Principles in Decompression-Table Calculations 



The first consideration in calculating decompression tables is that, when a 

 diver ascends from depth, the PI in his tissues not exceed a certain value 

 above that of the ambient pressure. It is generally accepted that the ratio 

 between the inert gas in the tissues and in the ambient pressure, called the 

 supersaturation ratio, should not exceed 1.5/1 in the slowest tissue on sur- 

 facing (defined hereafter) . It might be mentioned that oxygen partial pressure 

 in the tissues is believed not to be a consideration in decompression, since O2 

 is, in general, so rapidly utilized in tissue metabolism that inert-gas bubble 

 formation is not appreciably affected by it. 



The second factor of vital concern in decompression-table computation is the 

 rate at which the various bodily tissues become saturated with and eliminate 

 inert gases. These rates are generally accepted as being exponential, and are 

 typically described in terms of half-saturation time or tissue half-time--that 

 is, the interval required for a tissue to respond to a change in the PI of the 

 inspired breathing mixture by saturating (or desaturating) to half the gradient 

 formed between the gas in the tissue and in the breathing mixture. 



The slowest tissue — the one requiring the longest time to saturate or 

 desaturate--is the controlling one in any decompression procedure in saturation 

 diving. Except for blood, however, the half-times of specific human tissues 

 (Ti-) are, unfortunately, not known. The inherent difficulty in making accurate 

 determinations is that tissue samples cannot easily be analyzed for gas uptake 

 and elimination, although certain animal studies have suggested T'S for analagous 

 human tissues. 



Diving-table calculations are therefore predicated on an assumed value (in 

 minutes) for the slowest T}^ and on the amount of excess PI--supersaturation-- 

 which that tissue can withstand at all increments during the decompression 

 procedure. 



Decompression can be accomplished in two manners. The first is incremental, 

 through a series of "stops" at various pressures, which are calculated to 

 raise the slowest tissue's supersaturation ratio to its maximum safe value at 

 each stop. The duration of each stop is calculated so that a safe PI is reached 

 before "ascent" to the next stop and the reestablishraent of another maximum 

 supersaturation ratio. Or, alternately, decompression can be carried out at a 

 continuous rate of ascent, so that maximum safe tissue supersaturation is never 

 exceeded at any time. 



Empirical data have confirmed that the degree of supersaturation that tissues 

 can withstand varies with depth. Greater pressure changes are better tolerated 

 at deep stops than at shallow ones. Furthermore, breathing 1007o oxygen 

 increases the rate of inert-gas elimination and, therefore, the safe rate of 

 ascent; hence it decreases decompression time. 



The calculation of wholly adequate decompression tables in saturation diving 

 is hampered by lack of accurate values for the slowest tissue half-time. As 

 an example of the inadequacy of existing data, the table that was initially 

 calculated and tested for use in the TEKTITE I 50-ft (so-called) mission 



IX-16 



