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A stated depth turns a sealing claim into a number you can convert to pressure, and a stated duration turns it into a statement about leak rate. Together they move the claim from a category that sounds reassuring to a specification you can hold against the job in front of you: one metre is about 9.8 kPa of gauge pressure, thirty minutes is a short single exposure, and anything beyond either of those is outside what was demonstrated. Without both numbers you have a label. With both, you have something checkable.
A depth figure is a pressure figure in disguise
Water depth is the convenient way to express a load that the seal actually feels as pressure. The conversion is density times gravitational acceleration times height, so with fresh water at 1000 kg per cubic metre and gravity at 9.81 metres per second squared, every metre of depth adds roughly 9.81 kPa of gauge pressure to everything below it. That is the entire physical content of a depth claim.
| Stated depth | Gauge pressure at that depth | What it corresponds to in practice |
|---|---|---|
| 150 mm | About 1.5 kPa | The shallow end of a laboratory immersion, a puddle, a shallow tray |
| 500 mm | About 4.9 kPa | A sink, a bucket, standing water in a plant room |
| 1 metre | About 9.8 kPa | The default condition for the temporary immersion level |
| 1.5 metres | About 14.7 kPa | A common declared figure where a maker has gone beyond the default |
| 3 metres | About 29.4 kPa | A tank or a sump, well outside any default condition |
Seen this way, the differences between declared depths are modest. Going from one metre to three metres triples the load on the seal but only takes it from about 9.8 kPa to about 29.4 kPa, which is still far below what a hose nozzle delivers against a gasket edge. The useful work a depth figure does is not to impress but to let you decide whether your own situation sits inside or outside it, and to make two instruments comparable on the same axis.
Why the continuous immersion level means nothing on its own
The temporary immersion level comes with default conditions written into the standard, so a manufacturer who claims it has claimed something definite even if the bullet says nothing more. The continuous immersion level is deliberately open. The standard says the conditions shall be more severe than the temporary level and shall be agreed between the manufacturer and the user, which means the depth and the duration are supplied by the maker or they do not exist. A bare claim at that level with no numbers beside it is therefore weaker than a claim at the level below it, which is the opposite of how the ladder reads to a buyer.
This is also why the pairing matters more than either number alone. A declaration of two metres for one hour and a declaration of one metre for twenty four hours are both more informative than a bare level, and they describe different capabilities: one is about pressure, the other is about leak rate over time. A seal with a slow weep passes the short deep test and fails the long shallow one, while a seal with a marginal compression at a flange face does the reverse.
What a duration adds that a depth cannot
Ingress through a real enclosure is rarely a breach. It is a slow transport of liquid along a path that is almost closed: a hair of grit on a gasket face, a microscopic channel in a potting compound, a cable jacket that has shrunk slightly away from its gland. The quantity that arrives inside is approximately the leak rate multiplied by the time, and the leak rate is roughly set by the pressure. Depth therefore sets the rate, and duration sets the total.
That structure explains why the acceptance criterion in the standard is written in terms of harmful quantity rather than any quantity. A design can admit a few milligrams of water vapour over half an hour without any consequence, and the same path over a working day deposits enough liquid to bridge a track on a circuit board. Doubling the declared duration at a constant depth is a stronger statement about build quality than doubling the declared depth at a constant duration, because it probes the slow failure mode rather than the sudden one.
Temperature belongs in this picture too. The test is carried out with water close to the temperature of the sample precisely so that the trapped air inside does not contract and create an internal depression. In service that protection is absent, and a warm instrument entering cold water develops a pressure difference that works in the wrong direction for the whole time it takes to reach equilibrium. A long declared duration gives no information about that case either, which is why the sequence of operations matters as much as the figures.
A depth on a moisture meter is a sensing depth, not an immersion depth
The word depth appears on instrument listings in two unrelated roles, and mixing them up leads people to put instruments into water that were never intended to go near it. The Proster pinless moisture meter states that it detects up to three quarters of an inch below the surface, a maximum of 19 mm, with selectable ranges of 5 to 70 percent for wall and masonry and 5 to 75 percent for soft and hard wood. That 19 mm is how far the sensing field penetrates a solid material. It is a measurement range, and it tells you nothing about whether the case can be wetted. The same listing gives no ingress figure at all: Not stated in the listing bullets.
The same gap appears across instruments that routinely meet damp conditions. The Fluke 1AC II voltage detector publishes a detection range of 90 V to 1000 V alternating, a continuous self test, an audible and silent mode and a measurement category rating, and no ingress figure. The measurement category on an electrical instrument is a statement about surviving a transient from the installation, which is a completely separate hazard from water, and the presence of one rating does not supply the other. The Temtop M2000C air quality monitor states carbon dioxide, particulate at 2.5 and 10 micrometres, temperature and humidity, an audio alarm and export of readings as CSV over USB, and again publishes no ingress figure. It also has to draw ambient air across its sensors, so an unsealed case is a design requirement rather than an omission.
Matching a stated pair to the job in front of you
Start from your own worst case rather than from the label. Write down the deepest the instrument will go, the longest it will stay there, the liquid it will be in and the temperature difference at the moment it enters. Convert your depth to pressure so it is on the same axis as the declared figure. Then compare: if your duration is longer than the declared one, the slow leak path has never been probed for your case, and if your liquid is anything other than clean fresh water, neither figure applies. A declared pair that comfortably exceeds your own worst case is the only configuration that lets you stop thinking about it.
When no pair is published, the honest conclusion is that the question was never answered rather than that the answer is reassuring. In that situation the useful move is to keep the instrument out of the liquid entirely and change how you take the measurement: a non contact detector held above a wet surface, a probe on an extension that enters the liquid while the electronics stay dry, or a measurement of the consequence rather than the liquid itself. Every one of those is cheaper than discovering the limits of an undeclared seal with the instrument in your hand.
Frequently Asked Questions
Why does the seventh level have default conditions and the eighth level not?
Because the seventh level describes one ordinary accident, so the standard could pick a single representative case and fix it at one metre for thirty minutes. The eighth level describes continuous immersion, and there is no single representative case for that. A sensor in a drain and a sensor in a tank face very different pressures and durations, so the standard requires the manufacturer to declare the conditions and requires them to be more severe than the seventh level, rather than inventing a number that would suit nobody.
How much pressure does one metre of water actually apply?
About 9.8 kPa of gauge pressure, from density times gravity times height with fresh water taken at 1000 kg per cubic metre. Three metres is about 29.4 kPa. These are small pressures by industrial standards, which is why a depth figure on its own rarely sounds impressive and why the duration matters so much. Most seals that fail an immersion specification fail by weeping slowly, not by bursting.
Does a longer duration at the same depth prove a better seal?
It proves a lower leak rate, which is usually the thing you care about. Ingress through a marginal seal behaves more like a slow flow than a sudden breach, so the quantity admitted grows with time at roughly constant pressure. A design that is held under the same head for four hours instead of half an hour has demonstrated that its leak path, if it has one, is small enough not to matter across a much larger exposure.
Is a sensing depth on a moisture meter the same kind of number?
No, and the shared word causes real confusion. A pinless moisture meter states how far below the surface of a material its field reaches, which is a measurement range. An immersion depth states how far below the surface of a liquid the instrument may be taken, which is a survival condition. One describes what the instrument can see, the other describes what it can endure, and a listing can state either without implying anything about the other.
What if a listing gives a level but no depth or duration?
Treat it as the default case if it is the seventh level, because that level has published default conditions. Treat it as unverified if it is the eighth level, because that level has none. In both cases the conditions you actually face still have to be compared against the declared ones, and a figure that exists only in a marketing bullet rather than in a manual or declaration is worth less than one that appears in both.
Do the numbers tell me anything about repeated immersion?
Very little. Both levels are demonstrated as a single exposure on a sample that was new at the start of it. Repeated wetting and drying cycles a seal through swelling and shrinking, pumps air in and out through any breathing path, and gradually carries dissolved solids into the sealing face. A declared depth and duration is the best information available, and it is still a statement about one event rather than about a service life.