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Air flow rate data & instruments: this article defines air flow rate or cubic feet per minute (CFM) as the term is used to describe building air conditioners, heating systems, or building air movement rates. We describe the types of devices or instruments used to measure air flow, comparing the features, operation, and accuracy of each approach. We include examples of manufacturer's air flow rate or CFM data for HVAC equipment like air conditioners and furnaces. We also include a list of air flow rate measurement instrument or tool suppliers - where to buy CFM measurement equipment. Also see INCREASING RETURN AIR. Page top photo illustrates an example of a vane anemometer produced by Extech, the Extech ExTech SDL300 Anemometer and data logger - www.extech.com [permission requested 9/12/12]
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How do we measure air flow in CFM (cubic feet per minute) in an HVAC system such as air conditioners or warm air heaters?
How is CFM measured? - Anon.
Reply: Definition of Air Flow Rate & Flow Rate Measurement
Air flow rates for HVAC systems are expressed as a volume of air being delivered at some rate, typically cubic feet per minute (CFM) or m/sec (meters per second), ft/sec (feet per second), or ft/min (feet per minute).
A nice clear technical answer of how we measure flow rate is provided by Flow Kinetics:
Our HVAC air duct register photos above and below illustrate two common air flow measurement points in a duct system: at the return air inlet (unsafe in the above photo left) and at a supply air register (below left).
If I held up a one-foot square sensor in front of an air source (say an air supply register) and the sensor measured air velocity at 12 inches per minute, I'd be measuring 1 CFM of airflow. (One cubic foot = 12 x 12 x 12 inches).
Or if we measured an air velocity at an air supply register of one foot per minute and we knew that the duct work was a 12-inch square duct, we'd figure we were seeing one cubic foot per minute of air supply at that location.
Actually here are more than one answer to your question about how airflow is measured in an HVAC system because there is a range of air flow measurement instruments on the market.
The measuring devices vary in price, accuracy, and in operating principle, and there are also of course multiple sources of CFM data: manufacturers specifications, theoretical numbers, and actual measurements. We are most interested in the last category.
Vane or Fan Blade Anemometers, for Fan type air flow measurement: these are the most commonly used lower-cost CFM measurement devices used by home inspectors and HVAC technicians. At left is a wind speed anemometer - Wikipedia creative commons. At page top we illustrate the Extech ExTech SDL300 Anemometer and data logger available from www.extech.com. 
Some anemometers are comparatively small inexpensive (and less precise) air flow measurement devices that use a hand-held fan like instrument such as the Kanomax vane anemometers 6800 series or the ExTech SDL300 shown at page top) to measure air flow in CFM or equivalent rates on other scales.
A hand-held portable fan blade anemometer device is held in the air path and moving air rotates a fan blade. The instrument measures fan blade rotation to calculate a flow rate or pressure equivalent that is combined with the known cross sectional area of the measurement device. An advantage of measuring CFM with an anemometer is that you don't need to correct the measurement for temperature (variation in air density).
Swing Vane Anemometers: using a vane or ball that moves along a curved scale are used to measure low velocity air (25 to 400 feet per minute) for checking wind speed or for measuring the air flow rate in duct work, at air filters (is the air filter dirty and needing replacement?), and to meet safety ventilation requirements for OSHA and the US EPA for safety exhaust hoods, spray booths and similar applications.
Pitot tube probes: a Pitot tube (invented by Henri Pitot (1732)) is a device that measures air (or other flowing gas or liquid) pressure when the tube is inserted or placed in the proper position (pointed into the direction from which air flow emanates) for sensing airflow.
The pressure is converted to a flow rate by considering the cross-sectional area of the duct or opening through which air is being delivered. (There are some assumptions behind this including that air flow rate is uniform across the cross section of the opening.) By comparing the dynamic (moving air) pressure to static (non-moving air) pressure a pitot tube can give very accurate air flow velocity data.
Pitot tube image, Wikipedia creative commons. 
Quoting Flow Kinetics who offer instruments for air flow measurement as well as excellent technical publications on this topic illustrate a device used fro CFM measurement by measuring air pressure.
Of course in our case the "fluid" is air and we're interested in air movement through ductwork or out of a supply register into a building space. Pitot tubes are familiar to air travelers who have noticed that little tube sticking down and pointing forward from the bottom of many aircraft where the pitot tube is used to measure the air speed of the craft. Indeed pitot tubes are used for high velocity airflow measurements where a vane anemometer could not possibly be up to the task.
Pitot tubes are the most accurate technology for measuring air flow rates and are generally used to provide the accuracy standard for comparison with other CFM measurement devices.
Pressure transducers: also measure pressure from a flowing gas or air and permit conversion to CFM measurements in the same manner as a pitot tube - knowing the cross sectional area of the duct or opening. Pressure sensors measure the force exerted by a "fluid" including air or liquid by measuring the force that would be necessary to stop that movement. These devices are also called pressure transmitters, pressure senders, pressure indicators, piezometers, and in HVAC equipment and testing, manometers.
Actual measurements of airflow in an HVAC system or at air supply registers are expressed in cubic feet per minute and are most often made in the field using a hand held flow meter through which air moves. The flow meter is calibrated based on the its input area and the resistance offered by its own fan blades. As air, say coming out of an air supply duct, blows through the handheld device it causes the device fan or sensor to move, giving a measurement of calculated air flow in cubic feet per minute at that location and time.
An anemometer type device that uses a heated wire and measures the cooling effect of low velocity air flow can also be used to estimate air flow rates provided that air temperature is also considered to provide a correct estimate of air flow rate. T
The GrayWolf Advanced Sense HVAC differential pressure manometer works on this principle using a hot wire probe inserted into the HVAC duct.
Also see the Kanomax A031 hot wire anemometer (photo at left, kanomax-usa.com, described below. ) 
Note: all of the air measurement instrument manufacturers listed in this article produce a range of air flow rate monitoring instruments (and other test equipment) providing a variety of functions, accuracy, and of course, price.
Capture Hoods can be used to make accurate measurements of air flow rates at HVAC system air supply registers. Capture hoods cover the entire supply air register and use a differential pressure device or a hot wire device to obtain an air flow CFM number.
Liquid Column gauges - liquid column manometers are a special form of liquid-column manometer used to measure low velocity air flow by comparing air pressure inside and outside of two spaces. At left the U-shaped plastic tube filled with a blue liquid is connected at its left end to the interior of a 6" plastic vertical exhaust duct forming part of a radon mitigation system.
The right end of the liquid column gauge is simply open to the atmosphere of the room, in this case a basement. The differential in air pressure between the two ends of the tube is marked on a scale indicating the air flow rate inside of the column.
The difference in height between the two ends of the column of blue liquid is always in direct proportion to the difference between the two air pressures (inside & outside of the exhaust duct). If no air were flowing inside of the white exhaust duct, the two ends of the blue liquid would be at the same level.
In this application, air flowing past the end of the flexible plastic tube inserted into the column interior causes a reduction of air pressure in the tube that is a function of the speed of air flow past the tube opening. In this application the liquid column gauge reading of differential air pressure does not have to be precise as its function is simply to indicate that there is some difference in air pressure between the room interior and the exhaust duct interior.
As long as the room is at higher pressure than the column interior, the exhaust system is working and any radon gas below the floor slab (in this application) tends to exhaust through the duct rather than enter the room.
Just hold a tissue or piece of toilet paper near the inlet grille face.
If air is moving into the grille the tissue will be pulled against the opening.
This toilet paper or tissue test can confirm air flow as well as the direction of air movement at an HVAC air supply or return air register, and is a useful, if trivial, demonstration that can help confirm air movement when air flow in the system is weak or uncertain.
Obviously, this is a subjective, non-quantitative test for air movement at a building location.
Fans such as a blower assembly, are rated at a cubic feet per minute of air that the fan can move, presuming a particular rotating speed.
Watch out: the true CFM of a squirrel cage blower fan in a central warm air heating or cool air conditioning system can be 50% less than rated if the fan blades are dirty however.
CFM measurements on HVAC systems should be considered an approximation not precision measurements. There are a number of sources of uncertainty even in the measurement itself. For most HVAC air flow troubleshooting or air balancing applications, we are more interested in comparison measurements of air flow between different locations in the HVAC duct system than in high precision in statement of air flow itself.
OPINION: Therefore while pitot-tube type instruments and some electronic air flow measurement instruments can offer both precision and accuracy in HVAC or building air flow measurements, all of the instruments described in our article above can work suitably for heating and air conditioning design and maintenance.
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