Amongst the many features and specs touted by watchmakers, one of the most prominent is water resistance. And with summer coming up, there are ample opportunities to put those numbers to the test; even if you aren’t a certified diver, it’s still nice to wear a watch to the pool or beach without having to take it off before a swim. Surely a watch with, say, a 30-meter water resistance should handle these conditions swimmingly; after all, 30 meters below the water’s surface is pretty damn deep, and you’re not likely to be swimming down there.

The problem is that your 30-meter water-resistant watch might not actually be able to handle 30-meter depths. In fact, most watch manufacturers don’t want you to submerge that 30-meter watch at all. In the world of water-resistant watches, depth ratings can’t always be taken literally.

Today, standards for water-resistance ratings are stipulated by the International Organization for Standardization (ISO), which has two separate standards regarding the underwater performance of watches: ISO 6425 — specific to dive watches — and, as of 2010, ISO 22810 for everything else. ISO 6425 is well-known in the watch world, and includes specific parameters for how water-resistant a dive watch needs to be before it can be considered a “diver”(100 meters at the very least ), specific criteria for testing pressure resistance, a condensation test that follows pressure testing, immersion and temperature shock testing, and more. While most of these can be performed on just a sampling of the total production run, every watch meeting ISO 6425 needs to be pressure-tested.

The result is a watch that can really be realistically worn at incredible depths, and in some cases, divers have been shown to surpass their ratings. For example, in 2014, Seiko strapped two 1,0000-meter dive watches — one quartz, one mechanical — to an ROV. The quartz diver continued to function to 3,248 meters, while the mechanical lasted until 4,299 meters.

ISO 22810 is a bit different, and the differences between an honest-to-god dive watch and one merely billed as water-resistant are staggering. The standard doesn’t specify an exact depth rating for watches; rather it sets forth criteria for testing them for water-resistance. The idea is to clear up any ambiguity about what you can and can’t do with a watch in water and create a standard procedure for water-resistance testing. But a significant level of discretion in testing is still left up to the manufacturer.

According to Hodinkee, unlike ISO 6425, not every watch needs to receive pressure testing to meet the standard, but rather just a sampling of the production run. Further, a manufacturer is free to use an air chamber for pressure testing rather than using water immersion, and the minimum amount of time a watch is subjected to pressure testing is only 10 minutes (dive watches are tested for two hours). The tests for temperature shock resistance are also less stringent, and there are no tests and criteria for shock resistance or salt-water resistance.

The Doxa Sub 300 Black Lung has a depth rating of 300 meters. Read the review here.

This is all fine and good, and the result is that theoretically your 30-meter watch can withstand swims of up to 30 meters and your 50-meter watch can withstand swims of up to 50 meters, but these are assuming perfect lab-like conditions, short immersion periods, and a brand new watch with perfect gaskets and seals. The ocean is decidedly not like this ideal test environment, and gaskets and seals wear out and degrade over time. Further, the ISO standard stipulates that manufacturers are “responsible for stating whether a specific activity falls within the field of use of a particular watch. Similarly, he defines the warranty conditions and the precautions to be taken to maintain the quality of the watch over an extended period of time.”

As such, to cover their bases most watch manufacturers suggest that you don’t take your non-dive watch anywhere near the depths they’re ostensibly rated for. For example, charts from brands like Seiko, Oris and Longines indicate watches rated for 30 meters are only really splash-resistant, suitable for wear when washing hands or in the rain. Similar charts for 50-meter watches indicate that you can swim with them, though usually only in still waters. Once a watch is rated to 100 meters, most manufacturers say you can wear it skin diving and for water sports like surfing.

Does this mean you should freak out if your 30-meter watch falls into the sink or you forget to take your 50-meter watch off before jumping in the Atlantic? No, but the point is that you should exercise some level of caution with respect to where you wear your watch. Is it worth risking a watch that you paid thousands for when you could go out and buy a beach-ready beater dive watch for $200? Probably not.

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