Air Flow Measurement

Measurement of airflow is critical in many industries including furnace control, fume extraction, heating and ventilation.
 
This article seeks to explain some methods of doing this.  More often than not, other parameters are measured and from this, actual flow rate is derived.

Volumetric flow (such as m³ per minute, standard cubic feet per hour (SCFH) or litres per second) is the most common requirement and this can be measured or derived from…. 

1. Velocity Measurement
2. Pressure Measurement
3. Anemometers
4. Mechanical ‘sail’ switches
5. Variable Area Flowmeters

Measuring Velocity 

If one knows the velocity of a gas, and one knows the cross-sectional diameter or rectangular dimensions of a pipe, it’s a relatively simple job to calculate flow volume.  There are many on-line calculators to convert velocity to volume.  In the example below we have selected a pipe diameter of 500 mm and entered a flow velocity of 20 metres per second. 
   
Illustrated below are some examples of velocity instruments suitable for ducts etc.
 

Dwyer VTT Air Velocity Transmitter	Dwyer 641 Transmitter

Measuring Pressure Pitot Tubes & Static Ports

A pitot tube (named for its inventor Henry Pitot) is a simple tube that points into the flow of air.  One might see these in various velocity measurement situations such as the example below, as a speed indicator on a RNZAF Skyhawk and a cargo plane.
 
To measure velocity with a pitot tube, we measure the static pressure and dynamic pressure.  This is done either with two separate tubes or with a compound device.
 
The dynamic tube points into the air flow while the static tube measures the background pressure in the duct.  The differential between these ports allows the velocity figure to be derived.
 
These tubes may be connected to a liquid filled manometer, a pressure transmitter or a differential pressure gauge (e.g. a Magnehelic which may have a dial calibrated in flow).
 

Pitot Tube connected to an Incline Manometer	Close up of a Magnehelic gauge calibrated for volume flow
Dwyer PAFS averaging flow sensor with differential output	A Dwyer FLST duct mounted airflow measurement station.

Anemometers

Center 330 Vane Anemometer	AQTI-AP1 Hotwire Anemometer	AQTI-VP2 Wireless Vane Anemometer

Anemometers measure velocity using spinning vanes or the cooling effect of moving air (hot wire anemometer).  Combination units (such as the Dwyer AQTI above) can use both types of velocity measurement along with the ability to measure relative humidity and pressure. (The one base instrument can accept any of the available sensors)
 

Phone App	Bluetooth Gateway	AP2 Hot Wire Anemometer Probe	RP2  Temp/Humidity Probe	VP2 Vane Anemometer Probe

There’s an app for that!  Android and IOS apps are available that communicate via a Bluetooth gateway with the AQTI wireless sensors. 

Air Flow Switch

‘Sail’ switches from Dwyer (left) and Honeywell (right) are used for flow or no-flow indication of medium and low speeds respectively.

Variable Area Flowmeters

Direct acting or magnetically coupled variable area flowmeters (Rotameters) are a very cost effective means of measuring gas flow in smaller pipes.  They may be ordered for specific gases (eg Nitrogen, Ozone etc).  Some models have flow control needle valves included.
 
For a more in depth look at how Variable Area flowmeters work, it is explained here in an earlier article.