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.
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.
The straps are usually made of an elastomer that can sometimes cause hair tangling. This can be prevented by replacing the rubber strap with a customized neoprene strap or by covering the existing strap with a neoprene cover.
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.
It is quite common for water to leak into the mask, which can be annoying, or interfere with clear vision, and the diver needs to be able to get rid of the water quickly and effectively. Reasons for the leakage include poor fit or fitting, leaking via head or facial hair, movement of the facial muscles causing temporary leaks, or impact of external objects against the mask, which may distort it temporarily, or move it so that it leaks, or in extreme cases dislodge it entirely from the diver's head.
The methods of clearing differ between the conventional recreational diver's half mask, which covers the eyes and nose, and the full-face mask, which also covers the mouth.
The pressure changes during ascent and descent may affect gas spaces in the diver and diving equipment. A change in pressure will cause a pressure difference between the gas space and environment which will cause the gas to expand or compress if that is possible, and constraining the gas from expanding or compressing to balance the pressure may cause damage to the surrounding material or tissues by over-expansion or crushing. Some gas spaces, such as the mask, will automatically release excess gas as it expands, but have to be equalized during compression.
Equalizing of the mask is an essential skill for any form of diving. Goggles that do not cover the nose can not be equalised and are unsuitable for diving.
Types of diving mask
A twin-lens, low-volume diving mask with wide peripheral view of the type favoured by many Underwater Hockey players.
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". Participants in the sport of underwater hockey are required to use twin-lens masks of this type for their own safety - the sport uses a heavy lead puck similar to an ice hockey puck but skilful players can give the puck considerable height when making passes which leads to the possibility of accidental puck contact with other players. Should a puck hit the lens of a single-lens mask there is a good chance it will break the glass and pass through the aperture into the face and eyes, but with a twin-lens mask though the glass may still break the frame will prevent the passage of the puck any further.
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.
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 - usually 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.
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