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Extraction cleaning - reading your report
A grease test on your ductwork comes back as a set of numbers - micron figures at points along the system, maybe a mean and some high spots. If you are the person responsible for the kitchen, those numbers need to mean something to you: whether a clean is due, how urgent it is, what the fire risk is. Here is how to read a wet film thickness result and know what it is telling you to do.
The short answer
A wet film thickness result gives the grease depth in microns at points along your system, and reading it comes down to comparing those figures to the recognised thresholds. A mean under 200 microns means the system is within an acceptable range; as the figures rise toward and past 200, a clean is becoming due. A reading of 500 microns at any single point means that spot has a heavy local build-up that needs attention regardless of the average. And after a clean, figures below 50 microns confirm the clean has worked. So the numbers tell you three things: whether the grease has built up past the acceptable level (is a clean due), where the worst build-up is (which spots are of concern), and - after cleaning - whether the clean has genuinely brought the grease down. Reading your report against those thresholds turns the figures into clear decisions.
What the report gives you
A wet film thickness test produces a report of grease depths, measured in microns - thousandths of a millimetre - at points along your extraction system. Rather than a single figure, you typically get readings from several places: the canopy, points through the duct runs, near the fan, and any areas of particular interest. This is because grease does not build up evenly - it accumulates faster in some places than others, so a spread of readings gives a truer picture than one measurement. The report may also give a mean across the readings, summarising the overall level, and flag the highest individual readings, showing where the build-up is worst.
So the first thing to understand about your report is that it is a set of point measurements, not a single verdict - and reading it means looking at both the overall level and the individual high spots. The mean tells you how the system stands on average; the highest readings tell you where the problems are. Both matter, because a system can have an acceptable average but a dangerous local build-up at one point, or a generally raised level that needs a full clean. Understanding the report as a spread of readings, with an average and some peaks, is the starting point for reading what it means - you are interpreting a picture of the grease through the system, not a single number.
The thresholds to read against
The numbers mean something because there are recognised thresholds to read them against. The three that matter are these. A mean grease depth under 200 microns across the system is regarded as an acceptable level - a system within it is considered reasonably maintained, not needing immediate cleaning on grease grounds. A reading of 500 microns at any single point is taken as calling for a clean at that spot - a local build-up that heavy is a concern in its own right, whatever the average. And after a clean, the grease should be below 50 microns - that is the figure that confirms a clean has properly removed the grease. So the thresholds map the numbers onto meaning: under 200 mean is acceptable, 500 at a point needs cleaning, under 50 confirms cleaned.
Reading your report against these thresholds is what turns the figures into decisions. If your mean is well under 200, the system is in reasonable shape and you are managing the grease adequately. If the mean is approaching or over 200, a clean is becoming due - the grease has built up to the level where action is needed. If any single reading is at or over 500, that spot needs attention now, regardless of the mean, because a heavy local build-up is a local fire risk. And if you are reading a post-clean report, figures under 50 confirm the clean has done its job. So you read each number by asking which threshold it sits against - and that tells you what the number means for your decisions.
Average versus high spots
One thing worth understanding when reading the numbers is why both the average and the individual high spots matter, and why you cannot rely on the mean alone. The mean tells you the overall grease level, which is useful for judging whether the system generally needs cleaning. But the mean can hide a dangerous local build-up: a system could have a comfortable average while one section - a bend, a spot near the fan, a low point where grease collects - has built up to a heavy, high-reading level. That local build-up is a real fire risk at that point, even though the average looks fine. So reading only the mean would miss it.
That is why the 500-micron single-point threshold exists alongside the mean threshold: to catch heavy local build-ups that the average would mask. When you read your report, you should therefore look at the highest individual readings as well as the mean - a high peak reading is telling you about a specific problem spot that needs attention, regardless of how the system averages out. In practice, grease does tend to build up unevenly, so high spots are common and worth watching. Reading both the average and the peaks gives you the full picture: the mean for the overall state, the peaks for the local risks. Neither alone is enough - together they tell you what the system actually needs.
From numbers to fire risk
Behind the thresholds, what the numbers are really measuring is fire risk. Grease build-up in the ductwork is combustible, and the thicker the grease, the more fuel there is for a fire and the greater the risk. So the micron figures are, in effect, a measure of how much combustible grease is lining your ducts - and reading them against the thresholds is reading where your fire risk stands. A system under the acceptable level has its grease fire risk reasonably controlled; a system over it, or with heavy local build-ups, has a raised fire risk that the cleaning is there to bring back down. So when you read the numbers, you are reading the state of the fire risk in your ductwork.
This is why the numbers are worth taking seriously rather than filing away. A report showing raised figures is not just a prompt to book a clean for tidiness - it is telling you the combustible grease has built up to a level where the fire risk warrants action. And a post-clean report showing figures back under the threshold is confirming the fire risk has been brought down. So the practical meaning of the numbers, for you as the person responsible for the kitchen, is fire safety: they tell you whether the grease fire risk in your ducts is controlled or raised, and whether a clean is needed to control it. Reading them is a way of keeping an objective eye on a risk that is otherwise hidden inside the ducts.
The takeaway
A wet film thickness report is a set of grease-depth readings in microns at points along your system, with a mean and some high spots - and reading it means comparing those figures to the recognised thresholds. A mean under 200 microns is acceptable; a mean approaching or over 200 means a clean is becoming due; any single reading at or over 500 microns means that spot needs cleaning regardless of the average; and post-clean figures under 50 confirm the clean has worked. Read both the mean, for the overall state, and the individual high spots, for local build-ups the average would hide - both matter.
What the numbers ultimately mean is fire risk: they measure how much combustible grease lines your ducts, so reading them against the thresholds tells you whether that risk is controlled or raised, and whether a clean is needed. So do not just file the report - read it. The figures tell you whether a clean is due, how urgent, where the worst build-up is, and - after cleaning - whether the clean has done its job. That is what the numbers mean for you: an objective read on a hidden risk, turned into clear decisions. If your report is showing raised figures, it is telling you to act; if it shows figures under the thresholds, it is confirming the grease is under control.
Questions
They are grease depths in microns at points along your system, read against recognised thresholds: a mean under 200 microns is acceptable, 500 microns at any single point calls for a clean there, and below 50 microns after a clean confirms it has worked. So the figures tell you whether a clean is due, where the worst build-up is, and whether a completed clean has done its job.
Because grease does not build up evenly - it accumulates faster in some places than others, so a spread of readings from the canopy, duct runs and fan gives a truer picture than a single measurement. The report typically gives a mean across the readings plus the highest individual figures, so you can see both the overall level and where the build-up is worst.
Both. The mean tells you the overall grease level; the individual high spots tell you about local build-ups the average can hide. A system can have an acceptable mean but a dangerous heavy build-up at one point - which is why the 500-micron single-point threshold exists. Read both: the mean for the overall state, the peaks for the local risks.
It means that single point has a heavy local build-up that needs cleaning, regardless of the average. A build-up that thick at one spot is a local fire risk in its own right, so it warrants attention even if the system's mean looks acceptable. High spots like this are common because grease builds up unevenly, so they are worth watching on any report.
Everything, in effect - the grease being measured is combustible, so the micron figures measure how much fuel lines your ducts. Reading them against the thresholds tells you whether the grease fire risk is controlled or raised. Raised figures mean the risk warrants action; post-clean figures under the threshold confirm the risk has been brought down.
Read it against the thresholds and act accordingly. Mean well under 200 - the system is in reasonable shape. Mean approaching or over 200 - a clean is becoming due. Any single reading at or over 500 - that spot needs attention now. Post-clean figures under 50 - the clean is confirmed. The report turns a hidden risk into a clear decision, so use it to decide.
We test grease depth in microns and read it against the thresholds - so your report tells you clearly whether a clean is due, where the worst build-up is, and that a completed clean has worked.
