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11 Sep 2018, 14:12

Tips and solutions for air velocity measurements in ducts using Testo 440

Within HVAC (Heating, Ventilation and Air Conditioning) systems the air flow is one of the major points to consider (along with temperature, humidity and carbon dioxide) to ensure the performance of the system.It’s fairly simple to calculate the volume flow, by multiplying the air velocity by the cross section area of the duct, this will give you air volume flowing past a point within the duct per unit of time (normally measures in cubic feet per minute – CFM), However it’s not so easy to accurately measure air velocity across the ducts as a number of factors need to be taken into account, such as probe type, measurements taken across the duct and avoiding measurement errors.

Testo have created an article on “Tips and tricks for air velocity measurements in ducts” which can be downloaded by the link at the bottom of this page, but key points to take into account are:

The optimum probe for you application:

It’s important to choose the correct probe for your application and three common types are:

  • Pilot Tubes: for high air flow from 20 m/s or measurement of heavily contaminated flows with a high particle content:

  • Vane probes (with minimal diameters): for mean air flow 5 m/s to 20 m/s:

  • Thermal probes: for low air flow of up to 5 m/s

Acceptance test measurement to DIN EN 12599:

Air flow can vary across the cross sectional area of the duct, so various measurements need to be taken across the cross section from which a mean air velocity can be derived and from which the volumetric air velocity is then calculated.

The number of measurement and position of sample is dependent on the size and shape of the duct:

For a rectangular duct, the cross-section can easily be divided into equally sized measurement areas, with the measurement position being in the centre of each, where there is an even velocity profile across the duct a small number of measuring points can be taken, but for large differences in flow across the cross-section then the number of measuring points need to be increased.

 With circular ducts the cross-section is divided into rings of equal area, with measurements taken across the centroidal axes – an example of the measurement can be found in the Testo document at the bottom of this page.

Identifying and avoiding measurement errors:

  • Measurement uncertainties dependant on probe type:
    • Pilot tubes – error decreases significantly as air velocity increases
    • Vanes – have an intrinsic error of ±(0.1 to 0.2 m/s) and a sensitivity error of 1 to 2% of measured value
    • Thermal probes – have an extremely small intrinsic error of ±(2 to 5 cm/s), to which a sensitivity error of 2.5 to 5% of measured value should be added.
  • Influence of interference points : Air flowing through a duct is impacted by interference points in the ducting (such as bends), so when taking measurements you should observer a clearance from the interference (further details in the Testo document)
  • Blockage of the flow cross-section by the measurement probe: The probe itself can impact the measurements, for example a probe with Ø 16 mm in a large duct will have limited impact, but in ducts with small cross-sectional areas, the probe head can cause excessively high velocities to be measured.
    • Incorrect evaluation of the measurement results when using Pitot tubes: Below 5 m/s, use of pilot tubes are limited and measurements using thermal probes or vane sensors are recommended (further details and calculations in the Testo document).


The ideal solution for all air velocity and indoor air quality measurements (IAQ) is the Testo 440 Series:

The Testo 440 can be customised to provide an invaluable and unique tool for all air velocity & IAQ measurements, as illustrated through the following videos:

Product Overview Video

Key Features of the Testo 440 Series:

  • Intuitive – Clear structured menus
  • Wireless – Bluetooth probes (ease of use without cables)
  • Space Saving – universal handle for all probes
  • Clear Display – simultaneous display up to 3 readings
  • Reliable – internal memory with USB data export


Probe System Video: Demonstrates the flexibility of the probe system, including telescopic extension:

Demonstration video on Volume Flow measurements within ducts: This short video shows how to configure the Testo 440 for duct shape and measurement type

Demonstration video on Volume Flow measurements at outlets:

Demonstration video on indoor air quality and comfort level

Testo 440 Kits Include:

Item description

Testo item code

RS Stock Number

Testo 440 Hot Wire Kit

0563 4400


Testo 440 16 mm Vane Kit

0563 4401


Testo 440 Lux Kit

0563 4402


Testo 440 100 mm Vane Kit with Bluetooth

0563 4403


Testo 440 Humidity Kit with Bluetooth

0563 4404


Testo 440 CO2 Kit with Bluetooth

0563 4405


Testo 440 Air Flow ComboKit 1 with Bluetooth

0563 4406


Testo 440 Air Flow ComboKit 2 with Bluetooth

0563 4407


Testo 440 Indoor Comfort ComboKit with Bluetooth

0563 4408


Testo 440 delta P Air Flow ComboKit 1 with Bluetooth

0563 4409


Testo 440 delta P Air Flow ComboKit 2 with Bluetooth

0563 4410



With a background in electronics and electrical engineering, with a keen eye on innovation and how things work.

11 Sep 2018, 14:12