Understanding the latest EN 14181 flow requirements

James Eldridge

By Engaged Expert

James Eldridge

More content from this author

Learn about our Engaged Experts

With almost 20 years of experience in the industrial emissions sector, James brings a high level of technical expertise to the team.

Following the revision to the Environment Agency’s MID for EN 16911-2 released at the beginning of 2024, there are a number of mandatory requirements that we need to adopt when performing QAL2 and AST testing for Volumetric Flow Rate and Velocity as per the stipulations of EN 14181.


What revisions have been made?

The first major change required is a highly recommended one-off ‘pre-investigation’ test for all newly installed CEMS. This is also recommended for existing CEMS. The pre-investigation test aims to determine whether the installed CEMS is suitable for the profile encountered at the sampling plane at varying flow rates, and the outcome of this test dictates which type of QAL2/AST is to be performed. The two types of QAL2/AST calibration are:

  1. Pre-Investigation outcome is a Pass: This allows for a Type 1 QAL2 / AST – this is a reduced test QAL2, whereby the minimum number of paired tests performed can reduce from a minimum of 15 to 9 (4 for an AST). There is no need for the operator to vary the flow profile over the course of the QAL2 or AST.
  2. Pre-Investigation outcome is a Fail or has not been completed: This requires a Type 2 QAL2 / AST, which involves a minimum of 15 paired measurements (5 for an AST). The Type 2 QAL2 is split into different test routes, following an assessment of the flow variability during normal operation over the last calendar year.


If the lowest 10th percentile flow is determined to be >80% of the maximum flow, then the flow profile does not need to be altered by the operator during the QAL2 tests.

If the lowest 10th percentile flow is determined to be <80% of the maximum flow, then the operator must provide their testing partner with low flow conditions so a single test can be achieved at these conditions, before returning the flow to normal, as part of the QAL2 tests.


How to perform revised EN 14181 flow assessments

In order for an assessment of the varying flow over the last calendar year to be performed, contact your testing partner about submitting your CEMS data. This data should include the following:

  1. Time and date
  2. Volume Flow Rate value (corrected conditions)
  3. A tag or column highlights if the corresponding measurement is considered valid data (valid would be for example the plant operating normally / or is invalid due to start up or shut down / plant off or in maintenance cycle).

Upon receipt of the data, your testing partner will remove all invalid data, determine how varied your flow is over the normal operation, and then advise on what steps to take during your EN 14181 flow assessment.



The decision process for EN 14181 

The flow chart below demonstrates the decision process regarding EN 14181 flow assessments.

EN 14181 flow assessments


Summary of revisions and next steps

The recent revision to the Environment Agency’s MID for EN 16911-2, released in early 2024, brings significant changes to QAL2 and AST testing for Volumetric Flow Rate and Velocity under EN 14181. Collaboration with testing partners to assess flow variability over the past year is essential for EN 14181 flow assessment. These revisions highlight the importance of thorough testing and data analysis for reliable emissions monitoring systems.

Should you have any queries with the above information please contact our team or James Eldridge directly for clarification.

Find related Resources

About the author

With nearly two decades immersed in the industrial emissions business, James Eldridge is a highly experienced expert contributing his wealth of technical knowledge to the Element Environmental team.

From his beginnings as a Technician mastering stack emissions testing to his tenure as Operations Manager, James has honed his skills to become a trusted leader. Now, he not only shares his expertise with colleagues but also shapes the future of emissions experts through tailored training programs and participation in industry-standard courses.

Situated in Stockport, James delivers a personable service to clients, embodying professionalism at every turn.

James Eldridge

Environmental Testing and Compliance Content

Discover our environmental thought leadership content including articles, white papers, case studies and FAQs.

Written by our industry experts, we explore a range of laboratory and field-based topics related to how we help clients comply with environmental legislation and maintain safe operations.


Related Services

BS EN 14181

EN 14181 Calibration & Testing 

Element experts offer EN 14181 calibration services including QAL1, QAL2, QAL3 and ASTs to verify the performance of CEMS and ensure regulatory compliance.

CEMS emissions monitoring

Temporary CEMS Emissions Monitoring

For industrial sites encountering an unexpected breakdown of their CEMS emissions measurement instruments, our team will quickly devise a CEMS recovery plan based on your monitoring requirements and processes.

Stack Emissions Testing

Stack Emissions Testing

Find out how our stack emissions testing services help our customers to prevent exposure, reduce emissions, improve air quality and achieve regulatory compliance for the industrial installations they operate.

Read More
Acoustic Camera Survey

Environmental Impact Assessments

Element’s industry experts provide environmental impact assessments for air quality, noise, and vibration, which must be closely considered through the planning process.


Environmental Compliance

Serving Indiana, Southern Michigan and Western Ohio, Element's environmental labs operate extensive route and field sampling services, enabling us to quickly and efficiently route your sample to the lab best suited to meet your needs.

Read more
Stack Emissions Testing

Siloxane Monitoring

Element offers siloxane sampling and analysis services to accurately evaluate the levels of siloxanes in filter systems.

Read More
Emissions Biogenic Carbon 14 Monitoring

Biogenic Carbon 14 Emissions Monitoring

Find out more about our OFGEM approved Biogenic Carbon 14 Measurement services, used to determine the renewable energy content of waste-derived fuels.

Ambient Air Quality Modelling and Dispersion

Ambient Air Quality

Find out how we help our customers comply with regulations for ambient air quality to protect human health and the environment from the harmful effects of air pollution.


Our team of over 9,000 Engaged Experts in North America, Europe, The Middle East, Australia, Asia and Africa are ready to help you.