Service Insight: How is Airflow and Differential Pressure Measured?

Clearly air is not conducive to being measured. Although we cannot see it, its motion is three dimensional and turbulent. It would need a number of measurements of velocity and direction to integrate a total volume flow unless the air was first collected and then directed in a suitable (straight) test airway.

British Clinical Services perform test procedures for determining a fan’s performance that produces repeatable results under laboratory conditions. To achieve a repeatable and consistent measurement of fan pressure, we attach standardised duct sections to the inlet or outlet of the fan.

However, in normal conditions, there are obstructions within a particular system which cause resistance to flow. Most component manufacturers produce resistance to flow data and we are then able to estimate an overall resistance to flow and size a fan accordingly. Although there will always be instances where the system does not operate to the designed specifications and it is not uncommon for a fan to function inefficiently.

To correct issues at the source it is important to test for:

  • Leakage, recirculation, or other faults within the system;
  • Inaccurate estimation of flow resistance;
  • Erroneous application of the ‘standardised’ fan data;

For accurate testing, it is required that the airflow be swirl-free; checks should also be made to ensure that the fan and any associated components are functioning properly; and that the fan operating speed is stable and operating at steady conditions (˚C).

Measuring Pressure

For the purposes of measuring the pressure rise across the fan, static pressure should be measured in close proximity to the fan inlet and outlet. Where this is not possible, allowances must be made for duct friction effects between the fan and the measuring plane.

Pressure measurements are usually made relative to atmospheric pressure and are corrected to ‘Standard Conditions’ such that comparisons between readings are possible.

A tube placed in a duct facing into the direction of the flow will measure the total pressure in the duct. If frictional losses are neglected, the mean total pressure at any cross-section throughout the duct system is constant.

Static pressure can only be determined accurately by measuring it in such a way that the velocity pressure has no influence on the measurement at all. To do this, testing must be carried out by measuring pressure through a small hole at the wall of the duct; or a series of holes positioned at right angles to the flow in a surface lying parallel to the lines of flow.


To enquire about our Airflow and Differential Pressure Testing services or to book a test for your business or organisation use our contact form here.

British Clinical Services assists a number of pharmaceutical, healthcare, and educational companies across the United Kingdom in all aspects of compliance. It provides tailored services and adds value to businesses through the British Clinical Services range of product and services which include Safety Cabinet Installation and Maintenance, Fumigation and HTM Compliance.