TR19 & standards
How grease is measured inside the duct - the gauge, the test points, and the microns that decide whether your system passes.
The short answer
The Wet Film Thickness Test measures soft grease inside the ductwork in microns. A notched gauge is pressed into the deposit and read where the coating stops; the reading is taken at set points from canopy to fan. A mean over 200 microns means the whole system needs cleaning, any single point at 500 or more needs an immediate spot-clean, and after cleaning the surface should read under about 50 microns.
How the test works
The Wet Film Thickness Test (WFTT) is used for grease and other soft deposits; its companion, the Deposit Thickness Test (DTT), handles harder, carbonised build-up. Both report in microns - thousandths of a millimetre. The wet-film method was pioneered within the specialist ductwork-cleaning industry and carried into TR19 Grease, which Swiftclean's Gary Nicholls co-authored.
The gauge itself is a flat comb with teeth cut to calibrated depths. A clean gauge is pressed straight into the deposit so the end teeth touch the metal, then lifted off. The reading sits between the deepest tooth that came away coated and the next tooth that stayed clean - so a gauge showing coating to the 200 tooth but not the 500 tooth reads between the two. The gauge is wiped after every reading so grease from one point never skews the next. It is a quick, on-the-spot estimate rather than a laboratory figure, which is exactly what a working duct survey needs.
Where the readings are taken
A single reading proves nothing on its own, because grease does not build evenly. TR19 Grease sets standard test points so surveys are comparable and nothing hides between them. Readings are taken - where access allows - at the canopy or extract plenum behind the filters, in the duct one metre from the canopy, again at three metres, midway between canopy and fan, just upstream of the fan, and in the discharge duct downstream of the fan.
Any point that cannot be reached is recorded as no-access rather than skipped quietly, because a gap in the readings is itself information - it flags where access panels are needed before the system can be fully verified. The pattern of readings across the six points shows how grease migrates along the run, which is often heaviest near the canopy and around the fan.
Turning microns into action
The readings drive three decisions. If the mean average across the system tops roughly 200 microns - about half the thickness of a business card - the whole system needs cleaning. If any single location hits 500 microns or more, that area needs an immediate spot-clean regardless of the average. After the work, a post-clean reading should show the surface under about 50 microns, roughly half a human hair, backed by a visual check.
Paired before-and-after readings at the same points are what a report uses to prove the clean actually worked, not just that a visit happened. And the rate at which grease returns between visits is what refines your cleaning interval over time - the test is both a pass-or-fail gate and the evidence behind your schedule.
Questions
It is a way of measuring soft grease deposits inside kitchen extract ductwork in microns. A notched comb gauge is pressed into the grease and read where the coating stops, giving an on-the-spot thickness at each test point.
The Wet Film Thickness Test is for soft grease; the Deposit Thickness Test is for harder, carbonised deposits. Both report in microns and both are used to verify a TR19 Grease clean.
The mean across the system should stay under about 200 microns, and no single point should reach 500 microns. After cleaning, surfaces should read under roughly 50 microns and pass a visual check.
At set points from canopy to discharge: behind the filters, one and three metres from the canopy, midway to the fan, upstream of the fan and downstream of it - wherever access allows.
Paired readings at the same points prove the clean worked rather than just happened. They also show how fast grease returns, which is used to set and refine your cleaning interval.
Phoenix Duct Clean · by the numbers
We measure at the standard points and give you the figures in writing - proof, not guesswork.