Like cars, watches come with special features that make them more durable or easier to use – the horological equivalents of anti-rust sealant or power steering.
Water resistance is one such feature. There are different levels of water resistance, usually labeled on the watch dial. A watch labeled simply “water resistant” or “water resistant to 30 meters” can withstand splashes of water but should not be worn in the shower or submerged. A label of “water resistant to 50 meters” means the watch can withstand 50 meters of water pressure and can be worn swimming. One meter equals about 3.3 feet, so 50 meters is about 165 feet. A label of 100 meters (330 feet) means the watch can be worn snorkeling; 200 meters (660 feet), recreational scuba diving. A few watches have even higher levels of water resistance – 1,000 meters or more. These are designed for professional divers. Water resistance is also sometimes labeled in atmospheres, or ATM’s. An atmosphere is equal to 10 meters of water pressure, so 50 meters translates as 5 ATM’s, 100 meters as 10 ATM’s, etc. The word “bar” is synonymous with ATM.
It may seem odd that a water-resistant watch labeled “30 meters” can’t be worn swimming or even, for that matter, in the bathtub. Here’s why: The water-resistance level marked on the watch refers to the depth the watch would withstand in laboratory testing conditions, where watch and water are perfectly still. Any movement of the watch wearer’s arm greatly increases the pressure on the watch case and hence the likelihood of water seepage.
To many people in the watch business, water resistance is the defining characteristic of a sports watch, a huge category of watches that often share other features: rugged cases, casual styling, and, sometimes, special sports-related functions such as chronographs.
No watch should ever be called “water proof,” no matter how high its level of water resistance. That term suggests that no water can ever get into the watch, under any circumstances, a guarantee that no manufacturer or retailer can make.
Several features contribute to a watch’s water resistance. Gaskets, or O-rings, placed inside the case wherever there’s a seam or gap – such as where the case back is attached-are one feature. A screw-down crown, which can be screwed into the case, forming a watertight seal, is another. Many water resistant watches have screw-in backs. Many also have extra-thick cases, designed to stand up to water pressure. Finally, some have been treated on the inside with a special sealant that helps keep out water. (Divers’ watches, a distinct and very popular category of sports watch, combine most if not all of these features.)
Water-resistance can diminish or disappear over time as a watch’s gaskets become corroded. That’s why a water resistant watch must be tested periodically. Opening a watch’s case back can also compromise its water resistance – the gaskets could shift in the process – which is why some manufacturers recommend their watches be tested after every battery change, especially if the watch is worn diving.
Another special feature on many watches is a rotating bezel, which enables the wearer to measure elapsed time without having to do any mental arithmetic. At the start of the event to be timed, he turns the bezel so that the zero marker – often a triangular pointer-is aligned with the watch’s minute or seconds hand. When the event ends, he reads the elapsed time off the bezel.
Some rotating bezels are designed for purposes other than time measurement. A rotating slide-rule bezel lets the wearer perform arithmetic calculations. A rotating compass bezel allows the watch to be used as a solar compass.
Divers’ watches nearly always have unidirectional rotating bezels. As the name implies, this type of bezel rotates in only one direction: counterclockwise. That way, if the diver accidentally knocks the bezel while underwater, the watch will err on the safe side – overstating the duration of his dive, not understating it. (A bezel that rotates in either direction is called a bi-directional rotating bezel.)
There’s a category of watches equipped with another special feature: a second-time-zone indicator. These watches show both local time and the time in another time zone.
They come in many varieties and have many names. Some are called GMT watches. “GMT” stands for Greenwich Mean Time, the time at the Prime Meridian in Greenwich, England, the standard time for navigation and international radio broadcasts. GMT watches have an additional hour hand, set separately from the regular hour hand, which gives the hour in a second time zone using a 24-hour scale on the watch dial. Despite the watch’s name, which is a holdover from the days when pilots used Greenwich Mean Time for navigation, the second time zone can be any time zone in the world: it need not be Greenwich Mean Time.
Other so-called dual-time-zone watches show the second time zone via a digital display or an analog subdial. Still others have two separate dials, either adjacent to each other or, in rare instances, back to back. In the latter type, the wearer must turn the watch over to see the second time zone.
World time watches give the time in all of the world’s time zones simultaneously. Their bezels or dials bear the names of 24 cities throughout the world, one representing each time zone. These city-rings can be rotated against a 24-hour ring to show the time anywhere in the world.
Some watches, especially sports watches, have another special feature, luminosity, which enables them to be read in the dark. There are two types of luminosity – photoluminescence and radioluminescence. The former is often marketed under the trademarks LumiBrite, LumiNova and SuperLumiNova This type of luminescence requires that the watch be exposed to light before being worn in the dark. Its glow will disappear in a matter of hours.
Radio luminescence, produced by the radioisotope tritium, does not require recharging by a light source. Despite that fact, there has been a trend away from the use of tritium on watch dials. That’s because some consumers fear that tritium. a radioactive substance, is dangerous. (The fear is unfounded. A tritium watch emits a tiny amount of radiation – just 0.002% of what a person encounters in the environment.)
There’s another way of making a watch legible in the dark: electro-luminescence. Watches with this feature have dials that light up brightly when the wearer pushes a button. The energy to light the watch comes from the watch’s battery; therefore electro-luminescence is found only in quartz watches.
A power reserve indicator, which shows how many hours of running power remain in the watch’s mainspring, is another special feature on some mechanical watches. It’s usually a retrograde display (a display in which a hand moves through an arc, stops, then goes back to its original position).
Some motion-powered quartz watches also have displays that show how much power is stored in the watch’s capacitor, or energy-storage cell. Others have seconds hands that jump at two-second intervals when the power starts to get low.
A tourbillon (pronounced “TOUR bee ohn”) is a very rare feature found only on a few mechanical watches costing tens of thousands of dollars (or even more). Its purpose is to make a watch more accurate by eliminating timing errors caused by the effects of gravity on the balance when the watch is in different positions. It consists of a tiny, delicate cage containing the balance and escapement. The cage turns constantly at a fixed rate, usually once per minute. (This constant rotation is how the tourbillon got its name. The word is French for “whirlwind.”) The device is considered the greatest test of a watchmaker’s skill, which is why it commands such high prices.