Breathing performance of regulators
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.
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.
[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.
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.
[4] 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.
[6] 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.
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.
In a normal resting state the physiological work of breathing constitutes about 5% of the total body oxygen consumption.
U.S. Navy Experimental Diving Unit's unmanned cold water test procedures (1994) have been used as an unofficial standard for cold water testing by various military users and major equipment manufacturers.
The standard also set requirements for failure modes and effects analysis, and other issues relating to manufacturing, quality assurance and documentation.
This standard drew attention to issues with a lot of existing equipment, and led to major improvements in open circuit regulator performance.
The US Navy procedure has been to test regulators primarily at a depth of 190 fsw (58 msw) in water 28 to 29 °F (−2 to −2 °C) at a very high breathing rate of 62.5 lpm for a minimum of 30 minutes, with inlet pressure to the first stage of 1,500 pounds per square inch (100 bar), which results in an average second stage inlet temperature of around 7 °F (−14 °C), compared to an average of −13 °F (−25 °C) if 3,000 pounds per square inch (210 bar) would be used.
EN 14143: 2013 – Respiratory Equipment – Self-Contained Re-Breathing Diving Apparatus defines minimum requirements for rebreathers.
