Diving mask
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A diving 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.
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[edit] Construction
Diving masks have a 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. A strap keeps the mask 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.
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 plastic or low-quality 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, and air doesn't get in, then the fit is good.
[edit] 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 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.
It is a good idea to always rinse the mask inside and out with clean, fresh water after each day's use. Dry it off after washing. Do not store the mask in direct sunlight for long periods of time. This will enable the mask to last many years.
[edit] Types of diving mask
- The old type with one big oval pane. In the oldest diving masks this window was an ellipse.
- Similar to the previous, but the window is closer to the face, and the window has a big cutout below to fit over the nose, which is kept dry by an expansion of the rubber mask edging.
- Similar to the previous, but with two windows, one window for each eye, as shown above. This type can have the windows closer to the face than the one-window type and thus contain less space for the diver to have to blow water out of if the mask floods.
- The double-dome mask. This is a new invention 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 / 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); his vision will go myopic when he puts his head out of water with the contact lenses in.
- Other optional features:
- Blowoff valve.
- Indentations underneath for the diver to put a thumb and forefinger in to pinch the nose to aid ear clearing.
- The Data Mask, developed by Oceanic, is an eyes-and-nose diving mask with a built-in LED display which displays various dive and breathing set conditions including the function of a diving computer [1] [2] [3]. It is currently very expensive.
[edit] 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 (LiquiVision fluid goggles).
[edit] External links
- Deep Ocean Diving's Ocean Science
- Brief history of diving - From antiquity to the present.
