A pilot’s watch, I felt, ought to be a model of simplicity and legibility and accuracy, without any extraneous bells and whistles, and having a feature which aped a feature of a cockpit instrument panel seemed to betray the integrity of the pilot’s watch as a genre, on the most fundamental level. It took me a while to articulate to myself exactly why these date windows bothered me so much. Finally, I realized it was because they turned the pilot’s watch, into an illustration of a pilot’s watch rather than the real thing.

I nursed this impotent outrage for the better part of a decade, but over the years I have become more tolerant. Altimeter-style date windows undoubtedly say “aviation” to a lot of us and moreover, aviation in the pre-glass cockpit era, when analog dials ruled the day, not Garmin. (I would hasten to add that having been in the front seat of a small aircraft with a Garmin instrument display, I think glass cockpits are indisputably an improvement in legibility and safety.) They also give makers of pilot’s watches a wider range of design elements to work with. I mean, what was IWC supposed to do, just keep making Mark XIIs until they went out of business?

A couple of decades ago, Timezone’s Walt Odets called the Mark XII “every non-pilot’s favorite pilot’s watch” (that he’s a licensed pilot and former commercial aviator just gave the observation additional teeth). Technical watches nowadays – and this has been true for a very long time – no longer exist so much to serve a particular group of working professionals, as they do to give us a sense of connection to a particular group of working professionals.

Consider, therefore, the helium escape valve.

The reason behind them is simple. Helium escape valves are a solution to a problem encountered in the early days of saturation diving. Saturation divers work at such extreme depths that the amount of time necessary for decompression would reduce actual bottom working time to an unacceptable minimum. To get around this, divers enter a chamber on the deck of a support ship which is gradually brought to the pressure at working depths. To get to the work area, they descend in a pressurized diving bell, exit the bell and do their thing, and then re-enter the bell to go back to the living chamber. When they’re finished with their tour of duty, the pressure chamber is gradually de-pressurized to air pressure at sea level and they can exit safely.

It sounds horrendously dangerous but in general, divers seem to tolerate the whole procedure well. The enormous difference in pressure between the inside of the living chamber and the surface is so extreme that on the highly improbable chance that you are involved in a catastrophic decompression accident, you’re playing a harp before you can say Jacques Cousteau.

So what’s the deal with helium? The air we breathe is about 78 percent nitrogen and 21 percent oxygen (the rest is trace gasses). The problem with nitrogen is that it can induce a kind of delirium called nitrogen narcosis if you breathe it under too much pressure – this is one of the limiting factors in depths you can reach breathing a standard air mixture. Helium can be substituted for nitrogen and for the small price of making you sound like Donald Duck, you can dive hundreds of feet deeper, and stay down longer than you can breathing a nitrogen-oxygen mixture.

Helium atoms (helium is monoatomic, it won’t even bond with itself) are small enough to work their way into the interior of a watch case past the water-tight gaskets. The helium itself won’t damage anything, but during decompression, the gas, which can be under pressure several times that of air pressure at sea level, can’t get out again as fast as external pressure is going down and this can blow the crystal off a watch – as saturation diver and later, Rolex executive T. Walker Lloyd testified in ads for the Rolex Sea-Dweller when Rolex began to sell them to the general public (and as Sealab diver Bob Barth confirmed in an interview with Jason Heaton in 2012).

(After typing that helium atoms are much smaller than water molecules for many years and leaving it at that, I am pleased to announce that after several decades of being a watch enthusiast, I finally looked it up. The atomic radius of a helium atom is 0.49 angstroms and that of a water molecule is about 2.75 angstroms, I suppose depending on how you measure it as the molecule has a v-shape. An angstrom is 10 to the minus-tenth-power of a meter, or 1 / 10000000000 of a meter. One thinks of Robert Burns’ “To A Mouse” which begins, “Wee, sleekit, cowrin, tim’rous beastie.” Or perhaps I’m getting old faster than I think.)

There are two solutions to the exploding watch problem. The first is just to make the case so thoroughly sealed that helium can’t enter in appreciable amounts in the first place. Often this is done through the use of special gaskets – nitrile rubber, according to some sources I’ve seen, has a very low coefficient of permeability for helium – as well as, often, monocoque cases and it was the solution used by both Seiko and Omega (in the Ploprof) as well as Citizen, whose current-production Professional Diver 300M is engraved, “For Saturation Diving” on the caseback.

The other solution, which involves a less massive case and probably gives you a watch easier to sell to ordinary consumers, is a one-way gas release valve that vents overpressure gas inside the watch case to the outside atmosphere during decompression.

A lot of modern watch enthusiasts don’t care for the helium escape valve. The reason is straightforward: no one who buys a modern dive watch with an HEV needs one. This is, you might say, a completely watertight argument. The chances of anyone who buys a dive watch actually needing an HEV is probably lower on any given day than the odds of being struck by lightning, hit by a meteorite, and winning the Mega-Millions lottery all at the same time, and so having another opening in the case, and sometimes even an unnecessary external projection, seems pointless at best.

However, the fact is that no one who buys a modern dive watch really needs one at all. A 200m water-resistant watch which is compliant with the dive watch standard, ISO 6425, is overdoing it in the same way that a helium escape valve is overdoing it. You’re not diving to 200m (at least most of you aren’t) and I’m not diving to 200m. That, however, is beside the point. The fact is that it’s nice to know you could dive with your dive watch even if you never do, and this ultimately has nothing to do with practicality, it has to do with feeling a part of a decades-long tradition of human exploration of the deep blue sea.

The helium escape valve may only be genuinely useful for saturation diving. But it is genuinely fun to know your watch could be used for saturation diving. Wearing an honest-to-goodness full-spec saturation diver’s watch is a connection to a different world, where a miscalculation can be instantly fatal but the reward is experiencing a world few humans ever see. It is, in a word, a symbol of authenticity and integrity, and considering how short the supply of both is in so many of our lives these days, I for one remain a fan. HEV, I salute you. Long may you fart.

For a deeper dive on saturation diving, check out Technical Perspectives: What “Saturation Diving'”Really Means. For a lifelong diver’s take on the HEV, see Jason Heaton’s Too Much Hot Air About The Helium Escape Valve. For a very close look at a highly focused collection of watches made for saturation diving, watch Danny Milton’s Talking Watches With Grahame Fowler, Supreme Collector Of COMEX Rolex Divers. And don’t sleep on Louis Westphalen’s Reference Points: Understanding The Rolex Sea Dweller.