Posted By Merv Newman Can anyone tell me how much air we breath ? volume of air per inhalation,, rate per minute etc ? Thanks Merv Newman

Posted By Ciaran McAleenan Merv The following extract from 'Confined Spaces Expert' should help with your enquiry. "From birth we breathe in the region of 18-20 times per minute under normal conditions, although this may vary according to an individual's metabolism. We can however approximate the rate of air consumption for various activities in order to estimate an average ratio of work rate against air consumption. This information will give an appreciation of the volume of air we need to cope with different circumstances. The table below outlines what is generally accepted as air consumption rates across a range of activities. At rest (in bed) 8l/min = approx. air consumption At rest (standing) 10.5l/min = approx. air consumption Walking (3.2kph) 18.5l/min = approx. air consumption Walking (4.8kph) 25l/min = approx. air consumption Walking (6.4kph) 38l/min = approx. air consumption Walking (8kph) 61l/min = approx. air consumption It is usually assumed that the air consumption for the average working rate is roughly equivalent to the air consumed when walking at a rate of 6.4kph." Regards Ciaran

Posted By Stuart Nagle Hi, Merv Oxygen. Normal atmosphere is 20.8% Oxygen, usually refered to as 21% Normal Person: On inhaling will absorb approximately 3.5% of the available oxygen, and exhale approximately 17.5% of the oxygen breathed in. Thats why mouth to mouth works so well !! This is also the reason why we humans can become affected so quickly by drops in the oxygen content in the atmosphere. For information gas monitors for confined spaces working are set to alarm when oxygens levels reach a depleted level of 19% and also go off when oxygen levels are enriched at 23% (too much can be just as dangerous as too little !!). Hope this sorts out the gobbledegook for you merv. Best regards... Stuart Nagle

Posted By Adrian Watson Oxygen is necessary for all metabolic processes. It is used to metabolise food to produce energy. The amount of oxygen consumed depends upon the individuals basal metabolism and the persons work load. The process of metabolising food produces carbon dioxide. This is mainly removed through the respiratory process. There are a number of methods of calculating oxygen consumption from surface area and/or body mass and workload. Oxygen consumption for a seated resting person ranges from 0.25-0.3 l/min and for a low work rate ranges from 0.5-1.0 l/min to 4-4.5 l/min for high work rates. Atmospheres with Oxygen concentrations below 13% are dangerous in that a persons judgement becomes faulty and they become emotionally unstable, and when the oxygen concentration falls below 6% people rapidly lose consciousness, fall into a coma and die. Fitting the task to the human 5th Edn A textbook of human ergonomics KHE Kroemer and E Grandjean p 118 table 6.6 Indoor Climate DA McIntyre p 92 Hunter’s Diseases of Occupations 9th Edn PJ Baxtor, PH Adams, Tar-Ching Aw, A Cockcroft & J M Harrington

Posted By Nigel L As has been suggested, the use of percentage by volume as an indicator for acceptable oxygen levels, can be problematic. The crucial issue is the partial pressure of the oxygen, a common alarm threshold is 190 millibar, 160 millibar is a commonly used threshold for the initiation of hypoxia. This may seem an overly complicated method, however air at sea level contains roughly 21% oxygen (210 millibar), take a cylinder filled with this to altitude where the ambient pressure is say 0.5 bar and that cylinder still contains 21% oxygen but now at 105 millibar (enough for the breather to be showing serious symptoms of hypoxia), Of course conversely take the same cylinder to 140 metres seawater and that 21% cylinder is now 3150 millibar PO2, enough for the breather to suffer a grand mal seizure from acute oxygen toxicity. Interestingly, in normal circumstances oxygen levels play little part in the volume of gas that is being exhaled or inhaled, this normally toggles on the CO2 levels creating the stimulus to inhale/exhale. In the UK it has been for some time standard practice to inflate the PO2 in Offshore Divers emergency reserve gas (called a bailout) upto 2500millibar, which increases the level of saturated oxygen for metabolism (should the diver passout). I am interested in the consumption figures for gas and their origin. Primarily as I am struck by the thought that should a diver go onto his emergency gas he will return immediately to the Bell, he will know his primary and secondary supplies have failed and be fully aware of the gravity of his situation, if he is not swimming then he will be running across the seabed in wellington boots, over obstacles, through mud before climbing his umbilical upto the bell, I would propose that he is expending considerably more energy than a chap walking at 8kph. Yet the common UK norm for emergency gas reserve is 42.5 ltrs. Norway has a statutory level of 62 ltrs, though I have heard it said that this is so much gas that it is difficult to physically consume it through Breathing Apparatus. Obviously the more gas reserve the better, yet the diver has to carry it and so there must be balance for commercial deep diving, cylinders that would keep a sports diver happily breathing for a couple of hours on a coral reef will only contain enough gas for a couple of minutes at the bottom of the North Sea. Regards Nigel