AIR SPEED FACTS
Air speed / air flow
About air velocity, air flow, supply and extract air, sensors for different types of measurements and practical tips on how to measure in ducts, over diffusers and grilles and make reference diffuser measurements.
Air velocity is the speed of the air over a fixed point, e.g., the ground. a sensor. Usually expressed in m/s (although the instruments are often referred to as air flow meters, as it is almost always the air flow that you want to measure).
Airflow is a defined amount of air (volume) passing a given point during a certain period of time and is usually expressed in l/s or m³/h.
Sensor for measuring air speed.
Hot wire sensors.
The principle of hot wire sensors is based on a heated element, from which heat is dissipated by a cooler airflow.
The temperature is kept constant via a control circuit. The control current is directly proportional to the air speed.
When hot wire sensors are used in turbulent flows, the heated element is affected from several directions, which can affect the measurement result.
This can largely be avoided by ensuring that there is a sufficiently long straight section of a channel before and after the measurement point.
Pitot tube (prandtl tube).
The front opening of the pitot tube is subjected to the total pressure (dynamic + static pressure) in a channel and leads it to point (a) which is connected to the plus nipple of a diff-pressure measuring instrument or sensor.
The static pressure is received through the side openings of the pitot tube and is conducted to point (b) which is connected to the negative nipple of the instrument. The resulting diff pressure is directly proportional to the air velocity in a duct and is converted according to the formula below.
v = Air speed in m/s
s = Pitot tube factor
r = Air density in kg/m3
p = Differential pressure in Pascal
ATTENTION! Air density is normally quite unaffected by humidity and absolute pressure. However, the temperature can have a significant impact if it is very different from the room temperature.
Flywheel sensors.
The measuring principle of vane sensors is based on the conversion of rotary motion into electrical signals.
The fluid causes the vane to rotate. An inductive limit switch “counts” the revolutions of the vane sensor and generates a pulse sequence which is converted in the measuring instrument to a speed value (usually m/s).
- Large diameter vane sensors (ø 60 mm and ø 100 mm) are suitable for measuring turbulent flows (e.g. at ventilation grilles) at lower or intermediate speeds.
- Small diameters are more suitable for measurements in channels, whose cross-sectional area must be 10 times larger than the diameter of the sensor.
- Sensor with 16 mm diameter. has proven to be very versatile. It is large enough to have good starting characteristics and small enough to withstand speeds of up to 50 m/s.
Important! The position of the sensors in the air stream.
For all sensor types – hot wire, vanes and pitot tube – it is very important that they are angled straight towards the airflow. Therefore, turn the sensor until the maximum value is displayed on the meter. Only then can the measured value be read. Especially important is the right angle to the air stream when measuring with pitot tubes.
Air flow measurement.
There are several methods to determine the volume of air (l/s or m³/h) passing through a ventilation duct or out or in from a diffuser or grille. In the case of duct measurement, it is most common to measure with hot-wire, pitot tube or vane wheel sensors in the absence of fixed measuring devices. For exhaust air diffusers, a connector with a sensor mounted in its narrow part is usually used. Measuring on supply air grilles requires a special supply air nozzle with large openings on each side, partly to cover the entire surface of the grille and partly to avoid creating an excessive pressure drop in the ventilation system.
Measurement in canal.
If possible, the measurement should take place on a straight section of the canal. The straight section of duct should be at least 5 times as long as the inside diameter of the duct before the measurement point and 2 times as long after the measurement point. The flow profile shall not be disturbed in any way by flaps, sloping sections, angles, etc. Since the airflow is different in different parts of the duct cross-section, measurements should be taken at several different points and averaged before converting to volume flow. Also note that the diameter of the sensor should not be greater than 1:30 of the duct diameter to minimize the impact of the sensor on the airflow.
Measurement over ducts and grids.
Most diffusers and grilles now have fixed measuring outlets where the air flow can be calculated by measuring the pressure. If these are not available or if it is difficult to find the specifications of the jack, measurements with different types of jacks are used.
Exhaust air diffuser: the air flow is measured across the diffuser with a combination of an exhaust air nozzle and an air velocity measuring instrument. The airflow is calculated in the instrument using a shock factor.
Air diffuser: requires a special nozzle with a certain length and large openings that minimize the influence of pressure drop and turbulence on the measurement. The measurement result is presented directly in l/s or m³/h.
Reference drone measurement.
When adjusting slightly larger ventilation systems, you need to have control of the device that is furthest out in the system, the so-called reference device. Previously, you had to have a person who monitored this device and reported its readings to the adjuster. Now there is modern technology that continuously monitors the reference sensor and relays the measured values to the adjuster via the mobile network and apps that deliver customized information.
Swedish standard SS-EN 16211:2015.
Air treatment – Field methods for the measurement of air flow rates
This document describes the standards and methods for measuring air flows. Also covers more details and methods than described above. It can be ordered from SIS, the Swedish Standards Institute or from Svensk Byggtjänst.
Example of an air flow meter from Nordtec.
Manual instrument
Testo has a wide range of portable handheld instruments for measuring air velocity and air flow. In addition, a wide range of sensors and accessories for almost every conceivable application.
Stosar
In addition to a number of smaller probes for exhaust air measurement, there is also the testo 420 which can be used for both supply and exhaust air measurement. Airflow rectifiers provide accurate measurement results.
Reference tone meter
CelsiVent monitors the reference sensor in real time with no distance limitations. Provides accurate readings directly to your phone. Shows the flow rate in l/s or m³/h and as a percentage of the design value.
