Diving regulator
A diving regulator or underwater diving regulator is a pressure regulator that controls the pressure of breathing gas for underwater diving. The most commonly recognised application is to reduce pressurized breathing gas to ambient pressure and deliver it to the diver, but there are also other types of gas pressure regulator used for diving applications. The gas may be air or one of a variety of specially blended breathing gases. The gas may be supplied from a scuba cylinder carried by the diver, in which case it is called a scuba regulator, or via a hose from a compressor or high-pressure storage cylinders at the surface in surface-supplied diving. A gas pressure regulator has one or more valves in series which reduce pressure from the source, and use the downstream pressure as feedback to control the delivered pressure, or the upstream pressure as feedback to prevent excessive flow rates, lowering the pressure at each stage.
The terms "regulator" and "demand valve" are often used interchangeably, but a demand valve is the final stage pressure-reduction regulator that delivers gas only while the diver is inhaling and reduces the gas pressure to approximately ambient. In single-hose demand regulators, the demand valve is either held in the diver's mouth by a mouthpiece or attached to the full-face mask or helmet. In twin-hose regulators the demand valve is included in the body of the regulator which is usually attached directly to the cylinder valve or manifold outlet, with a remote mouthpiece supplied at ambient pressure.
A pressure-reduction regulator is used to control the delivery pressure of the gas supplied to a free-flow helmet or full-face mask, in which the flow is continuous, to maintain the downstream pressure which is limited by the ambient pressure of the exhaust and the flow resistance of the delivery system and not much influenced by the breathing of the diver. Diving rebreather systems may also use regulators to control the flow of fresh gas, and demand valves, known as automatic diluent valves, to maintain the volume in the breathing loop during descent. Gas reclaim systems and built-in breathing systems use a different kind of regulator to control the flow of exhaled gas to the return hose and through the topside reclaim system, or to the outside of the hyperbaric chamber, these are of the back-pressure regulator class.
The performance of a regulator is measured by the and added mechanical work of breathing, and the capacity to deliver breathing gas at peak inspiratory flow rate at high ambient pressures without excessive pressure drop, and without excessive dead space. For some cold water diving applications the capacity to deliver high flow rates at low ambient temperatures without jamming due to regulator freezing is important.
Purpose
The diving regulator is a mechanism which reduces the pressure of the supply of breathing gas and provides it to the diver at approximately ambient pressure. The gas may be supplied on demand, when the diver inhales, or as a constant flow past the diver inside the helmet or mask, from which the diver uses what is necessary, while the remainder goes to waste.The gas may be provided directly to the diver, or to a rebreather circuit, to make up for used gas and volume changes due to depth variations. Gas supply may be from a high-pressure scuba cylinder carried by the diver, or from a surface supply through a hose connected to a compressor or high pressure storage system.
Types
An open circuit demand valve provides gas flow only while the diver inhales, a free flow regulator provides a constant flow rate at the delivery pressure, reclaim and built-in-breathing-systems regulators allow exhaust outflow only during exhalation. Rebreathers use demand regulators to make up a volume deficit in the loop, and may use constant mass flow regulators to refresh the oxygen content of the loop gas mixture. A scuba diving regulator is used to supply a scuba diver from a scuba cylinder, while a diving helmet demand valve may supply gas from surface supply or a bailout scuba cylinder.Open circuit demand valve
A demand valve detects the pressure drop when the diver starts inhaling and supplies the diver with a breath of gas at ambient pressure. When the diver stops inhaling, the demand valve closes to stop the flow. The demand valve has a chamber, which in normal use contains breathing gas at ambient pressure, which is connected to a bite-grip mouthpiece, a full-face mask, or a diving helmet, either direct coupled or connected by a flexible low-pressure hose. On one side of the chamber is a flexible diaphragm to sense the pressure difference between the gas in the chamber on one side and the surrounding water on the other side, and control the operation of the valve which supplies pressurised gas into the chamber.This is done by a mechanical system linking the diaphragm to a valve which is opened to an extent proportional to the displacement of the diaphragm from the closed position. The pressure difference between the inside of the mouthpiece and the ambient pressure outside the diaphragm required to open the valve is known as the cracking pressure. This cracking pressure difference is usually negative relative to ambient, but may be slightly positive on a positive pressure regulator. Once the valve has opened, gas flow should continue at the smallest stable pressure difference reasonably practicable while the diver inhales, and should stop as soon as gas flow stops. Several mechanisms have been devised to provide this function, some of them extremely simple and robust, and others somewhat more complex, but more sensitive to small pressure changes. The diaphragm is protected by a cover with holes or slits through which outside water can enter freely. This cover reduces sensitivity of the diaphragm to water turbulence and dynamic pressure due to movement, which might otherwise trigger gas flow when it is not needed.
When the diver starts to inhale, the removal of gas from the casing lowers the pressure inside the chamber, and the external water pressure moves the diaphragm inwards operating a lever which lifts the valve off its seat, releasing gas into the chamber. The inter-stage gas, at about over ambient pressure, expands through the valve orifice as its pressure is reduced to ambient and supplies the diver with more gas to breathe. When the diver stops inhaling the chamber fills until the external pressure is balanced, the diaphragm returns to its rest position and the lever releases the valve to be closed by the valve spring and gas flow stops.
When the diver exhales, one-way valves made from a flexible air-tight material flex outwards under the pressure of the exhalation, letting gas escape from the chamber. They close, making a seal, when the exhalation stops and the pressure inside the chamber reduces to ambient pressure.
The vast majority of demand valves are used on open circuit breathing apparatus, which means that the exhaled gas is discharged into the surrounding environment and lost. Reclaim valves can be fitted to helmets to allow the used gas to be returned to the surface for reuse after removing the carbon dioxide and making up the oxygen. This process, referred to as "push-pull", is technologically complex and expensive and is only used for deep commercial diving on heliox mixtures, where the saving on helium compensates for the expense and complications of the system, and for diving in contaminated water, where the gas is not reclaimed, but the system reduces the risk of contaminated water leaking into the helmet through an exhaust valve.
Open circuit free-flow regulator
These are generally used in surface supply diving with free-flow masks and helmets. They are usually a large high-flow rated industrial gas regulator that is manually controlled at the gas panel on the surface to the pressure required to provide the desired flow rate to the diver. Free flow is not normally used on scuba equipment as the high gas flow rates are inefficient and wasteful.In constant-flow regulators the pressure regulator provides a constant reduced pressure, which provides gas flow to the diver, which may be to some extent controlled by an adjustable orifice controlled by the diver. These are the earliest type of breathing set flow control. The diver must physically open and close the adjustable supply valve to regulate flow. Constant flow valves in an open circuit breathing set consume gas less economically than demand valve regulators because gas flows even when it is not needed, and must flow at the rate required for peak inhalation. Before 1939, self contained diving and industrial open circuit breathing sets with constant-flow regulators were designed by Le Prieur, but did not get into general use due to very short dive duration. Design complications resulted from the need to put the second-stage flow control valve where it could be easily operated by the diver.
Reclaim regulators
The cost of breathing gas containing a high fraction of helium is a significant part of the cost of deep diving operations, and can be reduced by recovering the breathing gas for recycling. A reclaim helmet is provided with a return line in the diver's umbilical, and exhaled gas is discharged to this hose through a reclaim regulator, which ensures that gas pressure in the helmet cannot fall below the ambient pressure. The gas is processed at the surface in the helium reclaim system by filtering, scrubbing and boosting into storage cylinders until needed. The oxygen content may be adjusted when appropriate. The same principle is used in built-in breathing systems used to vent oxygen-rich treatment gases from a hyperbaric chamber, though those gases are generally not reclaimed. A diverter valve is provided to allow the diver to manually switch to open circuit if the reclaim valve malfunctions, and an underpressure flood valve allows water to enter the helmet to avoid a squeeze if the reclaim valve fails suddenly, allowing the diver time to switch to open circuit without injury. Reclaim valves for deep diving may use two stages to give smoother flow and lower work of breathing. The reclaim regulator works on a similar principle to the demand regulator, in that it allows flow only when the pressure difference between the interior of the helmet and the ambient water opens the valve, but uses the upstream over-pressure to activate the valve, where the demand valve uses downstream underpressure.Reclaim regulators are also sometimes used for hazmat diving to reduce the risk of backflow of contaminated water through the exhaust valves into the helmet. In this application there would not be an underpressure flood valve, but the pressure differences and the squeeze risk are relatively low. The breathing gas in this application would usually be air and would not actually be recycled.