OMEGA: “Measuring Air Flow in Ducts, Pipes, Hoods and Stacks”

This practical reference guide provides an excellent introduction to one of the most fundamental activities undertaken by LEV engineers and occupational hygienists: measuring airflow. Although produced for the North American market, the principles described are directly applicable to LEV testing and commissioning activities carried out throughout the UK.

The document explains the practical use of anemometers, Pitot tubes and duct traverses for determining airflow rates within ducts, hoods, stacks and ventilation systems. It also discusses common measurement techniques, calculation methods and the factors that can influence measurement accuracy. For anyone involved in LEV testing, commissioning or performance evaluation, it provides a useful overview of airflow measurement fundamentals.

View Original OMEGA Doc

OXYL8 Video – How to Use a Pitot

Why This Guidance Matters

Every LEV system ultimately exists to move air.

Whether assessing a captor hood, commissioning a new installation or carrying out a Thorough Examination and Test (TExT), the ability to accurately measure airflow is fundamental to determining whether a system is capable of providing adequate control.

The document provides a useful reminder that airflow measurement is not simply about placing an instrument into a duct and recording a single reading. Air velocity varies significantly across most duct cross-sections, meaning representative measurements require appropriate traverse techniques and averaging methods.

Understanding these principles helps ensure that airflow measurements are both accurate and repeatable.

Pitot Tubes and Duct Traverses

For many LEV engineers, Pitot tube traversing remains the preferred method of determining duct airflow.

A Pitot tube measures:

Total pressure
Static pressure
Velocity pressure

From these measurements, air velocity can be calculated and ultimately converted into volumetric airflow.

The guide explains why multiple readings are required across the duct cross-section and why averaging techniques are essential when determining representative airflow values. This remains one of the core skills taught on LEV testing and commissioning courses.

The Importance of Representative Measurements

One of the key messages within the document is that airflow is rarely uniform.

Velocity profiles can be influenced by:

Duct bends
Branch connections
Dampers
Fans
Changes in duct size
System obstructions

As a result, a single reading rarely represents actual airflow conditions. Proper traverse techniques are required to obtain meaningful results and reduce measurement uncertainty.

This principle applies equally to LEV systems, HVAC systems and industrial process ventilation.

Practical Relevance to LEV Testing

Although written for a wider ventilation audience, the guidance aligns closely with the practical skills required by LEV engineers.

The techniques described are routinely used during:

LEV commissioning
Thorough Examination and Test (TExT)
System troubleshooting
Performance verification
Fan performance assessments
Airflow balancing exercises

Many of the airflow calculations encountered during P600, P601, P602 and P604 training programmes are based upon the same underlying principles.

Key LEVCentral Takeaways

Accurate airflow measurement is fundamental to effective LEV testing.
Pitot tubes remain one of the most important instruments used by LEV engineers.
Multiple measurement points are required to obtain representative airflow values.
Air velocity profiles are rarely uniform across a duct.
Duct traverses help improve measurement accuracy.
Airflow calculations underpin commissioning and Thorough Examination and Test activities.
Understanding measurement uncertainty is an important part of competent LEV practice.
Good airflow measurements support better exposure control decisions.

Further Resources

Related LEVCentral Resources:

Recommended Learning

Thought Leadership

Many engineers become highly proficient at operating airflow instruments without fully understanding the principles behind the measurements. Yet the quality of any airflow assessment depends not on the instrument itself, but on how and where the measurements are taken.

This guide serves as a useful reminder that good airflow measurement remains a blend of science, engineering and practical experience. Whether using a Pitot tube, anemometer or flow hood, the objective remains the same: obtaining reliable data that allows informed decisions to be made about ventilation performance and worker protection.