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Breathing performance of regulators

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A typical graph produced when testing the breathing performance of a diving regulator

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 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 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. Although these factors may be judged subjectively, it is convenient to have a standard by which the many different types and manufactures of regulators may be compared.

Various breathing machines have been developed and used for assessment of breathing apparatus performance.[1] 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.[2] Publishing results of the performance of regulators in the ANSTI test machine has resulted in performance improvements.[3]

Applications

Breathing performance of the regulator is relevant in all circumstances where a demand regulator is used to provide breathing gas. 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 will provide a low work of breathing at high RMV.

  • Scuba diving – All breathing gas is carried in high-pressure cylinders by the diver
    • Recreational scuba diving – Air and nitrox at ambient pressures up to about 30 msw
    • Technical diving – Ambient pressures may significantly exceed 30 msw
      • Mixed gas – Breathing gases containing helium to limit narcosis and work of breathing
      • Decompression gas – Breathing gases with high oxygen partial pressures, generally not very high ambient pressure
  • Surface-supplied diving – Breathing gas supplied from the surface at a wide range of depths
    • Open circuit – Gas is released into the environment and lost on exhalation
    • Breathing gas reclaim systems – Helium based breathing gas is returned to the surface for recycling via an exhaust regulator to save expensive gas
  • Built-in breathing systems in hyperbaric environments – High oxygen content gas is vented to the exterior via an exhaust regulator to avoid high fire risk. High performance not generally needed, as the user is normally resting.
  • Oxygen administration for first aid in diving accidents – High oxygen fraction at surface pressure, fairly low flow rate, but user may be injured and have difficulty breathing.
  • Breathing apparatus for work in unbreathable atmospheres – Usually at ambient pressures close to normal atmospheric pressure, breathing air. Work rate can be high but should not be extreme. Positive pressure breathing may be used in toxic atmospheres to reduce risk of contamination due to leaks.
    • Self-contained breathing apparatus (SCBA) for rescue and firefighting – Users may have to work hard in difficult conditions, but pressure range is generally close to normal atmospheric pressure. Work rate may be extreme in emergencies. Positive pressure masks may be used, which will offset the pressure graph, but not necessarily increase the net work of breathing.
  • Emergency breathing gas supply in submarines (BIBS) – Survival conditions, at unpredictable pressures.
  • Oxygen supply for unpressurized aircraft – Low ambient pressure due to high altitude. Air supply enriched by additional oxygen. Flow rate not expected to be very high

Relevance

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, the namely 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.

Measurement

Work of breathing

Main article: Work of breathing

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.[4][5]

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[6] or constraints on gas flow imposed by breathing apparatus, ambient pressure, or breathing gas composition.

ANSTI machine

The ANSTI Breathing Simulator is rated to a maximum working pressure of 100 msw. It uses a piston mechanism to provide an accurate and repeatable volume displacement with a sine wave drive mechanism. It has adjustable tidal volume and breathing rate settings which can providing ventilation rates from 10 to 180 litres per minute.[7]

EU Standards

In the European Union the standard EN250:2000 Respiratory equipment. Open-circuit self-contained compressed air diving apparatus. Requirements, testing, marking defines minimum performance standards for "Open-circuit self-contained compressed air diving apparatus",[8] and BS 8547:2016 defines requirements for demand regulators to be used at depths exceeding 50 m.[9] EN 13949: 2003 – Respiratory Equipment – Open Circuit Self-Contained Diving Apparatus for use with Compressed Nitrox and Oxygen – Requirements, Testing, Marking defines requirements for regulators to be used with raised levels of oxygen.[10]

The standard contains limits on inhalation and exhalation pressures and overall work of breathing. It specifies the following, under test conditions of a breathing rate of 62.5 litres (2.2 cu ft) per minute and an ambient pressure of 6 bars (600 kPa):

  • Work of breathing: <3.0 joules per litre
  • Peak respiratory pressure: ±25 mbar (±2.5 kPa) (inhalation or exhalation)
  • Inhalation work of breathing: <0.3 joule per litre
  • Pressure spikes with no measurable positive work of breathing: <10 mbar (1 kPa)
  • Pressure spikes with measurable positive work of breathing: <5 mbar (0.5 kPa)

Although a regulator meeting the above limits will supply sufficient air where the first stage feeds a single second stage, it is not necessarily capable of supplying sufficient air in all circumstances when a single first stage feeds two second stages simultaneously.[11]

In Europe, EN 250: 2014 – Respiratory Equipment – Open Circuit Self - Contained Compressed Air Diving Apparatus – Requirements, Testing and Marking defines the minimum requirements for breathing performance of regulators,[10] and BS 8547:2016 defines requirements for demand regulators to be used at depths exceeding 50 m.[9] EN 13949: 2003 – Respiratory Equipment – Open Circuit Self-Contained Diving Apparatus for use with Compressed Nitrox and Oxygen – Requirements, Testing, Marking.[10] defines requirements for regulators to be used with raised levels of oxygen.

EN 15333 – 1: 2008 COR 2009 – Respiratory Equipment – Open-Circuit Umbilical Supplied Compressed Gas Diving Apparatus – Part 1: Demand Apparatus. and EN 15333 – 2: 2009 – Respiratory Equipment – Open-Circuit Umbilical Supplied Compressed Gas Diving Apparatus – Part 2: Free Flow Apparatus.[10]

I.S. EN 14143: 2013 – Respiratory Equipment – Self-Contained Re-Breathing Diving Apparatus[10] defines minimum requirements for rebreathers.

US Military

In the United States Military, the standard for single-hose scuba regulators was MIl-R-24169B, now withdrawn.[12][13][14][15]

See also

References

  1. ^ Reimers, S. D. (1973). "Performance Characteristics and Basic Design Features of a Breathing Machine for Use to Depths of up to 3000 Feet of Sea Water". US Navy Experimental Diving Unit Technical Report. NEDU-20-73. Retrieved 2008-06-12.
  2. ^ "Demand Regulator Test Station". Ansti Test Systems Ltd. 2003-06-19. Retrieved 2009-09-14.
  3. ^ "The ANSTI Machine: Evaluating A Regulator's Breathing Characteristics - Scuba Diving Magazine". Archived from the original on 2010-02-09.
  4. ^ Medical Dictionary for the Health Professions and Nursing. S.v. "work of breathing." Retrieved September 8, 2015, from http://medical-dictionary.thefreedictionary.com/work+of+breathing
  5. ^ Medical Dictionary. S.v. "work of breathing." Retrieved September 8, 2015, from http://medical-dictionary.thefreedictionary.com/work+of+breathing
  6. ^ Mosby's Medical Dictionary, 8th edition. S.v. "work of breathing." Retrieved September 8, 2015, from http://medical-dictionary.thefreedictionary.com/work+of+breathing
  7. ^ staff. "Life Support Equipment Test Facility" (PDF). Retrieved 18 November 2016.
  8. ^ ISBN 0-580-35713-9 British Standards Institution
  9. ^ a b Committee PH/4/7 (31 March 2016). BS 8547:2016 - Respiratory equipment. Breathing gas demand regulator used for diving to depths greater than 50 metres. Requirements and test methods. London: British Standards Institute. ISBN 978 0 580 89213 4.{{cite book}}: CS1 maint: numeric names: authors list (link)
  10. ^ a b c d e Staff (August 2014). "Diving Breathing Apparatus" (PDF). Diving Standards. Dublin: Health and Safety Authority. Archived from the original (PDF) on 2016-11-18. Retrieved 18 November 2016.
  11. ^ Anthony, T G; Fisher, A. S.; Gould, R. J. "HSE publishes research on performance of 'octopus' regulators for scuba diving" (PDF). Research Report 341. Norwich, UK: HMSO. ISBN 0 7176 6101 6. Retrieved 18 November 2016.
  12. ^ Middleton, JR (1980). "Evaluation of Commercially Available Open Circuit Scuba Regulators". US Navy Experimental Diving Unit Technical Report. NEDU-2-80. Retrieved 2008-06-12.
  13. ^ Morson, P. D. (1987). "Evaluation of Commercially Available Open Circuit Scuba Regulators". US Navy Experimental Diving Unit Technical Report. NEDU-8-87. Retrieved 2008-06-12.
  14. ^ Warkander, D. E. (2007). "Comprehensive Performance Limits for Divers' Underwater Breathing Gear: Consequences of Adopting Diver-Focused Limits". US Navy Experimental Diving Unit Technical Report. NEDU-TR-07-02. Retrieved 2008-06-12.
  15. ^ Staff (22 February 1982). "MIL-R-24169 › Regulator, Air Demand, Single Hose, Diver S". US Department of Defence. Retrieved 27 November 2016.
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