Half-Time — What It Is and Why It Matters
Half-time is the time required for a tissue compartment to absorb or release half the difference between its current gas tension and the ambient gas pressure. A tissue with a 5-minute half-time reaches 50% saturation in 5 minutes, 75% in 10 minutes, 87.5% in 15 minutes, and is considered effectively saturated (over 98%) after six half-times (30 minutes).
The same math applies in reverse during off-gassing on ascent. Dive computers model multiple compartments with different half-times (from 4 to 635 minutes in Bühlmann ZHL-16C) to account for the fact that different body tissues absorb and release gas at vastly different rates.
Frequently Asked Questions
Why do faster half-time compartments matter for deep dives?
Fast compartments (4 to 12.5 minute half-times) saturate quickly and reach their safe limits first on short, deep dives. They are the controlling tissues that determine your NDL when you are deep but have not been there long. Ascending relieves them quickly.
How long does it take to fully off-gas after a dive?
The slowest compartment (635 minutes) takes about 6 half-times — roughly 63 hours — to reach effective equilibrium. This is why flight-after-diving guidelines recommend waiting 12 to 24 hours and why multi-day diving gradually reduces NDLs as slow tissues accumulate nitrogen.
Affiliate Disclosure: DiveComputers is a participant in the Amazon Services LLC Associates Program and eBay Partner Network. Links on this page may earn us a commission at no extra cost to you.