Phoenix Journal · Ductwork
A kitchen extract system can look spotless and still carry enough grease to fail a fire risk assessment. Wet film thickness testing is the measurement that settles the argument in microns rather than opinions.
A kitchen extract system can look spotless to the eye and still be carrying enough grease to fail a fire risk assessment.
Grease is a quiet problem. It builds in thin films on the inside of ductwork, in the canopy plenum and across the fan, and for a long while it does not announce itself. A quick look up into an access panel tells you very little, because a wiped-looking surface and a genuinely clean one can be a hair apart to the eye and worlds apart in fire risk. That gap between looking clean and being clean is exactly where accidental kitchen fires start, and it is why the industry stopped trusting the eye and started trusting a number.
That number comes from wet film thickness. It is a simple measurement borrowed from the paint and coatings trade, and when it is applied to grease it turns a vague judgement - "that looks about right" - into a figure in microns that either sits under the threshold or does not. This is the measurement sitting behind every honest grease clean, and understanding it tells you a great deal about whether the certificate you are handed actually means anything.
The framework behind those figures is TR19® Grease, the BESA specification for fire risk management of commercial kitchen extraction. It sets out how grease should be measured, how clean is clean enough, and who is competent to sign it off. Central to it is the idea that grease depth is controlled to a mean average that should not exceed 200 microns between scheduled cleans. To put 200 microns in perspective, it is roughly half the thickness of a business card - not much grease at all, which is the point. Grease ignites and carries fire far more readily than most people expect, so the safe ceiling sits low.
Wet film thickness is how that ceiling is checked. A wet film thickness test, sometimes written WFTT, is used for the soft, tacky deposits - grease and cooking oil - that make up most of what accumulates in an extract system. Where the deposit has baked into a hard carbonised crust, a deposit thickness test, or DTT, is used instead. Both do the same job: they turn the state of a surface into a measured depth so it can be compared against the threshold rather than argued over. When a technician records readings comfortably below 200 microns across a system, and under 50 microns on the surfaces they have just cleaned, they have evidence rather than an impression.
The tool itself is disarmingly simple. A wet film comb is a small stainless gauge with a row of teeth. The two outer teeth sit level with each other, and the teeth between them step progressively away from that baseline, each one marked with a depth. You press the comb into the deposit at 90° to the surface until the outer teeth touch the metal beneath, hold it for a moment so the grease can wet the teeth, then withdraw it straight out. Some teeth come away coated, some come away clean. The reading is the gap between the last tooth that was wetted by the grease and the first one that stayed dry - that is the depth of the film at that spot.
One reading on its own proves little, because grease never lands evenly. It builds thickest where air slows and changes direction - behind the filters, at bends, around the fan - and stays thinner on the long straight runs. So a proper survey takes readings at multiple representative points through the system and works to a mean, while still flagging any single hot spot. A localised patch of 500 microns or more calls for immediate spot cleaning even if the average looks acceptable, because a heavy deposit in one place is a fire path in one place. This is also why the filters matter so much: they are the first line of defence, catching grease before it travels, and when they stop shedding it the load downstream climbs. If you have noticed that your filters will not come clean however hard you scrub, that is often the earliest visible sign that the wider system is carrying more than it should.
The measurement also keeps everyone honest about access. A reading can only be taken where a technician can physically reach the internal surface, which is why access panels are fitted at sensible intervals and why any length of duct that cannot be reached and measured has to be recorded as a limitation rather than quietly assumed clean. A number you cannot take is not a number you can claim.
The value of measurement is that it survives after the technician has packed up and gone. A compliant post-clean verification report under TR19® Grease is built around it: before-and-after photographs taken from matching positions, wet film or deposit thickness readings confirming grease sits safely below the threshold, a written summary of the works, notes on any areas that could not be accessed, drawings showing what was cleaned, and a hygiene certificate carrying the date, the location and the name of the technician. The specification also sets a competency bar, expecting the work to be carried out by someone holding the BESA Grease Hygiene Technician qualification, so the person taking the readings knows what a genuine result looks like.
That paperwork is not box-ticking. It is what your insurer looks for, what a fire risk assessment relies on, and what stands behind you if a claim is ever questioned. A verification report with real readings says the system was measured and found clean on a given date. A certificate with no readings says only that someone visited. The difference is precisely the gap between looking clean and being clean that the measurement exists to close - and it is worth remembering that a surface passing the eye can still be hiding trouble in more ways than one, since the same warm, greasy voids that build fire load also give pests a place to settle behind clean-looking equipment. Measurement is how you see past the surface in both cases.
So when you commission a grease clean, ask what the readings were, not just whether it was done. A figure in microns, tied to photographs and a named technician, is the whole point of the exercise. It is the thing that turns a clean from a promise into a record.
Questions
Wet film thickness is a measurement of how deep a soft film sits on a surface, given in microns. Borrowed from the coatings trade, it is used in kitchen extract cleaning to measure grease deposits so their depth can be checked against a safe threshold. It turns a vague judgement about how clean a duct looks into a hard figure that either passes or fails.
Under the TR19 Grease specification, grease depth should be controlled to a mean average that does not exceed 200 microns between cleans, which is roughly half the thickness of a business card. Any localised deposit of 500 microns or more warrants immediate spot cleaning. After a proper clean, remaining grease on the cleaned surfaces should measure under 50 microns.
A technician uses a wet film comb, a small toothed gauge, pressing it into the deposit at ninety degrees until it touches the metal, then withdrawing it. The depth is read from the gap between the last tooth coated in grease and the first tooth left dry. Readings are taken at several representative points through the system and averaged, with any hot spots flagged separately.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.