THE FACE MASK
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Diving face masks are crucial to the well being of the sport. It is most important that the mask fits properly. There are several points to look for when purchasing a face mask:
The principal behind keeping a mask from fogging up during the dive is to get a thin coating of a material to cover the lens without distorting the vision. Spit, shampoo, raw potato juice, commercial anti-fogging products, etc. have a slimy material that will leave a thin layer on the glass. If the diver coats the lens with one of these materials, and gives the mask a light rinse fog should not form as long as the mask does not dry out. It won't underwater, but it will if you coat the lens and then put the mask down while doing something else. The best thing to do is coat the lens last, put the mask on the face, and enter the water. Then let a little water in from the lake or ocean, slosh it around, and then dump it out. All this should be done with the mask on the face. The principle behind fogging is: When a diver breathes out into the mask the moist air hits the cooler glass and little droplets of water form on the lens. This is the fog. If the glass were coated with a syrupy liquid the droplets would just go into the coating and spread out instead of forming a droplet that interferes with vision.
There is one other consideration here: Surface tension. Water molecules are bipolar. They have a positive and negative side. The 2 hydrogens on either side of the oxygen atom are positive and the oxygen is negative. The hydrogens are at a 105 degree angle to the center of the oxygen. The official name for water could be Dihydrogen Monoxide.
The hydrogen side of the water molecule is positive and the oxygen side is negative.
At any surface, such as the top of the ocean, the outside of a droplet, and the inside of a bubble, the molecules line up positive to negative unlike their random movement down in the liquid because there are infinite orientations. This surface positive to negative "lockup" presents a barrrier that allows razors to float, bugs to walk, and jumpers to get killed. Do a belly-flop to see what I mean. Break the lockup with pointed fingers during a well-executed dive from a high cliff allows a painless entry. The outside of a fog droplet on your facemask is like elastic underwear. That droplet does not want to lay flat. If we could get the surface positive-to-negative forces (surface tension) to be destroyed you could belly-flop and love it. Ammonia, soap, detergents, etc. do that! Put a razor blade or a piece of string on the top of water in a glass. Put a drop of liquid detergent on the water's surface and they will sink. Adding ammonia or detergent to your wash will allow the water molecules to penetrate the fabric without surface tension. Adding some antifog to the interior of the face mask will prevent those annoying droplets to cloud your dive.
Be gentle with mask straps. They can and do break when pulled too hard. It's best to put the mask on the face first and then pull the strap down behind the head but not so low it hits the ears.
Because light has to travel through water, the mask glass, and the air inside the mask before it enters your eye it creates a magnification at close distances (1-15') underwater. Things can appear 33% larger and 25% closer than they really are. This creates problems for the underwater photographer, spearfisherman, etc. What you thought was a legal catch may get you into trouble when the true fish size becomes apparent after surfacing. It you want to see this effect, put one eye underwater and look at one of your fins that is partially sticking out of the water. Underwater the fin will look larger and closer.
Clearing a mask of water while underwater is a very important skill. It may sound easy to come to the surface if you get water in your mask just to dump it out. Surfacing from a dive takes time and is inconvenient to you and your buddy especially if it is for a problem that could be easily corrected underwater. So, mask-clearing underwater is a major component of any scuba course. It is a simple process, especially if you are able to breathe underwater. The best method is to place the hand lightly on the top of the mask (this keeps the air from escaping), look straight ahead, and then blow gently through the nose. The water should leave through the bottom of the mask because the trapped air pushes it out. If it does not all go out on the first try, inhale more air and do it again. The position of the head during this is very important. If you look down too far the water will stay against the glass. If you look up to much water will get in the nose when the air is not coming out. Some divers find it easier to open the bottom of the mask slightly WHILE AIR IS COMING OUT OF THE NOSE. This allows the water to exit without having to build up pressure in the mask.
Since the mask fits so closely to the face and makes a seal, there is the possibility it will be squeezed against the face when the diver descends. After all it is an air space that is going to be crushed to 1/2 its size at 33 feet. If the diver does nothing about this mask squeeze dire consequences may result. The mask is at lower pressure than the pressure inside the diver's body. The excess molecules under the skin will push the skin into the mask. The eyeballs will bulge into the interior of the mask. The pressure inside the skin of the nose becomes greater than the pressure in the nostrils. Divers have come up with a face that looks like it's been battered. In fact, the problem looks worse than it really is. The nose may bleed, the eyes may have blood vessels break so the whites will turn red, and the skin may look like someone slapped the hell out of it. All of this may be avoided by simply putting a little air in the mask through the nose as you descend in the water. Although it would be more hydrodynamic, divers cannot wear goggles while diving because there is no way of pressurizing the inside of them?
Professional masks for diving are made of glass, not plastic. Glass is hard to scratch. The glass should be tempered. Usually the word, "Tempered" is found on the top of the glass. Tempered means the glass will break in very little pieces rather than in large sharp shards. I have seen a diver fall head first into the ocean off a dock and hit the mask head on. The mask shattered but the diver was not cut (fortunately). Diving into the water with the mask on, dropping the mask in a shower, or packing it tightly in a suitcase may cause it to shatter and become worthless. Placing one hand over the glass lens of the mask when doing an entry is advised.
Lastly, when you store a mask it is important to not have pressure against it. If you were to pack a mask under a weight, for example, the mask may retain the shape it has been forced into for a prolonged period of time. That is why mask boxes are a wise investment.
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