shakeybrainsurgeon
Contributor
These questions came up on a nitrox thread and I thought they were interesting... I seached recent threads about ascent rates and could find no clear answers:
1) What is the maximum ascent rate of an uncontrolled ascent for a typical diver, from say, 200 feet? Not the maximum SAFE ascent rate, there have been numerous discussions on that, the actual rate. For example, a man in a 7 mil suit plans a wall dive to 120 ft but is distracted, loses buoyancy control and finds himself descending. At 200 ft he dumps his weights but doesn't dump his air and thus winds up in an uncontrolled buoyant ascent. How fast will he be going at 100 ft? 60 ft? At the surface? Just as a skydiver hits a terminal velocity due to air resistance after falling a certain distance, the diver may reach a maximal ascent rate and fairly quickly --- resistance goes up by the cube of velocity in water, only the square of the velocity in air. Will he reach that velocity in 200 ft or will he still be accelerating as he hits the atmosphere? In a thread posting last summer, a "rocket rate" of 180 fpm was quoted, three times the maximum safe ascent rate, but no source was given. Could rates as high as 300 fpm be possible? This would be hard to calculate and would have to be measured using a weighted mannekin, but someone may know the answer.
2) At what ascent rate does pulmonary barotrauma become problematic EVEN WITHOUT BREATH HOLDING? Several facts:1) in the final 30 ft of ascent, it takes only about 4 ft of ascent with breath holding to cause gas embolism --- put another way, the lungs can withstand only about 10-15% increases over atmospheric pressure before barotrauma occurs and 2) the healthy airways, at full forced expiration, can empty 70 % of lung volume in one second at 1 atmosphere, and 90% in about 3 seconds. Lungs afflcited with emphysema or asthma, due to airway obstruction, exhaust air much slowly, taking up to 5 seconds or even longer to exhaust half the lung capacity. Moreover, the denser air at depth takes a bit longer to exhaust.
Thus, even with an open glottis and forced muscular expiration, it takes normal lungs several seconds to equalize and diseased lungs even longer. Thus, if the "rocket" ascent rate equals or exceeds 4 ft/sec (240 fpm), even healthy lungs may suffer barotrauma (and diseased lungs almost certainly will) during an uncontrolled ascent because the rate of equalization, even with expiration, is too slow to match the rate of ascent.
It would be interesting to look at the cases of uncontrolled ascents to examine the risk of AGE as a function of a) pulmonary health (smokers, asthmatics vs normal) and b) the depth at the time of ascent, since that determines the maximum rate of ascent.
1) What is the maximum ascent rate of an uncontrolled ascent for a typical diver, from say, 200 feet? Not the maximum SAFE ascent rate, there have been numerous discussions on that, the actual rate. For example, a man in a 7 mil suit plans a wall dive to 120 ft but is distracted, loses buoyancy control and finds himself descending. At 200 ft he dumps his weights but doesn't dump his air and thus winds up in an uncontrolled buoyant ascent. How fast will he be going at 100 ft? 60 ft? At the surface? Just as a skydiver hits a terminal velocity due to air resistance after falling a certain distance, the diver may reach a maximal ascent rate and fairly quickly --- resistance goes up by the cube of velocity in water, only the square of the velocity in air. Will he reach that velocity in 200 ft or will he still be accelerating as he hits the atmosphere? In a thread posting last summer, a "rocket rate" of 180 fpm was quoted, three times the maximum safe ascent rate, but no source was given. Could rates as high as 300 fpm be possible? This would be hard to calculate and would have to be measured using a weighted mannekin, but someone may know the answer.
2) At what ascent rate does pulmonary barotrauma become problematic EVEN WITHOUT BREATH HOLDING? Several facts:1) in the final 30 ft of ascent, it takes only about 4 ft of ascent with breath holding to cause gas embolism --- put another way, the lungs can withstand only about 10-15% increases over atmospheric pressure before barotrauma occurs and 2) the healthy airways, at full forced expiration, can empty 70 % of lung volume in one second at 1 atmosphere, and 90% in about 3 seconds. Lungs afflcited with emphysema or asthma, due to airway obstruction, exhaust air much slowly, taking up to 5 seconds or even longer to exhaust half the lung capacity. Moreover, the denser air at depth takes a bit longer to exhaust.
Thus, even with an open glottis and forced muscular expiration, it takes normal lungs several seconds to equalize and diseased lungs even longer. Thus, if the "rocket" ascent rate equals or exceeds 4 ft/sec (240 fpm), even healthy lungs may suffer barotrauma (and diseased lungs almost certainly will) during an uncontrolled ascent because the rate of equalization, even with expiration, is too slow to match the rate of ascent.
It would be interesting to look at the cases of uncontrolled ascents to examine the risk of AGE as a function of a) pulmonary health (smokers, asthmatics vs normal) and b) the depth at the time of ascent, since that determines the maximum rate of ascent.