This started out as a segue in a reply that I was beginning to post on a post in "scams" regarding a DSSD and how deep it wad been tested to for waterproofing, and as I continued my thoughts I realized it would have more value as a separate thread, so here it is:
The thoughts that I was responding to had to do with "how deep do you need to test" to prove that a watch can go to "x" depth. Many people think that if you want to wear a watch to, say... 100 meters, you need to test each one to that depth. Nothing could be further than the truth.
So, here goes:
I worked in the undersea industries for a long while, and did final waterproofing testing on instrument and electronics packages that were being taken to Benthic depths, so take the following for what it's worth:
Waterproofing raises two concerns to the designers/users:
1: Physical strength, IE: "Will it be crushed physically by the pressure, or more to the point, will it be deformed by pressure to the point where the physical dimensions change sufficiently to allow the seals to be breached?" This is a matter of the physical strength of the housing, or for a watch the physical strength of the case, back, and crystal.
2: Sealing integrity: This is the concern of a nicked O-Ring, missing gasket, cat-hair across a gasket, etc etc. Basically this deals with leakage past a seal, even though the case structure itself hasn't been breached or it's dimensional changed by pressure.
In a watch, the concerns of (1): above are designed into the watch, IE: "We don't get to change the thickness of the back, or the material of the case, and only rarely can we do anything about the material of the crystal". Basically, we get what we get. The good news is that our common Rolex clones are so physically similar to the Gens that they essentially piggyback on the structural engineering of the originals to the point that "crush depth" concerns are almost nonexistent. The bottom line is that it's not terribly likely that you'll ever dive deep enough to literally crush the back into the movement, stopping the watch. I'm using Rolex as a model here, but the concept is identical to other watch cases. One thing to note is that crush-depth testing on a watch will nearly always result in a back deforming to the point where the movement stops, but that the cases do not leak water. Why? Imagine the cross section of the back sealing gasket area. The sealing surface is OUTSIDE of the threaded area, while the much larger actual back is INSIDE the threaded area. You can take a case without a movement, put it into a pressure chamber, pressurize it so much that the back bends so far that it touches the crystal (an acrylic crystal will also bend in until the crystal and back actually touch each other), yet NOT cause a leak to form. This has been proven again and again.
The takeaway from this ^^ is that "Full-Depth" testing (IE: To 200/1000/2000 meters) is something that tests the physical strength of the assembly. The engineering is either adequate or inadequate. You do not (likely) need to re-prove the strength margins of the case yourself. It doesn't change from example to example. And even with the differences in steel used between Gen and any reasonable quality Gen, there's no real difference in strength. Take this for what it's worth: Gold Subs have the same depth rating as a stainless one. The cheapest Chinese stainless is going to be FAR stronger than that Gen Rolex gold...
The seals of the watch are what we really need to test, both after receipt and after any servicing. Really, you're testing for the actual presence or absence of a gasket, and then you're testing for physical damage to gaskets and debris trapped across a sealing surface. Let's look at each in turn:
Leak point one:
1: Absence of a gasket: In MANY of the rep watches I've seen leak, the leak is from the fact that there's no seal between the case and the crown tube. I've seen this on a LOT of the vintage Rolex rep cases, other reps might vary. The "why" is irrelevant, and the fix is easy: Remove the tube and reinstall it with a bit of Loctite sealant. The depth of test needed to see this leak is about (hold your breath here)... five feet. Yup, the leak is so bad that you "barely" need to pressurize a watch tester to find it. This would be identical to what you would see with a missing back gasket.
Leak point two:
2: Damaged seals. The seals in a watch come in two types: Static and Dynamic. So let's look at these in detail:
(A): Static seals are "seal it once and then it never moves", which is the type of seal that you have at the crystal and at the back. Once it's closed, it's closed and there's no further motion between the parts you're trying to seal.
SOME static seals need to be lubricated with grease (silicone), but for two different reasons. On a STATIC seals we lubricate THREADED CLOSED seals (case backs) sparingly so that the friction of the rotation of the closure doesn't pull a seal by stretching it and damaging it. On seals that are not closed with a rotational effort (an underwater camera back seal for instance) modern practice is to use these seals without lubrication, as the silicone tends to collect sand and grit. The takeaway is that GREASE DOES NOT MAKE THE SEAL. Grease *prevents damage to the seal during the closure process*. Note that in every rep I've taken apart the back seals are put in dry. BUT also note that if the back seals, whether or not it was lubricated to start with is irrelevant.
(B): Dynamic seals: These are seals that are moved in normal use. On a watch they are the crown seal and on a chrono the pushers. These "are moved" by the user after the watch is closed. Since in this case we have routine (or semi-routine) movement of the two metal surfaces with a gasket between the two, we ALWAYS want the seals sparingly lubricated upon assembly. And I mean SPARINGLY. The grease doesn't vanish over time, it just gets spread out on the sealing surface. A seal that looks "wet" is all it takes.
Again, if a gasket is damaged, you do not need to pressurize a case terribly deep to find a leak. You're not proving anything by testing to 100 meters, if it's going to leak at 100 meters, it'll leak at 10. it just takes longer to see.
Leak point three:
(3): Debris in a gasket. This is the "cat hair across the gasket" leak. It's a VERY common reason for a flooded housing. Old dried lubricant not cleaned off after a reassembly, a hair across a seal, etc etc.. These also produce VERY obvious leaks that you do not need to test for at extreme depth.
All of this leads up to the "how do we test then? question. Before we go there, let's look at HOW we test a watch. Watches are usually tested by a two step process;
Step 1: Pressurize the watch IN THE DRY and let it "soak up" any air that might penetrate into the case by letting it sit under pressure for some time period.
Step 2: Depressurize it quickly and then immerse it in water, and look for bubbles escaping from it. Any gas that went IN past a bad seal will come back OUT past that same leak.
Watch testers do this by hanging the watch from a movable rod, over a cup of water, in a test chamber. You pressurize the chamber, wait, depressurize it, lower the watch into the water, and look at it.
So, "How deep do you need to test?" AKA "how in the world do I have confidence in a watch only tested to 5 atmospheres when I want to use it at (20, 100, 200) artmospheres (which is silly, but work with me here).
The answer is "not terribly deep".
1: You do NOT need to test to extreme depth to ensure a good seal at extreme depth. The physical strength of the original engineering doesn't need to be validated at home. Assuming that a rep basically meets the original design, it'll be plenty strong. On cheap reps this may not be the case, so.... yes there's some ambiguity, but even so... forget about it.
2: You DO need to test the gasket sealing of these things. The crystal to case seal, the back to case seal, the tube to case seal, and the actual crown'stem seal to the tube, BUT!! To test these you do NOT need top test to extreme depth. Depth changes the RATE of a leak, but not "if" it'll leak. The bottom line is that a 4 atmosphere test HELD FOR AN ADEQUATE LENGTH OF TIME is far more than adequate to check a watch for use in water depth far greater than 4 atmospheres. The key is to let the watch sit at pressure for an hour or two, to give time for that SLIGHT leak in a gasket to reveal itself, and then to watch the case for leaks for another hour or two upon depressurization.
The takeaway from this is that a PATIENT test to FAIRLY SHALLOW DEPTHS is a valid test for use in much deeper water. The error many people make is to not allow an adequate time period under dry pressure nor an adequate study period in the water looking for leaks after depressurization. If you hand your watch to someone for testing and they give it back to you in five minutes, you should be confident to go take a shower. Take that same pressure test but maintain it for an hour with a good result and be confident enough to go scuba diving.
Bottom line: Within reason, it's the LENGTH that's more important than the DEPTH.
And with that double entendre' I Ieave it for discussion.