SAC Rate and the Diver Who Wants to Quit
A certified diver on a day boat asks you, the divemaster, why their air lasts less than their buddy's. Their computer log shows a dive to an average depth of 18 m for 32 minutes. They started with 200 bar in an 11.1 L cylinder and ended with 70 bar. Their buddy, using a 12 L cylinder, ended with 90 bar on the same profile. The diver is physically fit, 45 years old, and recently certified. They ask whether their SAC rate is dangerous and why a deeper dive to 30 m would be even shorter. The diver is worried they are 'unfit for diving' and asks whether they should quit the sport. You have rescue-level first aid training but are not a physician. The boat is returning to shore in 40 minutes, so you have time for a short explanation. You have the dive profile and cylinder sizes in front of you.
1. Calculate the diver's SAC rate in L/min, showing each step.
Read the worked answer
Use surface air consumption (SAC) in litres per minute at the surface:
- Gas used in bar: 200 − 70 = 130 bar.
- Gas used in litres: 130 bar × 11.1 L = 1,443 L. This is the volume at surface pressure that was consumed.
- Average ambient pressure: 18 m ÷ 10 = 1.8 ATA above surface, plus 1 ATA = 2.8 ATA.
- SAC: 1,443 L ÷ (32 min × 2.8 ATA) = 1,443 ÷ 89.6 = 16.1 L/min.
Interpretation: 16 L/min is a common, moderate SAC for a recreational diver; it is not 'dangerous' by itself. It is higher than a very relaxed diver might see (12–15 L/min) but well within the range many divers show. The buddy's lower consumption may reflect body size, fitness, relaxation, trim, or simply less finning-not necessarily a better diver.
2. Explain why the same diver would use gas faster at 30 m.
Read the worked answer
Gas consumption at depth scales with ambient pressure. At the surface, 1 ATA; at 10 m, 2 ATA; at 30 m, 4 ATA. If the diver's SAC is 16.1 L/min at the surface, at 30 m the same effort uses about 16.1 × 4 = 64.4 L/min at depth. An 11.1 L cylinder filled to 200 bar holds 2,220 L of surface-equivalent gas. At 64.4 L/min, that is about 34 minutes total if the diver used every litre, leaving no reserve; with a 50-bar reserve (555 L), usable gas is 1,665 L, or about 25 minutes at 30 m. The diver's 32-minute dive at 18 m used 1,443 L, but at 30 m the same duration would require roughly 32 × 64.4 = 2,061 L-nearly the whole cylinder. This is why deeper dives need shorter bottom times or larger cylinders, and why gas planning must use the planned depth, not the average depth alone.
3. What practical advice would you give, and what would you not conclude?
Read the worked answer
Advice: (1) Plan gas using turn pressure, not just 'back on the boat with 50 bar.' For this diver, a 70-bar turn at 18 m was reasonable, but the dive should have been planned to reach that earlier. (2) Improve relaxation and trim: slow down, avoid unnecessary finning, streamline, and breathe normally-never skip breaths or hold breath. (3) Check weighting; overweighting causes drag and extra effort. (4) Compare SAC over several dives before drawing conclusions; one dive is a data point, not a diagnosis. (5) If the diver wants a personalised number, use a SAC calculator or dive computer average. Do not: tell them they are unfit to dive, diagnose asthma or heart disease, or advise them to ignore safety stops to save gas. If they have breathlessness at rest or a medical condition, refer to a physician. A higher SAC is a planning issue first, a medical issue only if there are other symptoms.