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Forgive me adding a bit to recent comments/questions on this subject.

The old (1943?) U.S.Navy air table, with depth/time combinations designed
for surface-supplied diving and 25 feet per minute ascent rate, was in
widespread use until the late 1950s when the current USN tables were
published. The deepest no decompression dive on the old table was 130 feet
for 15 minutes.

Also, as Alex Brylske points out in the February issue of Dive Training
magazine, in the early days of 1800-2000 psi tanks, at 130 feet  a single
tank would only last about 10-15 minutes.

Brylske also mentions that 130 feet/ 40 meters is about 132 feet = 5 ATA,
so PO2 is equivalent to 100% O2 at the surface - perhaps the threshold for
toxicity problems. He also cites OSHA's "legal" definition of recreational
diving as no decompression diving to  a maximum of 132 feet.

Finally, as many writers commented, 130 feet falls about where many experts
think N2 narcosis becomes a potential threat to typical air divers.

Aloha

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