Today, diving computers are becoming increasingly powerful and cheap, so few divers use mechanical diving watches. However, not so long ago, these watches provided vital information underwater!
Waterproof and diving watches are often mixed up. True, diving watches must be waterproof but this is not the only criterion they must meet.
Indeed, diving watches must resist to an underwater dive at 100 m or below, feature a time control system and meet light, readability, shock-resistance, chemical resistance to salt water, magnetic field resistance and strap robustness criteria. The very rigorous norm ISO 6425 precisely defines these constraints.
But, in practice, what does it all mean ?
WHY CHOOSING 100 M IN DEPTH ?
A depth of 100 m corresponds to a static overpressure of 10 bar. Even if the majority of divers do not go that deep, the corresponding dynamic overpressure - i.e. taking into account the movements of water - may be reached above 100 m deep.
WHAT IS THE PURPOSE OF THE TIME CONTROL SYSTEM ?
Time control systems usually designate the rotating bezel of the watch. To understand its purpose, you need to know what happens in the body of a diver who dives deeply for a long period of time.
The air breathed in contains nitrogen which - because of the pressure related to depth - is absorbed by the blood and body tissues. For example, nitrogen narcosis, also called rapture of the deep, is due to excess nitrogen levels in the blood stream.
If the pressure goes down slowly enough during surfacing, nitrogen in the blood stream is gradually rejected out of the lungs by breathing. However, if the pressure goes down too rapidly nitrogen creates bubbles in the blood stream. Decompression sickness occurs then and its consequences can be extremely severe: nitrogen bubbles circulating through the body can cause serious circulatory problems and, as a consequence, deprive the brain, spinal cord or vital organs from oxygen.
That is why divers must make pauses during surfacing: this is called decompression stops. Maximum diving durations and the amount of decompression stops are listed in tables that divers must perfectly be acquainted with.
The rotating bezel indicates the duration of the diving session to calculate decompression time. To evaluate the duration of the diving session, you simply need to put the “zero” or the triangular index of the bezel in front of the minutes hand.
The bezel features 5-minute graduations except for the first 5 minutes which is generally graduated by the minute to enable the diver to precisely measure shorter intervals - especially during decompression stops.
If the bezel only rotates one way, it is called unidirectional. This is a safety feature : when manipulated accidentally, the duration of the diving session can only be increased, thus increasing the duration of the decompression time. Rotating the bezel the other way would decrease the diving session duration and, consequently, would also decrease the corresponding decompression time and may cause decompression sickness.
WHY DO SOME WATCHES FEATURE AN HELIUM VALVE ?
When professional divers swim very deep in chambers, nitrogen is replaced by helium in the air mix to avoid the rapture of the deep cause by nitrogen.
Compressed helium molecules are very “thin” and can penetrate the watch. During surfacing, they dilate and can no longer escape from the watch. Therefore, they may cause the glass or the winding stem to explode. The helium valve featured on some watches helps prevent this from happening.
WHY ARE HANDS, NUMERALS AND INDEXES LUMINESCENT ?
As depth increases, light decreases. This phenomenon can happen even faster at night or when the water is murky. Therefore, in order to provide visible information deep into the water, the hands, numerals and indexes of the dial must be coated with a luminescent material.
Formerly, the materials used to do this were radio-luminescent - these were generally radium 226, promethium 147 or tritium. These radioactive materials, even if they are “imprisoned” in the watch, could cause irradiation/contamination risks. As a consequence, they have been gradually replaced with other harmless substances - called photoluminescent - such as Luminova (Nemoto & Co LTD) or Lumibrite (Seïko).
Radio-luminescent materials are “spontaneously” glowing whereas photoluminescent materials need to be exposed to some form of light to capture photons to, then, gradually liberate them for a duration ranging from tens of minutes to several hours.
WHICH STRAP SHOULD I CHOOSE FOR A DIVING WATCH ?
If you will be using your watch to do some serious diving, you should go for a water resistant, rot resistant, flexible and expandable strap. The latter feature allows you to secure the strap on your diving suit before diving so that it does not bother you while staying in place as the neoprene suit compresses when going deeper underwater.