8/28/2010

Characteristics of Dive Watches

Seiko 7002-7020 Diver's 200 m on a 4-ring NATO style strap made of ballistic nylon.
Citizen Hyper Aqualand ProMaster MA9004-21E digital Diver's 200 m (metric MS Windows version) on a rubber strap.
Many companies offer highly functional diving watches. Whilst diving watches are primarily tool watches, some companies offer models that can in addition to this be regarded by some as jewellery or fine mechanical devices. Diving watches can be analog or digital.



ISO 6425 standard for diving watches
The standards and features for diver's watches are regulated by the International Organization for Standardization in the ISO 6425 standard. Besides water resistance standards to a minimum of 100 meter depth rating ISO 6425 also provides minimum requirements for mechanical diver's watches (quartz and digital watches have slightly differing readability requirements) such as
:
  • The presence of a unidirectional bezel with at least at every 5 minutes elapsed minute markings and a pre-select marker to mark a specific minute marking.
  • The presence of clearly distinguishable minute markings on the watch face.
  • Adequate readability/visibility at 25 cm (9.84 in) in total darkness.
  • The presence of an indication that the watch is running in total darkness. This is usually indicated by a running second hand with a luminous tip or tail.
  • Magnetic resistance. This is tested by 3 expositions to a direct current magnetic field of 4,800 A/m. The watch must keep its accuracy to +/- 30 seconds/day as measured before the test despite the magnetic field.
  • Shock resistance. This is tested by two shocks (one on the 9 o'clock side, and one to the crystal and perpendicular to the face). The shock is usually delivered by a hard plastic hammer mounted as a pendulum, so as to deliver a measured amount of energy, specifically, a 3 kg hammer with an impact velocity of 4.43 m/s. The change in rate allowed is +/- 60 seconds/day.
  • Chemical resistance. This is tested by immersion in a 30 g/l NaCl solution for 24 hours to test its rust resistance. This test water solution has a salinity comparable to normal seawater.
  • Strap/band solidity. This is tested by applying a force of 200 N (45 lbf) to each springbar (or attaching point) in opposite directions with no damage to the watch or attachment point.
  • The presence of an End Of Life (EOL) indicator on battery powered watches.

Watch case
The watch cases of diving watches must be adequately water (pressure) resistant and be able to endure the galvanic corrosiveness of seawater, so the cases are generally made out of materials like stainless steel, titanium, ceramics and synthetic resins or plastics. The case must also provide an adequate degree of protection against external magnetic influences and shocks, though diver's watches do not have to be able to endure strong magnetic fields and shocks. To make mechanical watch movements themselves shock resistant various shock protection systems can be used.The cases of diving watches have to be constructed more stoutly than typical dress watches, because of the requirements necessary to withstand a seawater environment at depth. As a consequence diving watches are relatively heavy and large compared to dress watches made out of similar materials. Under water sheer weight is of less consequence than buoyancy, which a diver can address by a buoyancy compensator ("BC") vest. Before the introduction of other case materials diving watch cases were made out of stainless steel. Stainless steel is however still often used as case material in contemporary diving watches.


Elapsed time controller
Omega Seamaster 300 M Diver Chronometer. The second "crown" (at 10 o'clock) is a helium release valve.
Analog diving watches will often feature a rotating bezel, that allows for an easier reading of elapsed time of under one hour from a specific point. This is used to compute the length of a dive. (See Tachymeter.) When the zero on the bezel is aligned with the watch's seconds or (usually) minutes hand on the water entry moment a diver can read off the elapsed time from the bezel. This saves the diver having to remember the exact water entry moment and having to perform arithmetic’s that would be necessary if the watch's regular dial was used. On diving watches the bezel is "unidirectional", i.e., it contains a ratchet so it can only be turned anti-clockwise to increase the apparent elapsed time. This is an important "fail safe" feature. If the bezel could be turned clockwise this could suggest to a diver that the elapsed time was shorter than reality, thus indicating a falsely short elapsed time reading and therefore falsely low air consumption assumptions which are highly dangerous. Some diving watch models feature a lockable bezel to minimize the chance of unintentional bezel operation under water.
The exclusive use of a rotating bezel is considered a rudimentary diving technique in the 21th century, suitable for basic shallow single gas (air) diving only. Non-basic diving profiles and depths past 30 m (98 ft) require other more advanced timing and measuring methods to establish suitable decompression profiles to avoid decompression sickness. Besides for basic diving and as a backup for more complex diving, the one-way bezel can also be used for other situations in which a measurement of elapsed time of under one hour might be useful.Digital dive watches usually perform the elapsed timer function by use of a standard stop watch function. Digital dive watches may also feature a depth gauge and logging features, but are not usually regarded as a substitute for a dedicated dive computer.


Bezel markings

Most contemporary dive watches with conspicuous 15 or 20 minute markings on their bezels are the result of copying a Rolex bezel design of the 1950s. Back then divers typically planned a dive to a certain maximum depth based on now obsolete US Navy dive tables, and dove according to the planned dive profile. If the dive profile allowed a bottom time of 35 minutes the diver, upon entering the water, would set the marker on the bezel, 35 minutes ahead of the minute hand. The diver calculated this with the 60 - bottom time formulae (60 - 35 = 25, for 35 minutes bottom time the diver would align the 25 minute bezel-mark with the minute hand). Once the minute hand reached the main-marker on the bezel the diver would begin his ascent to the surface. The 15 or 20 minute scale helped with timing the ascent and whatever safety stop the diver deemed necessary. For contemporary diving methods the 15 or 20 minute "count-down" bezel is quite antiquated.


Crystal
Diving watches have relatively thick watch crystals. Sometimes domed crystals are used to enhance the pressure resistance of the watch. The typical materials used for crystals are acrylic glass, hardened glass and (synthetic) sapphire which all have their pros and cons. Acrylic glass is very break resistant but can easily be scratched. It has however the advantage small scratches can be buffed out with polishing compounds. Hardened glass is more scratch resistant than acrylic glass and less brittle than sapphire. Sapphire is very scratch resistant but less shatter proof than the other crystal options. Anti-reflective coatings are generally applied on sapphire crystals to enhance the legibility of the watch. Some manufacturers use sapphire/hardened glass laminate crystals, where the scratch resistance sapphire is combined with the better shatter resistance of hardened glass. Watch crystals can also be applied as display backs to view the watch movement. Display backs are however a rare feature on diving watches.


Crown
Citizen Promaster Eco-Drive AP0440-14F Diver's 200 m.
Analog diving watches must have a water resistant crown. Often the crown has to be unscrewed to set or adjust the time and date and afterwards retightened to restore the water resistance of the watch and minimize the chance of unintentional operation under water. There are also watch models where a locking handle, separate knob or an extra crown cover has to be manipulated before the crown can be operated. There are however models that have crowns that are operated like the crowns of non diver's analog watches. Screw down or otherwise locking crowns and traditionally operated water resistant crowns can not be operated under water.


Pushers
Digital and some analog chronograph diving watches - such as the Breitling Avenger Seawolf Chronograph or Sinn U1000 - have specially-designed push pieces that can be operated at depth without allowing water to enter the case.


Helium release valve
Some diving watches intended for saturation diving at great depths are fitted with a helium or mixed breathing gas release or escape valve to prevent the crystal from being blown off by a pressure build up caused by helium that has seeped into the watch in helium enriched environments as the watch and diver adjust to normal atmospheric conditions. Other helium safe/for mixed-gas rated diving watches can withstand the helium used in certain diving situations by using gaskets that simply do not allow helium gas to enter the watch case in a harmful way in the first place.


Strap/bracelet
Most diving watches feature a rubber, silicone rubber or polyurethane watchstrap or a metal bracelet of adequate length to facilitate wearing the watch over a diving suit sleeve. For this bracelets often have a (concealed) Divers extension deployment clasp by which the bracelet can be appropriately extended. One piece (NATO style) nylon fabric straps that slide under the watch case through both springbars are used to minimize the chance of losing the watch due to a springbar failure.


Legibility
Lume applied on a diver’s watch to make it readable in low light conditions.
The dials and markers on the watch face and bezel have to be legible under water and in low light conditions. An indication that the watch is running in total darkness also has to be present. For easy legibility most diving watches have high contrasting, non-cluttered dials and markers with a large, easily identifiable minute hand. The markers for 3, 6, 9 and (especially) 12 o'clock on the watch face and the zero marker on the bezel of analogue diver's watches are usually conspicuously styled to prevent disorientation induced read out errors. For low light conditions luminous phosphorescent non-toxic strontium aluminate based lume pigments marketed under brand names like Super-LumiNova or NoctiLumina and tritium based self-powered lighting devices called "gaseous tritium light source" (GTLS) is applied on the dials and markers. On digital diving watches lightened displays are used for low light conditions legibility.


Power reserve indicator
A diving watch with an electric battery powered movement must have an End Of Life (EOL) indicator, usually in the form of a two or four second jump of the second hand or a warning message on a digital display to safeguard against insufficient power reserve during underwater activities. Some electric and mechanical powered movement models have power reserve indicators that show the current power status of the watch.

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