard air formula in that is converts the airflow to mass flow because the vane measure independent of air density.

http://youtu.be/Pwtqt2obmto

While it is not inexpensive, it is the only meter on the market that can perform this calculation

10. I got the Testo 410 . Handy little thing. It is the size of a cell phone and comes with a cover and case.

11. Jim, could you explain the difference in formulae? Is this the "equation of continuity"? Wouldn't you need to calculate density, for an accurate reading with any method used? Or are you saying the vane does not need to calculate density due to it reading actual velocity, and not velocity pressure?

I see the vane anemometer used by traversing the return grille seems to be the method of choice.

What's your opinion on the hotwire probe in a duct? I know a duct traverse can be a bit more difficult than a grille traverse, due to the eddy currents and other turbulance, but which is more accurate? Could the hotwire be used with the same accuracy as a vane on a grille traverse? Correct me if I'm wrong, but doesn't the hotwire read "mass airflow"?

I'm asking all of these questions, because that nice kit is just not in my budget at this time, and changing from a pitot to a vane or hotwire is. I'm liking the hotwire cause it can be used for both duct and grille readings.
Last edited by mgenius33; 07-22-2011 at 09:47 PM.

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Originally Posted by mgenius33
Jim, could you explain the difference in formulae? Is this the "equation of continuity"? Wouldn't you need to calculate density, for an accurate reading with any method used? Or are you saying the vane does not need to calculate density due to it reading actual velocity, and not velocity pressure?

I see the vane anemometer used by traversing the return grille seems to be the method of choice.

What's your opinion on the hotwire probe in a duct? I know a duct traverse can be a bit more difficult than a grille traverse, due to the eddy currents and other turbulence, but which is more accurate? Could the hotwire be used with the same accuracy as a vane on a grille traverse? Correct me if I'm wrong, but doesn't the hotwire read "mass airflow"?
For residential airflow measurement, the vane anemometer is the ideal airflow measurement tool. Vane anemometers operate within the ideal measurement range, are non air density dependent, by design vanes are less influenced by turbulence than thermal probes (hot wires) or Pitot tubes. A fan is a constant volume machine; it will move the same CFM of air no matter what density of the air. If a fan will move a constant CFM, it will also measure a constant CFM. This is the primary reason behind the design of a vane anemometer. A vane anemometer is a miniature fan that is moved by airflow instead of moving air. This provides a cost effective solution for a quick and highly accurate airflow measurement in residential and light commercial systems. Vane anemometers have several advantages over any other method. The primary advantages are cost, speed, accuracy, and ease of use. Pitot tubes and hot wires both require calculation of air density for correction of the velocity measurement, and many hotwires are calibrated only for standard air and do not provide a means for compensation of the measurement. To measure velocity or volume of air, vane anemometers do not require air density compensation due to variations in air temperature, humidity, or atmospheric pressure – vane anemometers simply measure the air speed directly. Different size vanes are available for each step in the commissioning procedure. Remember, if you are using air measurements obtained with a vane to calculate cooling or heating capacity, air density will still have to be considered for accurate calculation involving the true mass flow rate across the heat exchanger or coil. A fan at a constant RPM is not a constant mass flow machine. Therefore, mass flow changes as the density changes. The mass flow is directly proportional to density change, while the volume flow (CFM) remains constant. As air density decreases, mass flow decreases and the effective cooling or heating will diminish proportionately. Therefore, equivalent mass flow is needed for equivalent cooling and heating calculations.

Using two wireless probes, and the return probe specifically to measure temperature and humidity; and a barometric pressure input by the user, the Testo 435 measures CFM and multiplies it times the measured airflow to get mass flow and then with a change in enthalpy the capacity is calculated.

A hot wire is calibrated for a specific density of air (some are adjustable) and its accuracy will be impacted by air outside of that it was calibrated or adjusted for. Also turbulent air impacts accuracy as multiple impacts from turbulent air molecules will influence the reading. While a hot wire is great for very low flows a vane is better for the typical ranges found in HVAC systems.

13. Originally Posted by jim bergmann
air density will still have to be considered for accurate calculation involving the true mass flow rate across the heat exchanger or coil. A fan at a constant RPM is not a constant mass flow machine. Therefore, mass flow changes as the density changes. The mass flow is directly proportional to density change, while the volume flow (CFM) remains constant. As air density decreases, mass flow decreases and the effective cooling or heating will diminish proportionately. Therefore, equivalent mass flow is needed for equivalent cooling and heating calculations.
Is it not correct to use the inverse of specific volume on the psychrometric chart to calculate Δ density for return and supply using wb° and db° of each?
Sorry for all the questions. I'm just used to the simple, less accurate methods.

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