Extech ExTech_SDL300_Anemomete air speed or air flow rate measurement device and data logger - Air flow rate (CFM) Measurement Methods
Tools, & air flow measurement data for buildings, air conditioners, warm air furnaces

  • AIR FLOW MEASUREMENT CFM - CONTENTS: how to measure air movement or flow rates in buildings; how to measure HVAC duct supply or return air flow rates in CFM or by other standards. What tools to use to measure air flow rate, accuracy, procedures, & where to buy. Definitions, Procedures & Tools for Measurements of Air Flow Rates (CFM) in Buildings
  • POST a QUESTION or READ FAQs about air flow rate or CFM measurement methods & typical rates in buildings and in HVAC systems
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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. Page top photo illustrates an example of a vane anemometer produced by Extech, the Extech ExTech SDL300 Anemometer and data logger - [permission requested 9/12/12]

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Definitions, Procedures & Tools for Measurements of Air Flow Rates (CFM) in Buildings

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipment

Article Contents

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:

Flow rate is measured by calculating an average velocity for the conduit of interest, and then, multiplying this velocity by the cross sectional area of the duct at the measurement location. The velocity value may estimated using a single reading, or a survey across the duct at a station. [12]

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).

Ceiling HVAC air supply register (C) Daniel Friedman

Here's a simplistic example of air velocity calculations

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.

Air Flow Rate CFM Measurement Devices & Approaches

Fan blade anemometer measuring wind speed - Wikipedia creative commons

How Vane / Fan Blade Anemometers are used for air flow rate measurements

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.

Above left is a wind speed anemometer - Wikipedia creative commons.

At page top we illustrate the Extech ExTech SDL300 Anemometer and data logger available from [19]

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). Another advantage is that some of these instruments are very low in costs.

Choose a unit that measures in the range of air speed in CFM specified by the standards against which you are measuring for your application.

Tip: always choose an instrument whose scale focuses most-closely on the measurement value range in which you're interested. That will generally give more-accurate readings.

For example, choosing an anemometer that can measure between 0.3 m/second = 5.15 cubic feet per minute - and 30 m/s = about 515 cfm - covers the necessary air speeds (20 to 100 cfm) for tests of bathroom and kitchen ventilation fans in North America.

If you were interested in the bath and kitchen vent fan standard and measurements for its compliance, using New York City's standard as an example the air flow speeds of interest are

Choosing an instrument that is designed to measure wind (air) speeds of 100 mph is likely to get one that is inaccurate over its scale for very low air speeds such as at a building vent fan.

Choosing an instrument that is designed to measure air speeds extending only up to 5 meters/second would, on the other hand, be unable to register higher wind speeds of interest to meteorologists and sailors.

Watch out: a more-accurate measurement of air flow in a duct system probably requires the user to insert an instrument actually inside the duct - such as a hot wire anemometer.

Swing Vane Anemometers used for air flow measurements

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 used for air flow measurements

Pitot Tube, Wikipedia creative commons 9/12/12

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. [25]

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.[12][13]

The (incompressible) velocity measured by a Pitot tube is calculated from the recorded differential pressure, Dp, and density, r, of the fluid. [12][13]

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.

How Pressure Transducers are used for air flow measurements

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.[14]

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.

Watch out however: measuring cfm at a supply register is not at all the whole story since air flow varies throughout the system as it is affected by internal resistances such as bends, crimps, surface smoothness, duct length, etc. And air flow through rectangular duct work is not identical to air flow CFM through a round duct of the same cross-sectional area.

"Hot wire" CFM measurements using a hot wire anemometer

Kanomax hot wire anemometer model K031 -

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.

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,, described below. ) [24]

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 for air flow measurements

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.

How Liquid Column gauges - liquid column manometers are used for air pressure or air flow measurements

Radon mitigation airflow measurement (C) Daniel Friedman

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.

Toilet Paper or Tissue Confirmation of Air Flow

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

A simple test for air movement at the return air inlet is illustrated in our sketch.

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.

Watch out: Obviously, this is a subjective, non-quantitative test for air movement at a building location. This test can tell you the direction of air flow but it is not accurate for much more than that.

Sketch above courtesy of Carson Dunlop Associates.

at AIR MOVEMENT in BUILDINGS we illustrate another toilet tissue test by taping a tissue to the bottom of an open window sash in a New York building.

Typical Manufacturer's Air Flow Rate CFM Specifications for HVAC equipment

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.

Accuracy of CFM or other air flow measurements on HVAC systems such as air conditioners or heaters

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.


Where to Buy Air Flow CFM Measurement Devices for HVAC Systems

Question: instruments for measuring of air flow rate in-line exhaust fans exist (at the end of a duct)

Could you please tell me is there any instrument for measuring of air flower rate in-line exhaust fans exist(at the end of a duct). We have not possibility and space for inside measuring at duct. We are thinking to find some way to measure from outside of duct. The diameter of duct( pipe) is 40-50 cm. Air temperature is 5-40 centigrade. Humidity =60-100 %

The duct is circular pipe with 40 Cm diameter. Do you think the suggested instruments could measuring accurately on turbulence air flow? Where we should put anemometer, before fan or after fan in Duct?

Please feel free to write me back for further questions.

I look forward to hearing from you. - D.K. by private email, 2016/03/21, The Netherlands


Dwyer MVA Anemometer measuring duct or vent air flow rates - Dwyer Instruments cited in this article

Above: the Dwyer MVA duct or vent airflow measurement probe system. Below the Dwyer DAFM airflow measuring probe.

Yes there are permanent, temporary, and hand-held probes that can measure air-flow inside of an air duct system.

I should have added you ought to be able to make an opening to insert a probe to measure airflow. That makes some older-style hand-held anemometers a bit large and inconvenient as you'd have to cut and perhaps seal around a larger opening, but other devices avoid that trouble as I'll cite below.

Dwyer instruments (and other HVAC duct airflow instrument manufacturers) make in-duct airflow measurement systems. You can install a permanently-mounted air flow measuring device in line in the duct system. Dwyer also has insertable probe devices.

Dwyer instruments DAFM air flow measurement probing device cited in this article

There are at least 3 options. Here are an excerpta from Dwyer Instruments.

  1. The Dwyer Instruments FLST series are permanently-mounted airflow sensors. These are mounted inline in a duct or vent system to permit continuous airflow rate monitoring & control.
  2. Dwyer Portable insertable hand-held anemometers from Dwyer include their MVA series.

    Handheld anemometers are an excellent, portable tool for performing tests on HVAC system performance; however, large rotating vanes can prevent easy access to ducts. Dwyer introduces the Model VT-200 Vane Thermo-Anemometer to eliminate this problem. Additionally, simple keypad programming enables the user to view volumetric flow rates in CFM or CMM.

    Data logging software is also available to easily record and view data on a PC or laptop.
    - Retrieved 2016/03/21, original s:
  3. Dwyer Portable insertable and probably more accureate (after the duct has been completed) series is DAFM The number of probes needed depends on duct shape and size but *round* duct need only 2 probes regardless of duct dimension.

    DAFM The Model DAFM Duct Air Flow Measuring Probe uses evenly distributed total and static pressure measuring points to deliver an accurate measurement of flows in a duct. The Air Flow Measuring Probe can be completely installed from outside of the duct making it very easy to install.

    With its lightweight and durable construction in addition to its ease of installation, this product lends itself to being used in the HVAC industry. These air flow measuring probes may be ordered for either round or rectangular ducts. In order to ensure accurate measurements you must determine the number of probes needed for your size duct. If the duct is rectangular, then consult the chart to determine appropriate quantity of probes.

    If the duct is round, it is only necessary to purchase two probes for any size of duct and mount them perpendicular to each other.
    - Retrieved 2016/03/21, original source:

As for probe location ... it depends on what we're studying.

If you want to focus first on what is being delivered to the exhaust fan destination by your 60 cm supply duct, insert the 2-probes or the hand-held anemometer I cited into the ductwork at a convenient accessible location downstream from the blower fan itself. By definition, in the supply duct means you are measuring "downstream' of the blower fan itself. Measuring in the supply plenum is more difficult (rectangular) and requires more probes.

Keep in mind that measuring near the exhausdt fan itself or ahead of the exhaust fan (less convenient in your design) is not going to tell us the effects of downstream bends, obstructions, air registers, or other variations in the system. In fact comparing a measurement near the air handler with those made at points of supply into rooms can be diagnostic. - if I correctly understand the problems you're solving.

These instruments are not (I think) designed to analyse turbulence per-se, but rather to measure the liters (or cubic meters) of air being delivered by a duct system. Measuring in the round duct a meter or more from the blower assembly / air handler itself should tell us what the system is delivering into the supply system.

Be advised: I am not an HVAC engineer. These resources may be of assistance:


Continue reading at AIR LEAKS in RETURN DUCTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR FLOW MEASUREMENT FAQs - questions & answers posted originally at this article


Or see AIR MOVEMENT in BUILDINGS for a discussion of factors affecting the direction and amount of air movement in buildings.

Suggested citation for this web page

AIR FLOW MEASUREMENT CFM at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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