A diving mask (also dive
mask or scuba mask) is an item of diving equipment that allows scuba
divers, free-divers, and snorkelers to see clearly underwater. When the human
eye is in direct contact with water as opposed to air, its normal environment, light
entering the eye is refracted by a different angle and the eye is unable to
focus the light. By providing an air space in front of the eyes, light enters
normally and the eye is able to focus correctly.
Construction
Diving masks may have a single, durable,
tempered glass plate in front of the eyes and a "skirt" of rubber or
silicone to create a watertight seal with the diver's face. Some masks may have
two lenses, which in some cases allows the user to fit prescription lenses. In
the case of freediving masks, which need to be low volume to minimise the
change of pressure that occurs with depth, the lenses may be made of
polycarbonate plastic. All masks have an elastic strap to keep them in
position.
Masks that are used at depth must be
constructed so that the diver can exhale through the nose into the mask to
prevent the "squeeze" caused by increasing pressure during descent in
water. The nose section of the mask also needs to be flexible to allow the
diver to perform an eqalisation maneuver such as the valsalva maneuver or the
frenzel maneuver when equalising pressure in the middle ear.
Some masks have a one-way purge
valve under the nose to let water out. The diver simply holds the mask upright
and exhales through the nose. While common in snorkeling, this feature is less
favored by SCUBA divers because of the possibility of the valve failing at
depth and leaving the user no means of clearing his mask. A simple and
effective method for clearing a diving mask while underwater is to look down,
place a finger on the top of the frame at either side, and slowly look up while
exhaling through the nose.
Sometimes masks are sold in
conjunction with snorkels and/or swimfins. Low quality snorkelling masks may
have a low-quality plastic or glass faceplate, and are not recommended for
anything but rare, casual use. Masks that have the snorkel built in are
considered hazardous as well.
Divers often test whether a mask is
a good fit by placing it on their face, without using the straps, and gently
inhaling through their nose. If the mask stays on without any help this
indicates that no air is being drawn in and that the fit is good.
Use
To prevent a mask from fogging up
due to condensation on the glass plate many divers spit into the mask, wipe the
spit around the inside of the plate and wash it out with a little water. There
are commercial products that can be used as an alternative to the saliva
method.
Refraction of light entering the
mask makes objects in salt water appear about 34% bigger and 25% nearer when
underwater, as shown in the underwater vision article. As the diver descends,
the water acts as a colour filter eliminating the red end of the visible
spectrum of the sunlight entering the water leaving only the blue end of the
spectrum. Depending on the depth and clarity of the water, eventually all
sunlight is blocked and the diver has to rely on artificial light sources to
see underwater.
A variety of prescription lenses can
be fitted inside the glass plate of the mask to correct some visual problems
underwater. Divers may be able to use contact lenses inside the mask but they
must keep their eyes closed if they remove the mask underwater to avoid losing
the lenses. Double-dome masks restore natural sized underwater vision and field
of view, while also correcting for a certain range of myopic vision. Mask
removal and refitting is a basic skill that all divers are taught so that the
diver can overcome floods or the mask being dislodged without panic.
When entering the water while
wearing the mask, the diver normally needs to place a hand over the mask to
disrupt fast water flow during entry. This prevents the mask from becoming
dislodged or the glass damaged. Alternatively, a diver can enter the water with
the mask off and then put it on or use an entry method such as the
"forward roll", where the diver rolls forward with head entering the
water first, which does not result in fast water flow over the mask.
Mask should always be rinsed inside
and out with clean, fresh water after each day's use, and dried off after
washing. A mask should not be stored in direct sunlight for long periods of
time as ultraviolet light degrades the silicone. A well-maintained mask will
last for many years.
Types
of diving mask
Older diving mask with one big window
Disassembled single-window, low-volume dive mask
A two-window, soft-silicone dive mask without purge valve
A HydroOptix Double-Dome mask
Mask with bifocal lenses for reading instruments
A diving mask showing the retaining strap
Older diving masks had a single elliptical
pane of glass. These masks have indentations underneath, either side of the
nose. Divers can then put a thumb and forefinger in to pinch the nose, when
performing a Valsalva maneuver to clear their ears. This design was improved by
bringing the window closer to the face, reducing the volume of air inside the
mask, thus making mask clearing easier. The window has a cutout to fit over the
nose, which is covered by the rubber or silicone material of the mask. This
facilitates pinching the nose when ear-clearing.
A further development is the mask
with two windows, one window for each eye. It can have the windows closer to
the face than the one-window type, and therefore contain even less space for
the diver to have to blow water out of if the mask floods. These types are
often called a "low-volume mask".
Recent innovations have produced
more complex designs, intended to provide extra features:
- The double-dome mask. This was invented by HydroOptix. Double-dome masks allow a wider field of view and avoid the refraction error in perceived distance and size of objects. Underwater the curved mask windows make the diver's vision effectively more hyperopic, or less myopic, and the diver must wear special contact lenses to compensate (unless his eyes are myopic to the right amount to compensate exactly for the refraction at the curved mask windows). The diver's vision will become myopic when he puts his head out of water with the contact lenses in.
- The "Data Mask", developed by Oceanic, is an eyes-and-nose diving mask with a built-in LCD display which displays various dive and breathing set conditions including the function of a diving computer. It is currently very expensive.
Related
equipment
There are several specialised types
of diving headgear or outerwear:
- full face diving mask - often worn by working divers who need underwater verbal communication ability.
- diving helmet - often worn by divers using surface supplied diving equipment.
- hard hat - part of the old fashioned standard diving dress.
- fluid filled mask - the need to equilibrate the internal pressure in the mask by exhaling air through the nose reduces the freediver capacity to dive deep. Masks or swimming goggles with high power lenses (40-200 diopters) have been developed in this view: they are filled with water or saline fluid.
More Diving Mask
Source :
http://en.wikipedia.org/wiki/Diving_mask
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