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Watch Lexicon

Always good to know

Our watch lexicon is designed to provide you with everything worth knowing about our watches. Do you want to find out more about the functionality of a wristwatch? Our watch lexicon guides you safely through the jungle of specialist timepiece terminology.

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An essential complication for aviators an altimeter measures altitude, or height above sea level. Recording ascent and descent, an altimeter can also be an important piece of equipment for climbers, walkers, mountaineers and other professionals where altitude is a needed measurement.


IndicatorA feature that indicates whether the indicated time is AM or PM. This feature can be found mostly (although not limited to) in watches with a GMT/Dual time display or a World Time Display to help know whether it is day or night in the other time zones.

Analog/Digital (Duo) Display

A dial or face of a watch that has the ability to display the time using rotating hands or other markers on a dial, (an analog display) and electronically by digital units (a digital display) on the dial simultaneously. This is also known as duo display or an AnaDigi watch.


The anchor (also known as balance spring) is the clock or the regulator in the watch. It briefly holds the clockwork and then releases it again, and this is heard as ‘tick-tock’. The anchor (balance spring), together with the balance wheel and lever forms the so-called escapement.

Annual Calendar

A complication showing the date, day and month. An extremely complicated and sought after movement. This watch will correctly adjust for short and long months; however, it will not correctly account for Leap Year, the 28 days of February once every four years.


The opening or window found on watch dials in which certain indications are given such as the date window on a Rolex Datejust.

Automatic winding

In a watch with automatic winding, the centrifugal and gravitational forces are used as an energy source. A semicircular weight called the rotor or balance weight winds the spring through the arm movements of the person wearing the watch. As an automatic watch is continuously being wound, the tension spring has a sliding clutch instead of an end hook. When the watch has been wound completely, the spring descends and overstraining is circumvented.


Balance Wheel

Analogous to the pendulum in grandfather clocks it is a weighted wheel in a mechanical movement that rotates back and forth separating time into beats. It is a key mechanism producing accuracy of time in a watch.


A ring on the top side of the case surrounding the crystal. It may be decorative or functional. The Bezel can be fixed, move only one way, Uni-Directional, or move both ways, Bi-Directional. Many functional Bezels are useful to calibrate time from any given point by rotating the Bezels 12 position to any given starting point of reference along the dial.

Bi-Directional Rotating Bezel

A bezel that can be moved both clockwise and counter-clockwise to perform mathematical functions using the dial of the timepiece as reference. A bi-directional bezel is similar to a slide rule and is extremely useful for aviators and aviator timepieces.

Blewed screws and hands

In the 16th Century the hands of a watch were blued for the first time, in part to improve the resistance of the hands, and in part for aesthetic reasons. A watch with blued hands was considered a sign of the utmost craftsmanship as the manual creation of these cornflower hued hands and screws required a lot of experience and skills.

The ‘bluing’ process is a thermal process whereby the hands and screws are evenly heated and made to glow incandescent before quenched by cold. With the uniform heating the blue color is created. If the material is either heated for too long or not long enough then the material is rendered useless.

Breguet spring

The Breguet balance spring was invented by Abraham-Louis Breguet. With this particular spring the last rotating hand is bent upwards. Until his invention they were placed flat next to each other. The upwardly curved hand enables the watch to breathe more evenly. A watchmaker would say: “the spiral breathes evenly”. Currently, most watches feature the Breguet spring as it guarantees a better accuracy of the watch, and it is also easier to adjust the watch. 


Caliber or Calibre

Since the early 18th Century, the calibre of a movement has denoted the position and size of its different components, notably the wheel train and the barrel. Today In watchmaking, the term refers to the specific layout and shape of a movement and the bridges, and its various components as well as the designer of the movement.


This refers to a Domed/Arched Crystal.


The container housing the movement of the watch and protecting it against dust, moisture, jarring and other hazards. Usually consisting of the case, the bezel, and the case back.

Chronograph or chrono

This watch tells the time and is also equipped with a timer mechanism or stopwatch. A chrono can be easily recognized by the extra push buttons, which release, stop and reset the chronograph hand. This hand turns once a minute, like a second hand on an auxiliary watch face. Further small hands count the minutes, half hours, hours and even tenth of seconds respectively.


A watch known as a chronometer is a highly accurate watch, and this watch must not run slower than 2 seconds or faster than 8 seconds per day. The Chronometer Certificate, also known as the Movement Certificate, is issued by the independent Chronometer Institute (COSC: Contrôle Officiel Suisse des Chronomètres). Each individual timepiece spends two weeks at the Institute and is tested for accuracy. Each certificate is unique and is individually numbered.

The German Chronometer Institute is based in Glashütte, near Dresden.


One or more features added to a watch in addition to its usual time-telling functions, which normally not only include the hours, minutes and seconds but also date and often the day of the week as well. Complications such as; perpetual calendars, moonphase displays, alarms, repeating mechanisms, quarter strikes as well as stop/start chronograph functions. Power reserve indicators are also usually regarded as ‘complications’.

This is the Swiss Official Chronometer Testing Institute that will certify the official chronometer status of a watch.


The cover over the watch dial is called the crystal. There are three types of crystals commonly used in watches: acrylic crystal is an inexpensive plastic that allows shallow scratches to be buffed out. Mineral crystal is composed of several elements that are heat-treated to create an unusual hardness that aids in resisting scratches. Sapphire crystal is the most expensive and durable, approximately three times harder than mineral crystals and 20 times harder than acrylic crystals. A non-reflective coating on some sport styles prevents glare.



A watch that shows both the day of the week and the date of the month.

Deployment (Foldover) Buckle

A three-folding enclosure that secures the two ends of the bracelet and allows enough room for placing the watch on the wrist when fully deployed. When closed, the buckle covers the two-piece folding mechanism.

Diving Watch
A watch that is at least 200M water resistant. A diving watch has a one way rotating bezel and a screw-on crown and back. Some watches have a helium escape button to release the pressure after the diving.

Also see ISO 6425 Certificate



End of Battery Life Indicator (EOL)
The EOL indicates when it is time to replace the existing battery. Different manufacturers use different methods to indicate a low battery, i.e. if a second hand usually sweeps, when the battery is low it will begin to tick.


The device at the heart of virtually all time-keeping mechanisms. The mechanism that “releases” the energy that maintains the oscillations of the balance wheel which governs the rate at which the escapement lets the wheels and hands of the watch revolve.

Equation Of Time or EOT

An Equation Of Time (aka EOT) complication indicates the difference between “true” solar time (that of nature) and “mean” solar time (that of man). As the earth orbits around the Sun in an elliptical (oval) shape & the axis is tilted – there are only 4 days a year when the day is exactly 24 hours long April 15th, June 14th, September 1st and December 24th. All other days of the year the days are shorter or longer – depending on the position of the earth. This watch will show the difference between the “mean” time & the “true” time. Since the number of the days are fixed year after year (at the same location) a watch can be manufactured to replicate the correction via a shaped cam which elongates & shortens the days accordingly.



Also known as complications. A term used to describe the various different tasks a watch can perform such as chronograph and countdown timer.


GMT Time Zone

Greenwich Mean Time (GMT) is also known as Zulu Time and UTC (Universal Time Coordinated). The standard by which all World Time is set was agreed at the 1884 International Meridian Conference at Washington DC, USA. It placed Greenwich on the Prime Meridian (Zero Longitude). Greenwich Mean Time or GMT is the time standard against which all other time zones in the world are referenced. It is the same all year round and is not affected by Summer Time or Daylight Savings Time. GMT was originally set-up to aid naval navigation when the globe started to open up with the discovery of the “New World” (America) in the Fifteenth Century. Generally, when the GMT term is used with watches it refers to the ability of the watch that shows local time and the time in at least one other time zone in a 24-hour mode. The reason for showing the additional time zone in 24 hour mode is to allow the wearer to know if the second time zone is in AM or PM.


Also known as Engine Turning. It is an engraving technique in which a very precise intricate repetitive patterns or design is mechanically etched into an underlying material with very fine detail. Specifically, it involves a technique of engine turning, called guilloche in French, after the French engineer “Guillot”, who invented the machine that could scratch fine patterns and designs on metallic surfaces.


Helium Escape Valve

A feature found on some diving watches. It provides functionality for professional divers operating at great depths for prolonged periods of time or under saturation, breathing Hypoxic trimix or other mixed gases with helium in them. Because helium is such a small molecule (the second smallest there is), over time in a pressurized diving bell, helium will sneak its way past the o-rings into the inside of a dive watch. While at depth this causes no problem, it will as the divers decompress the helium which is unable to escape the watch. With a standard dive watch, this would lead to the watch crystal popping out from internal pressure. To stop this from happening, high-end, professional diver watches have a helium escape valve or helium bleed valve to let out this extra pressure during decompression. This is a one-way valve which allows the helium to escape.

The art of making a Timepiece.

How does a watch work?

In the following video, Michael Michaels explains it in easy words:


ISO 6425

The standards and features for diving watches are regulated by the ISO 6425 – Divers' watches international standard. This standard was introduced in 1996.

ISO 6425 defines such watches as: A watch designed to withstand diving in water at depths of at least 100 m and possessing a system to control the time. Diving watches are tested in static or still water under 125% of the rated (water) pressure, thus a watch with a 200-metre rating will be water resistant if it is stationary and under 250 metres of static water. ISO 6425 testing of the water resistance or water-tightness and resistance at a water overpressure as it is officially defined is fundamentally different from non-dive watches, because every single watch has to be tested.



In watchmaking, a synthetic ruby used for making low friction bearing in which the delicate pivots of the movement wheels run in. In some deluxe watches, sometimes sapphires or garnets are used. Expensive watch movements are jeweled from the barrel to the balance, and all automatic work, date and complication movements are expected to bejeweled.


K1 Mineralglas

K1 mineral glass is a hardened mineral glass which is much harder than normal mineral glass. The glass is made by grinding and not by heating. K1 mineral glass is a hardened mineral glass which is much harder than normal mineral glass. The glass is made by grinding and not by heating.

The following comparison helps with orientation:

Vicker hardness test : sapphire - around 1900, K1 - around 700, mineral glass - around 380


The karat is the unit for measuring the fineness of gold alloys. We are familiar with 9K, 14K, 18K and 22K gold, as well as 24K gold, the purest form of gold, also called "fine gold". The gold most often used for watch cases is an 18 karat (18K) alloy, which thus contains 18/24 or 750/1000 pure gold.  When spelled as “carat” and abbreviated as "ct", the carat also denotes the international unit for the weight of precious and semiprecious stones. 



Luminous dials first appeared during the Great War when soldiers needed to tell the time in the dark. Early forms used Zinc Sulphide compound agitated by a radioactive salt. It was painted on hands and was potentially dangerous to those applying it. Its use was banned in the ’50s, since Tritium, a substance with low radioactivity, replaced it. Other methods have been devised. Timex’s ‘Indiglo’ uses electronic luminescence; a button on the side of the case causes a tiny current from the battery to the electrodes and gives off energy in the form of light. Seiko uses fluorescent material on the dial, activated by any exposure to light.


Double extension of the case middle by which a strap or bracelet is attached.


Marine Chronometer

Arguably the most accurate timepiece in the world, a Marine Chronometer is a mechanical or electronic timekeeper that is enclosed in a box and is used for determining the longitude on board a ship. Marine chronometers with mechanical movements are mounted on gimbals so they are in the horizontal position that is essential for their precision.


An indicator that keeps track of the phases of the moon. A regular rotation of the moon is once around the earth every 29 days, 12 hours and 44 minutes. Once set, the moon phase indicator accurately displays the phase of the moon.

Mother of pearl

Iridescence is an optical phenomenon that gives mother of pearl its characteristic shine and produces a rainbow of different colours depending on how light hits it. The inside of certain seashells is made up of multiple thin layers of material, which are responsible for creating this shimmering effect. Incidentally, pearls are created in a similar process. The thin overlaying layers break and reflect light differently, resulting in a colour spectrum that you can also find in a rainbow.


The means by which a watch keeps time, often including the power source. For example, a watch with mechanical movement uses a spinning balance wheel powered by a tightly wound spring, whereas a watch with quartz movement measures the vibrations in a piece of quartz and often is powered by a battery.



An octagonal watchcase shape.




The plate is a part of the movement that supports all of the mechanical components, including the dial. It has holes drilled in it at specific locations where various parts are screwed in.

Power reserve

This term refers to how long a watch can run. A hand-wound watch is usually equipped with a barrel and runs accurately between 30 and 35 hours. An automatic watch with a barrel will run for about 40-45 hours. There are currently wristwatches that can run for far longer due to multiple barrels. An example is the Patek Philippe ’10 Days’, which houses four multiple barrels.


In general there are two pushers on a chronograph. The top one is for the Start/Stop function. If you press the bottom pusher the stopwatch hand is reset to zero. Please also refer to: screw-down-pushers.

PVD plating

PVD stands for physical vapour deposition and offers the ultimate in robust and long-lasting colour and composition. In the PVD process the watch case or strap is placed in a sealed, pressurised chamber in which a material is vaporised, creating a saturated atmosphere. The basic substrate stainless steel becomes completely saturated by vaporised molecules, creating an even and deep deposition of colour. In contrast to traditional plating or lacquering techniques, which only coat the surface of the substrate, making it subject to abrasion and tarnishing through exposure to UV rays or moisture, a PVD coating does not discolour thanks to the complete penetration of the colour particles into the metal.  


Quartz Movement

A caliber that uses the vibrations of a tiny crystal to maintain timing accuracy. The power comes from a battery that must be replaced about every 2-3 years. In recent years, new quartz technology enables the watch to recharge itself without battery replacement. This power is generated via body motion similar to an automatic mechanical watch, or powered by light through a solar cell (Kinetic & solar-tech).


Rattrapante (double chronograph)
A watch with a double chronograph has two seconds hands. One hand is superimposed over the other. While one hand moves continuously, the other one can be stopped, started or reset to zero in order to estimate two separate events of different durations.

Rotating Bezel

A bezel (the ring surrounding the watch dial) that can be turned. Different types of rotating bezels perform different timekeeping and mathematical functions.


The part of an automatic (or self-winding) mechanical watch that winds the movement’s mainspring. It is a flat piece of metal, usually shaped like a semicircle, that swivels on a pivot with the motion of the wearer’s arm.


Sapphire Crystal

Sapphire crystal is a very hard transparent material commonly used for “scratch-proof” watch glasses. Made by crystallizing aluminum oxide at very high temperatures, it is chemically the same as natural sapphire and ruby, but without the small amounts of other elements such as iron, titanium or chromium that give the gemstones their colors. Sapphire (whether natural or synthetic) is one of the hardest substances, measuring 9 on the Mohs scale, a system for rating the relative scratch hardness of materials. (Diamond measures 10, the highest rating, and the hardest steels are 8).

Shock Absorber

A resilient bearing which takes up the shocks received by the watch’s balance staff and protects its pivots from damage. As defined by the U.S. government regulation, a watch’s ability to withstand an impact equal to that of being dropped onto a wood floor from a height of three feet.

Skeleton Case

A watch in which the case and various parts of the movement are of transparent material, enabling the main parts of the watch to be seen.

Stainless Steel

An extremely durable metal alloy (chromium is a main ingredient) that is virtually immune to rust, discoloration, and corrosion; it can be highly polished, thus resembling a precious metal. Stainless steel is often used even on case backs on watches made of other metals and is the metal of choice used to make high-quality watchcases and bracelets. It is also hypoallergenic because it doesn’t contain nickel.


Super-LumiNova luminescent pigments are the latest patented development in the field of non-radioactive luminescent pigments. Thanks to their highly improved light storage capacity, these pigments can be used as luminescent markers on watch hands and dials. In essence, the photoluminescent pigments work like a light battery. After sufficient charging with either sunlight or artificial light, the stored light energy is discharged in the dark over a long period of time. This charging and discharging process can be repeated indefinitely and does not deteriorate or weaken over time.


As a part of a move towards greater consumer protection and in order to combat fakes in the Far East that claim to be Swiss made, the Swiss federal council in 1993 laid down the rule that a watch has to satisfy before it could be described as Swiss made. The movement must be of Swiss origin, and must contain at least 50% Swiss parts. The watch must be cased in Switzerland and pass its final inspection in that country.

Swiss A.O.S.C.

A certificate of origin – A mark identifying a watch that is assembled in Switzerland with components of Swiss origin.

Swiss Parts

Since most watch companies are now around the globe, a variety of watches manufactured in a year increases dramatically as well.

All watches that use Swiss components but have movements that are not assembled in Switzerland are considered as Swiss Parts watches.

Many of these watches are either assembled in Asia or USA, which are also known for having smartwacht makers. These watches do not bear any official stickers or stamps to know the parts used. This could be learned by checking the product’s details in its manual.

Moreover, even if Asia has been known as the hub of the Quartz Movement, it has still become one of the top spots for watches using Swiss Parts and watch product photography services as well.



The tachymeter scale on the watch face of a chronograph is used to measure the speed, e.g. of a car over a 1-km distance. The chronograph is activated when the vehicle passes the starting point and deactivated when the vehicle has reached the final point. The figure shown on the tachymeter scale corresponds to the speed in km per hour. The speed must be equal over the whole test distance.


A telemeter scale enables the calculation of the distance between an acoustic signal and its own position. Or, put more simply, it can be used to determine the proximity of a storm. To do this, the chronograph is started when lightning strikes and stopped at the first clap of thunder. It is then possible to gauge how far away the storm is by reading the telemeter scale and using the second hand as a counter. The scale is based on the well-known sonic speed value (343 m/s or 1,235 km/h), and was originally employed in a military context. It was used to determine the enemy's position via muzzle flashes and cannon fire.

This type of watch is a complex piece of micro-engineering which results in the escapement of a watch rotating on its own axis; the aim is to cancel out the variations in running regularity which can be caused by the watch being in different positions; (a watch may gain in one position yet lose in another).

Tritium gaseous tubes

Tritium is a colourless gas otherwise known as extra-heavy hydrogen. The name originates from the Greek word ‘tritos’ meaning third, and refers to the three components of the atom (3H). Tritium has been used for decades in all sorts of applications where constant, independent and long-lasting light sources are essential. On the dials of military watches you will often find a red, circular symbol with the notation ‘3H’ that refers to the use of tritium. In ‘civil’ watches, the abbreviation ‘T25’ denotes the same thing. In the past, luminous tritium was applied directly to the dial. Today’s watchmakers are more careful and fill the gas into fine tubes made from borosilicate glass, a highly resilient and ISO-certified glass used in chemical engineering. These Gaseous Tritium Light Sources (or GTLS) are not only exceptionally safe, they also guarantee the watch wearer at least ten years’ constant luminance – without any external energy source.


Uni-Directional Rotating Bezel

An elapsed time rotating bezel, often found on divers’ watches, that moves only in a counterclockwise direction. It is designed to prevent a diver who has unwittingly knocked the bezel off its original position from overestimating his remaining air supply. Because the bezel moves in only one direction, the diver can err only on the side of safety when timing his dive. Many divers’ watches are ratcheted so that they lock into place for greater safety.


Vibration Per Hour or VPH

Movement of a pendulum or other oscillating element, limited by two consecutive extreme positions. The balance of a mechanical watch generally makes five or six vibrations per second (i.e. 18,000 or 21,600 per hour), but that of a high-frequency watch may make seven, eight or even ten vibrations per second (i.e. 25,200, 28,800 or 36, 000 per hour).


Water resistance

Often, our customers and watch fans wonder if a certain watch is really "water resistant" and what it can be used for. Wristwatches whose original condition is resistant to water penetration up to the specified depth are referred to as "water resistant". "Water protected", however, means that the watch is resistant to splashing water or everyday (hand) washing. The DIN standard 8310 regulates whether a watch is considered "water resistantt" or not. The criteria are typographic tests. Example: Under normal laboratory conditions, the watch can withstand a pressure that is 200 meters deep. This means to 20 bar or 20 ATM. The watch can be classified as a diver's watch and you can dive with it safely. Watches with 10 ATM, however, should rather be used only for swimming and snorkeling, while watches with 5 ATM are only considered "water-protected", whereby they are protected against splashing water and can be worn while bathing. If a clock is only awarded 3 ATM, you should do without bathing with this clock better. Splashing water should not be a problem.

World Time Complication

A dial, usually on the outer edge of the watch face, which tells the time up to 24 time zones around the world. The time zones are represented by the names of cities printed on the bezel or dial. The wearer reads the hour in a particular time zone by looking at the scale next to the city that the hour hand is pointing to. The minutes are read as normal. Watches with this feature are called “world timers."


Water Resistance Guide

A watch rated as water resistant may come in contact with water to a predetermined extent. Most watches have a measurement until which the depth of immersion is safe. It is important to remember that a water-resistance rating is based on optimum conditions in a laboratory. Real life experience & aging of the gaskets will effectively decrease the manufacturer’s specifications of water resistance over time. The worst scenario possible is that moisture is allowed to make contact with the movement – thus we strongly suggest that you always work well within the parameters of the manufacturer’s recommendations and have your watch tested at least once a year. Any competent watchmaker has the necessary equipment to test water resistance. It is important to remember that all watches have limits and no watches are waterproof.

Optimum water resistance on a watch is obtained by 3 important factors:

  1. Case back – this refers to how the case back is attached to the watch


Snap-on case backs are sealed by pressure and are considered the least water resistant. The slightest nick in the case or a deformity in a gasket (which will happen over time) will allow water to penetrate the case. Generally, these watches will have a water resistance of 30m/99ft maximum – which allows for contact with water, such as washing hands, but not immersion.

Case-backs attached with screws are the second level of water resistance. Having the case back attached with screws allows for a much tighter seal than a snap-on case back, however, a deformity in the gasket will still allow water to penetrate. Generally, these watches will have a water resistance of 100m/330ft maximum – which allows for light immersion such as swimming in a pool.

Screw-down case backs are threaded and screws into the actual case. This creates a double seal, using both the threading & the gasket as a seal coupled with the deep water pressure. Generally, diving watches with water-resistant ratings greater than 100m/330ft will have this type of case back.

   2. Crown – the single most important factor to ensuring water resistance.

The weakest link in a watch for water to penetrate is the crown-stem hole. The stem of the crown is attached to the movement through a hole in the case edge. As the crown is constantly moved to different positions, wound and turned to correct the time, the gasket is constantly compressed, chafed & stressed. The slightest variation in the shape of the gasket or if the crown is not pushed all the way in will allow water to penetrate the watch through the stem hole.

Screw-Down crowns are threaded & screwed shut to a matching threaded tube in the case. The crown has a gasket that is compressed & seals the opening when the crown is tightened – thus ensuring water resistance. A screw-down crown is an essential feature for any watch you intend on swimming with. As matter of fact, we do not recommend swimming with a watch that does not have a screw-down crown. No matter if the watch has a screw-down crown & chronograph pushers, the crowns & pushers are never to be pushed, adjusted or operated when the watch is immersed in water – unless otherwise stated by the manufacturer. An additional benefit of the screw-down crown is that the crown is somewhat more protected from accidental knocks.

    3. Gaskets:

“O” rings are 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 to protect from dust and dirt, check with your manufacturer if you are unsure as to whether the pushers on your watch can be operated in wet conditions, it’s not a law that chronographs must be operable under water.

Gaskets begin to erode and break down over time, diminishing the water resistance of a watch. It is important to test your watch once a year for water resistance. Any competent watchmaker should have the necessary basic equipment to test the watch – the cost involved should be minimal.

Real Life and Water Resistance

When a watch is tested by the manufacturer it is usually done in a laboratory under optimum conditions, such as a fresh gasket, sitting stationary in a pressured water tank and with still/motionless water. However, real-life action will produce completely different results. Here are a few scenarios:

Water temperatures in a hot tub or a hot shower will effect the shape of the gasket seals. Especially if the watch is taken from hot temperatures & immediately plunged into cold water – such as going from a hot tub into a pool.

Sudden & rapid changes in pressure – such as diving (even shallow diving) into a pool, the force of plunging your arm into the water while swimming, will stress the gaskets for a fraction of a second. If the gaskets are not up to specification they may rupture and cause the watch to take in water.

As the watch ages, the seals begin to erode & will not maintain the same water resistance levels.

Water Resistance vs. Water Proof

The U.S. FTC (Federal Trade Commission) which enforces the truth-of-advertising has deemed the term “Waterproof” inappropriate. In their opinion, a watch can never be 100% truly impervious to water, as the gaskets deteriorate over time & exposure, thus reducing the specified depth of water resistance. In the words of the FTC: “The word proof connotes a measure of absolute protection that unfortunately does not exist with respect to watches, especially over prolonged periods of time.” The FTC has found the term Water Resistant to be more appropriate.

Water Resistance Testing Methods

There are 2 commonly used water-resistance testing methods:

Dry Test – The watch is placed in a chamber and the air pressure is increased. The machine will detect the smallest variation in the case size. If the case expands, even slightly, then the watch is not water resistant.

Wet Test – The watch is placed in a chamber which is half filled with water and half air. Air pressure is increased while the watch is out of the water, then the watch is slowly immersed in the water. Once the watch is completely immersed, the air pressure is slowly released. If bubbles come out of the watch it means that air entered the watch prior to immersion & the watch is not water resistant. This method is generally used as a second test to pinpoint the problem area.

Water Resistancy Test

Water Resistancy Test

Play Video

Units of Water Resistance based on pressure

Meters/Feet: This is the most common way of measuring the water resistance of a watch. 100 meters is just over 328 feet.

ATM: This stands for Atmospheric Pressure. At sea level, the ATM rating is 1, which is equal to 10 meters.

Bar: Bar is a metric unit of pressure named from the Greek word Baros which means weight. It isn’t isn’t commonly indicated on a watch because meters/feet and ATM are more widely understood by the general public. 1 bar is equal to 100,000 Pascals, 14.5 PSI (Pounds per Square Inch) or 0.98 ATM.

Helium Escape Valve

The Helium Escape/Relief Valve is used only by deep diving expeditions when a diver operates from a diving bell. As the bell is lowered pressure begins to increase & helium is added to the breathing mix. Oxygen becomes toxic at a higher pressure, for example, pure oxygen would become toxic at 6 meters. Normal air would become toxic at the pressures technical divers and saturation divers usually work at. They usually use a mixture of three gases (Oxygen, Nitrogen and Helium), known as Trimix. Helium makes up a significant proportion of the mixture because it does not have a narcotic effect on the body, therefore reducing the risk of nitrogen narcosis (the bends) from occurring. Not only this, but helium has a lower density than normal air which means that it is also easier to inhale under water.

The problem is that helium is one of the smallest molecules & will seep into the watch through the seals until the air pressure in the watch equals the air pressure in the diving bell. As the diving bell surfaces & decompresses, the helium molecules inside the watch expand, if there isn’t a valve then the pressure in the watch will pop the crystal off. To avoid this, Omega and Rolex developed their own helium escape valve systems which allow the helium to escape. Omega uses a second crown which screws closed on all Seamaster watches except for the Aqua Terra which doesn’t have one and the Ploprof which uses the automatic helium escape valve design that is common on Rolex diving watches. Many brands use the escape valve in one design or another. Generally, the escape valve can be found on watches which have a water resistance rating of 300m or greater.

Interpretation of the Depth Ratings

Although a watch may be rated 30m/99ft water resistant, it does not mean that the watch can be immersed to that depth. The depth rating posted by the manufacturer is theoretical in nature and can only be achieved in a perfectly optimum environment of a laboratory – which is impossible to replicate in real life. It’s also important to consider that watches such as the Rolex Datejust and Datejust II models might well be rated to 100m and therefore great for swimming with, but they were not designed as full-on diving watches and therefore may not survive prolonged experiences of over 50 meters. You should also consider any chemicals that might be in the water and whether they can react to materials used in the construction of the watch.

Water Resistance Guide

No rating (Dustproof/moisture proof) – 30m/99ft

Does not allow contact with water

30m/99ft – 50m/165ft

Allows for contact with water such as washing hands and rain

50m/165ft – 100m/330ft

Allows for light pool swimming and swimming to the bottom of the deep end

100m/330ft – 200m/660ft

Allows for swimming and snorkeling

200m/660ft – 500m/1650ft

Allows for impact water sports such as surfing and scuba diving

500m/1650ft +

Appropriate for technical diving

Obviously, the higher the rating, the more appropriate the watch is for deeper diving.

IMPORTANT: We strongly recommend purchasing a watch with a screw-down crown if you intend on wearing the watch while you are in contact with water.

Our Recommendations

  • Have your watch water-tested once a year.

  • Never open, wind or operate the crown while in water.

  • Never press the buttons of a chronograph watch while in water – unless otherwise stated by the manufacturer.

  • Do not subject your watch to extreme temperature changes.

  • Do not subject your watch to sudden & rapid air-pressure changes.

  • Do not allow your watch to come into contact with corrosive chemicals, such as abrasive soaps & highly chlorinated water.

  • Ensure that the crown is always pushed in, and if you have a screw-down crown make sure it is always tightened. Double-check before immersing in water.


What about the bath or a shower?

You must not wear your watch into the washroom if you have a habit of daily washing, even if it is a diving watch. The warm air in the room will cause the seals in the watch to expand, the temperature shock of when you step outside into the colder air of the rest of the building will cause the seals will contract and allow a tiny amount of moisture inside the case. Over a period of time between services this can cause the hands and dial of a watch to rust, even if the watch is rated at 300m or more.

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