Have you ever asked to your instructor how was calculated/ came-up-with the ascent rate of the tables or computer that you used?
Do you think it reflects basic fact of human physiology?
Well, the short answer is no. The selection was made by the USA NAVY based on a compromised between work and safety.
At the end of the 50th, was well known that ascent was an important element in decompression, and different ascent rates can not be used with the same table. Which means that, for the NAVY, produced and man-test a bunch of tables with different ascent rates was economically and time consuming, totally impractical.
Let’s put this in a historical context, in 58 the NAVY was starting to work in a new air tables, the demolition dive team insisted that divers couldn’t possibly ascend as slowly as 25 ft./min; for them, more like 100 ft./min., or even faster.
The hard-hat divers insisted that such a procedure wouldn’t be practical for hauling up divers in suit and helmet.
“Facing such different opinions on diving’s ascent procedures they compromise with some middle decisions, 60 ft./min. In the making, procedure will be modified to accommodate certain circumstances accordingly with the type of diving being made.”
There you go!
Today’s ascent rate of 60 ft./min. is considered the highest velocity, the recommendation are 30 ft./min. with open circuit.
This procedure went into practice with the advent of the doppler ultrasound-imaging.
Now I’m going ask you: why a dive beyond 60 ft. is considered a deep dive?
If you want to read more, check the following sources:
- Sixty Feet Per Minute. Why? by the Rev. Edward H. Lanphier, M.D.
- Boycott, A.E., Damant G.C.C., Haldane, J.S. “The prevention of compressed air illness. J. Hyg. Camb, 1908; 8:342-443.2. U.S. Navy Department (Bureau of Ships).
- Diving Manual,NAVSHIPS 250-880 Washington: Superintendent of Documents, 1952.