Blonde Girl With A Diving Mask In The Sea

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.