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Extraction cleaning - measurement and evidence
How thick is the grease in your ductwork? It sounds like a vague question, but it has a precise answer - measured in microns, with a simple comb gauge, at points along the system. That measurement is what turns a grease clean from a matter of opinion into a matter of evidence: numbers that say when a clean is due and confirm when it is done. Here is how wet film thickness measurement works, and why it matters.
The short answer
Wet film thickness measurement is how grease build-up in a kitchen extraction system is measured objectively: a calibrated comb gauge is pressed into the grease deposit at points along the system, and the depth of grease is read off in microns (thousandths of a millimetre). The widely used thresholds are that a mean grease depth under 200 microns across the system is regarded as acceptable, a reading of 500 microns at any single point calls for a local clean, and after a clean the grease should be below 50 microns. So the measurement does two jobs: it tells you when a clean is due - when the grease has built up past the acceptable level - and it confirms when a clean is done - when the depth has been brought back below the post-clean figure. This is what turns a grease clean into something evidenced with numbers, rather than judged by eye - which matters for fire safety and for proving the system has been cleaned.
Measuring grease objectively
Grease build-up in a ductwork system can be judged by eye - a cleaner can see whether the ducts look heavily or lightly greased - but a judgement by eye is subjective and hard to record or compare. Wet film thickness measurement makes it objective. The method is straightforward: a calibrated comb gauge - a comb with teeth of graduated lengths - is pressed into the grease deposit on the duct surface, and the depth of the grease is read off from which teeth touch the surface beneath and which are held off by the grease. The result is a figure in microns, thousandths of a millimetre, giving a precise, recordable measure of how thick the grease deposit is at that point.
Taken at points along the system - at the canopy, through the duct runs, at the fan - these measurements give a picture of the grease build-up throughout, in numbers rather than impressions. A mean can be calculated across the readings, and individual high spots identified. This turns the state of the system into data: instead of a cleaner saying the ducts look dirty, there is a set of micron figures showing exactly how thick the grease is where. That objectivity is the point of the measurement - it makes the grease build-up something that can be measured, recorded, compared over time, and judged against defined thresholds, rather than a matter of one person's eye. And it is a simple, quick measurement to take, which is what makes it practical to use routinely.
The thresholds that matter
The measurement is useful because there are defined thresholds to judge the readings against - widely used figures that say what level of grease is acceptable and what calls for action. The commonly applied thresholds are these: a mean grease depth across the system of under 200 microns is regarded as an acceptable level, a system kept within it being considered reasonably maintained; a reading of 500 microns at any single point is taken as calling for a local clean at that spot, the build-up there having reached a level of concern; and after a clean, the grease should be brought below 50 microns, which is the standard for a properly completed clean. So the numbers have meaning: they map onto acceptable, action-needed, and cleaned.
These thresholds are what let the measurement drive decisions. If the mean is under 200 microns, the system is within the acceptable range; as it rises toward and past that, a clean becomes due. If any point reads 500 microns, that spot needs attention regardless of the mean, because a heavy local build-up is a local risk. And after cleaning, measuring below 50 microns confirms the clean has genuinely removed the grease, not just improved the appearance. So the thresholds turn the micron readings into a clear basis for action: measure, compare to the thresholds, and act accordingly - clean when the grease has built past the acceptable level, and confirm the clean by measuring it back below the post-clean figure. The numbers replace guesswork about whether the system needs cleaning or has been cleaned properly.
When a clean is due, and done
The wet film thickness measurement does two distinct jobs, at two points in the cleaning cycle. Before a clean, it tells you whether one is due: measuring the grease and comparing to the thresholds shows whether the system has built up past the acceptable level and needs cleaning, or is still within range. This takes the timing of cleaning off pure guesswork or a fixed calendar and ties it to the actual measured build-up - so a system loading fast is caught when the grease says so, and a system loading slowly is not cleaned before it needs to be. The measurement makes the cleaning interval responsive to the real grease level.
After a clean, the measurement does the second job: it confirms the clean is done properly. Measuring the grease depth after cleaning, and finding it below the post-clean figure of 50 microns, verifies that the clean has genuinely removed the grease throughout the system - not just where it was easy, or where it showed. A clean that looks good by eye but leaves grease above the threshold in the harder-to-reach parts is not a complete clean, and the measurement catches that. So the after-clean measurement is a quality check: it proves the clean has achieved the standard, giving confidence that the grease has actually been removed. Together, the two jobs - is a clean due, and is the clean done - make the measurement the objective backbone of the whole cleaning cycle, from deciding to clean to confirming it is complete.
Why the evidence matters
The reason all this measurement matters - beyond being tidy - is that it turns the grease clean into something evidenced, which is important for two connected reasons. The first is fire safety: grease build-up in the ductwork is a fire risk, and managing that risk means keeping the grease below a level where it is dangerous. The measurement is how you know the grease is being kept down - it gives an objective check that the build-up is within the acceptable range, and that cleans are genuinely reducing it, rather than assuming so. So the measurement is a fire-safety tool: it lets you manage the grease fire risk on the basis of measured fact rather than hope.
The second reason is proof. A duty holder responsible for a kitchen's fire safety needs to be able to show that the extraction system is being maintained and cleaned - to demonstrate, if asked, that the grease is being managed. The wet film thickness measurements, recorded before and after cleaning, are that evidence: they document the grease level, the fact that a clean was due, and the fact that it was completed to standard. A clean backed by measurements is a clean you can prove; a clean judged only by eye is one you can only assert. So the measurement provides the evidence that supports both the fire-safety management and the ability to demonstrate it - which is why professional grease cleaning is measured and recorded, not just done. The numbers are what make the clean count as evidenced, and the system as demonstrably managed.
The takeaway
How thick the grease is in your ductwork has a precise answer: measured in microns, with a calibrated comb gauge pressed into the deposit at points along the system. That measurement makes the grease build-up objective - data rather than impressions - and there are defined thresholds to judge it against: a mean under 200 microns is acceptable, 500 microns at a single point calls for a local clean, and below 50 microns confirms a completed clean. So the numbers have meaning, mapping onto acceptable, action-needed and cleaned.
The measurement does two jobs: before a clean it tells you when one is due, tying the timing to the actual measured build-up rather than guesswork; after a clean it confirms the clean is done properly, verifying the grease has genuinely been brought below the standard throughout. And it matters because it turns the clean into evidence - the basis for managing the grease fire risk on measured fact, and the proof a duty holder needs to demonstrate the system is being maintained. So wet film thickness is the measurement behind a grease clean: the numbers that say when to clean, confirm it is cleaned, and make the whole thing evidenced rather than assumed. A measured clean is one you can prove; a clean judged by eye is one you can only claim.
Questions
It is how grease build-up in a kitchen extraction system is measured objectively. A calibrated comb gauge - a comb with teeth of graduated lengths - is pressed into the grease deposit at points along the system, and the depth of grease is read off in microns, thousandths of a millimetre. It turns the grease build-up into recordable data rather than a judgement by eye.
The commonly used figures are: a mean grease depth under 200 microns across the system is regarded as acceptable; a reading of 500 microns at any single point calls for a local clean there; and after a clean, the grease should be below 50 microns. So the numbers map onto acceptable, action-needed and cleaned, giving a clear basis for decisions.
By measuring the grease and comparing it to the thresholds. If the mean is under 200 microns the system is within the acceptable range; as it rises past that, a clean becomes due, and any single point reading 500 microns needs attention regardless of the mean. This ties the cleaning timing to the actual measured build-up rather than pure guesswork or a fixed calendar.
By measuring the grease depth after cleaning and finding it below 50 microns, which verifies the clean has genuinely removed the grease throughout - not just where it was easy or visible. A clean that looks good by eye but leaves grease above the threshold in harder-to-reach parts is not complete, and the after-clean measurement catches that.
For fire safety and proof. Grease build-up in the ductwork is a fire risk, and the measurement is how you know the grease is being kept within a safe range and that cleans are genuinely reducing it. And the recorded measurements are the evidence a duty holder needs to demonstrate the system is being maintained - a measured clean is one you can prove.
It is far weaker than a measured one. A judgement by eye is subjective, hard to record or compare, and can miss grease left above the threshold in the parts that do not show. A measured clean gives objective figures before and after, against defined thresholds, so it can be verified and proven - which is why professional grease cleaning is measured and recorded, not just done.
Our extraction cleaning is measured and recorded - grease depth checked in microns before and after, so the clean is evidenced against the thresholds, not just judged by eye.
