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Breathing performance of regulators
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Breathing performance of regulators
The breathing performance of regulators is a measure of the ability of a breathing gas regulator to meet the demands placed on it at varying ambient pressures and temperatures, and under varying breathing loads, for the range of breathing gases it may be expected to deliver. Performance is an important factor in design and selection of breathing regulators for any application, but particularly for underwater diving, as the range of ambient operating pressures and temperatures, and variety of breathing gases is broader in this application. A diving regulator is a device that reduces the high pressure in a diving cylinder or surface supply hose to the same pressure as the diver's surroundings. It is desirable that breathing from a regulator requires low effort even when supplying large amounts of breathing gas as this is commonly the limiting factor for underwater exertion, and can be critical during diving emergencies. It is also preferable that the gas is delivered smoothly without any sudden changes in resistance while inhaling or exhaling, and that the regulator does not lock up and either fail to supply gas or free-flow. Although these factors may be judged subjectively, it is convenient to have standards by which the many different types and manufactures of regulators may be objectively compared.
Various breathing machines have been developed and used for assessment of breathing apparatus performance. Ansti Test Systems developed a turnkey system that measures the inhalation and exhalation effort in using a regulator, and produces graphs indicating the work of breathing at the set depth pressure and respiratory minute volume for the gas mixture used. Publishing results of the performance of regulators in the ANSTI test machine has resulted in performance improvements.
Breathing performance of the regulator is relevant in all circumstances where a regulator is used to control the flow of breathing gas in a supply on demand system. In some of these applications, a very basic regulator will perform adequately. In other applications, the performance of the regulator may limit the performance of the user. A high-performance regulator for a given combination of gas mixture and ambient pressure is defined as one which will provide a low work of breathing at high RMV, while supplying a flow of gas only when triggered by inhalation, and allowing an outflow of exhaled gas with minimum resistance.
Another aspect of breathing performance is demand regulator performance in cold water, where a high flow rate may cause chilling sufficient to lock up the mechanism with ice, which usually causes a severe free-flow with consequent loss of breathing gas, which can only be stopped by shutting off the cylinder valve.
A healthy person at rest at surface atmospheric pressure expends only a small amount of available effort on breathing. This can change considerably as the density of breathing gas increases at higher ambient pressure. When the energy expended to remove carbon dioxide produces more carbon dioxide than it removes the person will suffer from hypercapnia in a positive feedback cycle ending in unconsciousness and eventually death. Work of breathing is affected by breathing rate, breathing pattern, gas density, physiological factors, and the fluid dynamic details of the breathing apparatus, these being the frictional resistance to flow, and pressure differences required to open valves and hold them open to flow.
Breathing gas density can be reduced by using helium as the basic component, with sufficient oxygen added to suit the circumstances and retain a partial pressure sufficient to sustain consciousness but not so much as to cause oxygen toxicity problems. Frictional resistance to flow is influenced by the shape and size of the gas passages, and the pressure, density, viscosity, and velocity of the gas. Valve cracking pressure is a factor of design and settings of the valve mechanisms. The breathing performance of regulators assumes gas density is specified and measures the resistance to flow during the full breathing cycle with a given volumetric flow rate as a pressure drop between the mouthpiece and the exterior environment.
Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.
The total work of breathing when using a breathing apparatus is the sum of the physiological work of breathing and the mechanical work of breathing of the apparatus. In a normal resting state the physiological work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.
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Breathing performance of regulators
The breathing performance of regulators is a measure of the ability of a breathing gas regulator to meet the demands placed on it at varying ambient pressures and temperatures, and under varying breathing loads, for the range of breathing gases it may be expected to deliver. Performance is an important factor in design and selection of breathing regulators for any application, but particularly for underwater diving, as the range of ambient operating pressures and temperatures, and variety of breathing gases is broader in this application. A diving regulator is a device that reduces the high pressure in a diving cylinder or surface supply hose to the same pressure as the diver's surroundings. It is desirable that breathing from a regulator requires low effort even when supplying large amounts of breathing gas as this is commonly the limiting factor for underwater exertion, and can be critical during diving emergencies. It is also preferable that the gas is delivered smoothly without any sudden changes in resistance while inhaling or exhaling, and that the regulator does not lock up and either fail to supply gas or free-flow. Although these factors may be judged subjectively, it is convenient to have standards by which the many different types and manufactures of regulators may be objectively compared.
Various breathing machines have been developed and used for assessment of breathing apparatus performance. Ansti Test Systems developed a turnkey system that measures the inhalation and exhalation effort in using a regulator, and produces graphs indicating the work of breathing at the set depth pressure and respiratory minute volume for the gas mixture used. Publishing results of the performance of regulators in the ANSTI test machine has resulted in performance improvements.
Breathing performance of the regulator is relevant in all circumstances where a regulator is used to control the flow of breathing gas in a supply on demand system. In some of these applications, a very basic regulator will perform adequately. In other applications, the performance of the regulator may limit the performance of the user. A high-performance regulator for a given combination of gas mixture and ambient pressure is defined as one which will provide a low work of breathing at high RMV, while supplying a flow of gas only when triggered by inhalation, and allowing an outflow of exhaled gas with minimum resistance.
Another aspect of breathing performance is demand regulator performance in cold water, where a high flow rate may cause chilling sufficient to lock up the mechanism with ice, which usually causes a severe free-flow with consequent loss of breathing gas, which can only be stopped by shutting off the cylinder valve.
A healthy person at rest at surface atmospheric pressure expends only a small amount of available effort on breathing. This can change considerably as the density of breathing gas increases at higher ambient pressure. When the energy expended to remove carbon dioxide produces more carbon dioxide than it removes the person will suffer from hypercapnia in a positive feedback cycle ending in unconsciousness and eventually death. Work of breathing is affected by breathing rate, breathing pattern, gas density, physiological factors, and the fluid dynamic details of the breathing apparatus, these being the frictional resistance to flow, and pressure differences required to open valves and hold them open to flow.
Breathing gas density can be reduced by using helium as the basic component, with sufficient oxygen added to suit the circumstances and retain a partial pressure sufficient to sustain consciousness but not so much as to cause oxygen toxicity problems. Frictional resistance to flow is influenced by the shape and size of the gas passages, and the pressure, density, viscosity, and velocity of the gas. Valve cracking pressure is a factor of design and settings of the valve mechanisms. The breathing performance of regulators assumes gas density is specified and measures the resistance to flow during the full breathing cycle with a given volumetric flow rate as a pressure drop between the mouthpiece and the exterior environment.
Work of breathing (WOB) is the energy expended to inhale and exhale a breathing gas. It is usually expressed as work per unit volume, for example, joules/litre, or as a work rate (power), such as joules/min or equivalent units, as it is not particularly useful without a reference to volume or time. It can be calculated in terms of the pulmonary pressure multiplied by the change in pulmonary volume, or in terms of the oxygen consumption attributable to breathing.
The total work of breathing when using a breathing apparatus is the sum of the physiological work of breathing and the mechanical work of breathing of the apparatus. In a normal resting state the physiological work of breathing constitutes about 5% of the total body oxygen consumption. It can increase considerably due to illness or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.
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