Self-Contained Underwater Breathing Apparatus – Scuba

SCUBA demand valve (a) during inspiration and (b) exhalation.

The simplest form of breathing apparatus consisted of a gas source and a tap that the diver turned on to obtain each breath of air. This system was in use until the 1930s, but much of the diver’s time and concentration were taken up in operating the tap. In the most common breathing apparatus, the Aqualung or self-contained underwater breathing apparatus (scuba), the tap is replaced by a two-stage valve system. The flow of gas to the diver is triggered by the diver’s inspiratory effort and is closed by expiration or cessation of inspiration.

Figure 4.1 and Figure 4.2 (a) and (b) show the operating principles of a simple regulator and demand valve system. The air is stored in a cylinder at a maximum pressure that is determined by the design of the cylinder. For most cylinders this pressure, called the working pressure, is 160 to 300 bar (2300 to 4350 psi).

First stage reducer valve
Figure 4.1 First stage reducer valve: the gas escapes from the cylinder until the pressure above the piston increases to a level where the force on the piston can compress the spring, pushing the first stage valve seat down and shutting the gas flow off. The valve opens again when the pressure above the piston (and in the hose to the second stage valve) falls. This is normally because the diver has taken another breath.

The first stage of the valve system (see Figure 4.1) reduces the pressure from cylinder pressure to the equivalent of about 10 ATA greater than the pressure surrounding the diver, and it regulates its outlet pressure at this value. The valve is held open by the force of a spring until the pressure above the first stage piston builds up and forces the valve seal down on the seat, thus shutting off the gas. The first-stage valve opens and closes as gas is drawn from the system by the diver. In some regulators, the water can enter the water chamber and helps the spring to hold the valve open. In others, the ambient water pressure is transmitted indirectly. This adjustment of the supply pressure with water pressure is designed to prevent the flow decreasing as the diver descends.

When the diver inhales, he or she reduces the pressure in the mouthpiece chamber, or second-stage valve. As the diver does so, the diaphragm curves inward and depresses the lever (see Figure 4.2 (a)). The inlet valve opens and remains open until inhalation ceases. At this stage, the diaphragm moves back into the position shown in Figure 4.2 (b). The second-stage valve is usually called the demand valve.

SCUBA demand valve (a) during inspiration and (b) exhalation.
Figure 4.2 SCUBA demand valve (a) during inspiration and (b) exhalation. The arrows indicate air flow. During inspiration the diver decreases the pressure in the mouthpiece. This causes the diaphragm to curve in and tilt the air supply valve open. At the end of inspiration air continues to flow until the pressure in the mouthpiece equals the pressure in the water chamber, at this stage the diaphragm will return to the position shown in 4.2(b), and the air supply valve shuts. During exhalation the pressure in the mouthpiece is greater than that in the surrounding water. This pressure difference forces the exhaust valve open and allows exhaled air to escape. The purge button is used to trigger a flow of gas from the supply without the need to inhale from the regulator.

Expired gas passes out of the second stage through an exhaust valve. In the demand valve, gas flow increases with respiratory effort because the valve opens more, allowing the diver to breathe normally. The purge button allows the diver to open the inlet valve to force any water out of the regulator. The diver may need to do this if he or she takes the regulator from the mouth while underwater or if the seal around the mouthpiece is poor.

The scuba regulator is designed to provide the diver with a gas supply matched to his or her respiratory needs.

Most divers have little difficulty using scuba. However, when they first don it, the weight and bulk will make them awkward, and may aggravate back problems. In the water, the buoyancy of the set offsets its weight.

The diver’s lips should be sealed around the mouthpiece to prevent the entry of water. Water can enter through a hole in the mouthpiece if the mouthpiece is poorly attached or through the diaphragm or exhaust valve if either is faulty. A leak can generate an aerosol if the water reaches the inlet valve of the second-stage valve. The aerosol can cause distress to the diver and may sometimes cause a syndrome called salt water aspiration syndrome, or it may trigger other medical conditions such as asthma or possibly a cardiac dysrhythmia.

Another problem associated with demand valves is that they may cause pain in the temporomandibular joint. This condition is considered in Chapter 42.

In very cold water, the first stage of the regulator may ‘freeze up’. This occurs because the air cools as it passes through the first stage and can ice up with the piston frozen in the open position. The problem can be reduced by using a first stage that is designed for operation in cold water.

Because the first stage regulates the pressure to the second stage, the inspiratory effort required to cause a flow does not vary until the cylinder is almost empty. Then the pressure in the hose to the second stage falls and the flow decreases. The diver’s first warning that the cylinder is almost empty is increased resistance on inhalation. However, this warning may be minimal or absent with modern regulators.

Most divers have a console that includes a pressure gauge connected to the cylinder by a hose. Some modern systems transmit the cylinder pressure via radiofrequency signals rather than via a hose. The pressure gauge provides the diver with a measure of the remaining air supply. The contents of the cylinder are proportional to the pressure, so the gauge is often called the ‘contents gauge’. Divers tend to say they have 50 bar left, rather than the volume this represents. A major problem is that a diver who is entranced by the scenery, concentrating on a task or distracted may run out of air because he or she forgets to check the gauge. An audible low-air warning is incorporated into some systems and is valuable. A diver needs to ensure that he or she has a cylinder or cylinders with adequate gas supply for the planned dive, and an additional reserve.

A traditional and almost obsolete system to prevent divers from running out of air is a reserve valve. In operation it resembles a boiler safety valve; the air escapes to the diver until the cylinder pressure falls to the level at which the reserve valve seats. The remainder of the air can be released by pulling a lever that opens the reserve valve. One problem with this is that the valve lever may be inadvertently put into the ‘on’ position, causing the diver to use the reserve of gas without being aware of this. Another common problem is valve failure.