Diving and exposure to high pressures change the heat transfer from a diver’s body. In air, there is some insulation from the air trapped near the body, either by the clothes or the hair and the boundary layer. In water this is lost. The water adjacent to the skin is heated, expands slightly, and causes a convection current that tends to remove the layer of warmed water. This process is accelerated by movement of the diver or the water. The net result is that a diver cools or heats up much more quickly than he or she would in air of the same temperature.
Heat loss is also increased in warming the cooler inhaled air or gas. For a diver breathing air, most of this heat is used to humidify the dry air used for diving and is not sufficient to cause concern in most circumstances. However, the heat lost in a helium dive is more significant. Helium has a greater specific heat than nitrogen. The problem is compounded because at depth, the mass of gas inhaled is increased.
The heat transfer by conduction is also increased in a helium environment. The result is that a helium diver may need external heating to maintain body warmth at a water, or gas, temperature where external warming would not be required if the diver was in an air environment.
In warm environments, it is possible for a diver to suffer heat stress. A diver who is wearing a protective suit cannot lose heat by sweating because the sweat cannot evaporate. In a pressure chamber, the atmosphere can become saturated with water, and evaporative cooling is prevented. The heat stress for a given temperature is also increased if there is helium in the mixture.
Despite wearing thermal insulation in warm tropical waters, divers can continue to lose heat over several days of repetitive diving, and ‘silent’ hypothermia can develop, somewhat insidiously.
A diver in water or a helium-rich environment can cool or heat up at a temperature that would be comfortable in an air environment.
Even in the cleanest ocean water, only about 20 per cent of the incident light reaches a depth of 10 metres and only 1 per cent reaches 85 metres. Clean water has a maximum transparency to light with a wave length of 480 millimicrometres (blue). This variation of absorption with wave length causes distortion of colours and is responsible for the blue-green hues seen at depth. Red and orange light is absorbed most. Because of the absorption of light, the deep ocean appears black, and lights are needed for observation or photography. Because of the greater absorption of reds by water, some illumination is needed to see the true colours, even at shallow depths. Part of the appeal of diving at night is that objects that have a blue-green colour in natural light have a new brightness when they are illuminated with a torch.
Coastal water, with more suspended material, has a maximum transparency in the yellow-green band, about 530 millimicrometres. Absorption and scattering of light by suspended particles restrict vision and can tend to even out illumination. This can make the light intensity the same in all directions and is an important factor in causing loss of orientation.
When the eye focusses on an object in air, most of the refraction of light rays occurs at the air–cornea interface. In water, this refractive power is lost and the eye is incapable of focussing. A face mask provides an air-cornea boundary, which restores refraction at the cornea surface to normal. Refraction also occurs at the face mask surface, mainly at the glass–air boundary. This results in an apparent size increase of about 30 per cent and this makes objects appear closer than they are. Practice and adaption of the hand–eye co-ordination system allow the diver to compensate for this distortion, except when describing the size of fish.
Masks also restrict vision by narrowing the peripheral fields, and they distort objects that subtend large visual angles. Both absorption of light by water, which reduces apparent contrast, and scattering by suspended particles reduce visual acuity. Attempts have been made to improve the diver’s vision by modification of the face mask, the use of coloured filters, ground mask lenses and contact lenses. These can be relatively successful but can also impose their own problems.
Sound in water is transmitted as waves with a longitudinal mode of vibration. The speed of sound is about 1530 metres/second in sea water and 1470 metres/second in fresh water at 15°C. Water is a better transmitter of sound than air, so sounds travel greater distances under water. Low-pitched sounds travel farther than higher-pitched sounds. Transmission of sound is enhanced by reflection from the surface. This reflection also enhances the transmission of sound in air over water but reduces the transmission of sounds from air to water and from water to air.
Both high-pressure air and helium-oxygen mixtures cause speech distortion. This is greater when breathing helium mixtures and can render speech unintelligible. Distortion in air causes the voice to become more nasal and crisp as the pressure increases.
It is often thought that divers cannot talk underwater. This is not so if the diver has an air space to speak into. Helmet divers can communicate easily by touching their helmets together and using the air-metal-air pathway. Some scuba divers have mastered the art of talking by taking their demand valve from their mouth and speaking into an air space created by cupping their hands.