Thanks to the sports watch boom, water resistance has become so common in watches that we often take it for granted. But a closer look at this popular feature reveals some uncommon ingenuity.

What makes a watch water-resistant?

My divers' watch came with a leather strap. Will the water harm it?

1. What makes a watch water-resistant?

There are several features that help make a watch water-resistant. The most important is the gaskets, or 0 rings-made of rubber, nylon or Teflon which form watertight seals at the joints where the crystal, case back and crown meet the watch case. if the watch is a chronograph, the chronograph pushers will also have gaskets.

In addition, water-resistant watch cases are lined with a sealant, applied in the form of a quick-hardening liquid, which helps keep water out.

The thickness and material of the case is also a big factor in determining whether a watch can safely be worn underwater. The case must be sturdy enough to withstand pressure without caving in says John Sokol, technical directors at TAG-Heuer USA in general, this means a steel or titanium case or a steel case plated with gold, manufacturers say. Solid gold cases can be water resistant provided they are sufficiently thick.

A screw-in case back, as opposed to one that pushes in, also contributes to a watch's water resistance. A screw-in crown, a feature of many divers' watches, helps prevent water getting into the case through the watch-stem hole. When it is screwed down it forms a water tight seal much like the seal between a jar and its lid.

2. Why aren't watches ever labeled or described in advertising as "waterproof" even if they can be worn deep-sea diving?

According to guidelines issued by the Federal Trade Commission, watch marketers are not allowed to label their watches "water-proof." Even watches designed for deep-sea diving cannot claim to be water-proof.

3. My watch is labeled "water-resistant to 50 meters" but the manufacturer's instructions say I can only wear it swimming, not snorkeling or diving. Why is that?

The different levels of water resistance as expressed in meters are only theoretical. They refer to the depth at which a watch will keep out water if both watch and the water are perfectly motionless, says Scott Chou, technical director at Seiko Corp. of America. These conditions, of course, are never met in the real swimmer's or diver's world. in real life, the movement of the wearer's arm through the water increases the pressure on the watch dramatically; so it can't be worn to the depths indicated by lab testing machines.

4. What are the various levels of water-resistance?

Watches with the lowest level of water resistance are labeled simply "water-resistant." They can withstand splashes of water but should not be submerged. Above that (or below it, literally speaking), the most common designations are

-50 meters (1 meter is about 3.3 feet), which means the watch is suitable for swimming;

-100 meters; indicating it can be worn snorkeling;

-200 meters, suitable for recreational scuba diving and, believe it or not,

-1,000 meters (roughly three-fifths of a mile).

Watches in this last category can endure deep-sea diving. Their gaskets are made of materials that can withstand the helium used in decompression chambers. Some have valves that let the wearer release the helium that has seeped into the watch so the case won't explode as the watch and diver adjust to normal atmospheric conditions, says Peter Purtschert, technical director at Breitling USA.

Keep in mind that the depth specified on the watch dial represents the results of tests done in the lab, not in the ocean (see question 3).

5. I've seen the abbreviation ATM used in labeling degrees of water resistance. What does it mean?

It stands for "atmosphere" and it's equal to 10 meters of water pressure. Another word for "atmosphere" is "bar," which is often used in Europe.

6. Is water resistance permanent?

No. Water resistance depends on several factors (see question 1), some of which can be affected by wear or simply by time. Gaskets can become corroded or misshapen, cases denied or crystals loose or broken. That's why your watch, like your car and your teeth, needs preventive maintenance.

7. How often does water resistance need to be checked?

At least once a year. Most manufacturers say water resistance needs to be tested every time the case back is opened, because opening the case can dislodge the gaskets. This rule applies even to a simple battery change. (Many service centers also change the gaskets whenever a watch comes in for service.) You should take it only to a service center authorized by the manufacturer. Doing otherwise could invalidate your warranty.

8. How is water resistance tested?

There are basically two ways of machine-testing water resistance, referred to as "dry" and "wet" testing. In the former, the watch is subjected to air pressure and measurements taken to see whether the case expands as a result of air leaking into the case. If it does, the watch is not water resistant.

In one type of "wet" testing, the watch is first subjected to air pressure, then submerged in water. If air bubbles come out of the watch when it is underwater, it means air seeped into the watch before it was submerged, and it is therefore not water-resistant. In another type, the watch is placed in a small water-filled chamber which is then subjected to pressure from a piston. If water gets into the watch, it's not water resistant (this is obviously the riskiest form of water-resistance testing).

9. Can I wear my water resistant watch in the hot tub?

No. Exposing your watch to heat, whether it's in a hot tub or sauna can cause the gaskets to lose their shape and their ability to keep out water.

10. What besides extreme temperatures will jeopardize my watch's water-resistance?

Some chemicals can corrode the gaskets and make it vulnerable. Heavily chlorinated water can cause problems, as can spray-on perfumes and hairsprays that work their way into the watch's seams and damage the gaskets. (They can also damage your watch's finish.)

11. My divers' watch came with a leather strap. Will the water harm it?

It may. Wearing a leather strap in the pool or diving is not generally recommended, says TAG Heuer's John Sokol. "It's like wearing your shoes to go swimming." Instead, choose a metal, plastic or nylon strap, he advises.

However, some manufacturers do offer leather bands which have been specially treated to resist water and are guaranteed safe for swimming. Check your warranty before you wear your leather strap in the water.

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