You can’t judge a kitchen extract by looking into the canopy. Most of the grease sits where nobody sees it: behind the filters, along the duct, around the fan. Grease thickness testing is how a technician finds out what’s really there.
TR19® Grease uses measured readings, taken at points throughout the system, to decide when a clean is due and to show that a clean has worked. This guide explains the two test methods, the two figures BESA asks you to work to, where readings are taken and how to use them to set your cleaning interval.
Why grease is measured at all
Grease builds up when warm, fat-laden cooking vapour is drawn into the extract and cools on the inside of the duct. It sticks, thickens and, over time, can become a serious fuel load for a fire.
Measuring it does three jobs. It tells you whether the system needs cleaning now. It shows whether your cleaning interval is right. And it gives you hard evidence for your insurer, fire officer or landlord. A schedule backed by readings is much easier to defend than one based on the calendar alone.
Microns in plain terms
Grease deposits are measured in microns. One micron is a thousandth of a millimetre, so 200 microns is 0.2 mm and 500 microns is half a millimetre. That sounds thin, but spread across the inside of a duct it adds up to a lot of grease.
Because the numbers are small, readings are taken with gauges made for the job, not with a ruler or by eye.
The two figures that matter
BESA’s guidance on kitchen extract cleaning gives two figures:
| Reading | What it means |
|---|---|
| Mean of 200 microns or less | The level grease should stay within between cleans |
| Mean above 200 microns | A clean is due; the interval may need shortening |
| 500 microns or more at any point | Immediate spot cleaning needed at that point |
The 200 micron figure is a mean, an average across the readings taken. The 500 micron figure applies to any single point. So a system can have a reasonable average and still need urgent attention in one spot, often near a bend or the fan.
WFTT and DTT: the two test methods
BESA names two methods for measuring grease:
- Wet Film Thickness Test (WFTT). A comb-style gauge with teeth of different lengths is pressed into the grease. The teeth that come out coated show how deep the deposit is. It suits soft, wet grease.
- Deposit Thickness Test (DTT). The second method BESA names. It measures the thickness of the grease deposit itself, and a technician may use it where it suits the deposit better than a wet film gauge.
The technician chooses the method for the deposit they find and should record which one they used on the report. What matters to you is that readings are taken properly, recorded clearly and repeated in the same places each time.
Need it done? Duct Doctor cleans and tests kitchen extract and ventilation systems across London and the South East, with a certificate and photo report after every clean. Get a fixed quote.
Where readings are taken
A single reading at the canopy tells you very little. Readings should be spread through the system so they show what’s happening along its length. Typical points include:
- the canopy and plenum behind the filters;
- the first section of duct after the canopy;
- bends and horizontal runs, where grease settles;
- risers;
- at or near the extract fan;
- towards the discharge.
Each point should be identified on the report, ideally with a photo, so the next reading can be taken in the same place. Readings can only be taken where there’s access. If parts of the duct can’t be reached, that limits the picture, which is one reason to fit more access panels.
Readings before the clean
Pre-clean readings show how much grease built up since the last clean. They’re the best guide to whether the interval is right. If the mean is well over 200 microns, the system has been left too long. If it’s comfortably under, the interval is probably fine.
Pre-clean readings are often taken during a pre-clean survey visit, alongside photos and a check of the access panels, so the clean can be planned and quoted accurately.
Readings after the clean
Post-clean readings, taken at the same points, show that grease has been removed. Together with before and after photos, they’re the core of the post-clean report. They prove the clean reached the parts that matter, not just the canopy.
Check that your report shows readings both before and after, at named locations. If a report has no readings, or only one, ask why. Our post-clean verification page explains how independent checks work.
What readings can’t tell you
Readings are only as good as the places they’re taken. A section of duct with no access can’t be tested, so it’s a blind spot until panels are fitted. A reading also shows the grease on the day, not how fast it’s building. That’s why readings from several cleans, taken at the same points, are far more useful than one set on its own.
Readings don’t replace a visual inspection either. Photos of each part of the system show the type and spread of the deposit in a way a number can’t.
Using readings to set your interval
BESA’s minimum intervals of 3, 6 or 12 months are a starting point. Readings refine them. A simple approach:
- Start with the BESA interval for your hours of use.
- Take readings at each clean.
- If pre-clean readings are regularly above a 200 micron mean, shorten the interval.
- If any point reaches 500 microns, spot clean straight away and look at why.
Duct Doctor takes readings at every clean and records them on the report with the recommended next date. If you want readings between cleans, our grease thickness testing service can do that on its own.
Quick answers
What is a WFTT?
A Wet Film Thickness Test. A comb-style gauge is pressed into the grease and the coated teeth show the depth of the deposit in microns.
What grease level triggers a clean?
BESA’s guidance is that grease should not exceed a mean of 200 microns between cleans. A reading of 500 microns or more anywhere needs immediate spot cleaning.
What should readings be after a clean?
They should show grease has been removed at each test point. TR19 reports record before and after readings at named locations, with photos, so you can compare them.
Can grease testing be done without a full clean?
Yes. Grease thickness testing can be booked on its own to check whether a clean is due or to review your interval.
How many readings should be taken?
Enough to represent the whole system, from the canopy to the discharge, with extra readings at bends, risers and the fan where grease collects.
Related services
More advice
- TR19 Explained: Kitchen Extract Cleaning Standard
- How Often Should a Kitchen Extract Be Cleaned?
- TR19 Certificates: What They Should Show
- Pre-Clean Surveys: What to Expect
- All advice
This guide is general information. It isn’t legal or insurance advice: check your own policy wording and lease, and speak to a qualified adviser about your fire risk assessment.