Where "579 gallons" comes from: Catalina's aluminum 80, the industry's standard 80, holds an actual 77.4 cubic feet of air[2], which is 77.4 × 7.480519 = 579 gallons[11], measured at the surface. It is squeezed to 3,000 psi into 678 cubic inches[1], and 678 ÷ 231 is 2.9 gallons[11]. The first stage brings that down to about 138 psi above the water[3], and the second stage brings it down again, to the water's own pressure[3][5].
Where "4.1 psi a breath" comes from: a breath at light to moderate work is 0.40 gallons (the source gives 1.5 liters), 15 times a minute[9][11]. That is 0.0530 cubic feet[11]. Every psi on the gauge stands for 77.4 ÷ 3,000 = 0.0258 cubic feet of the tank's air[8], so at the surface one breath takes 0.0530 ÷ 0.0258 = 2.1 psi. At 33 feet the air and the water together press 29.4 psi, twice the surface's 14.7[6][7], so the same breath holds twice the air: 0.79 gallons of surface air, 4.1 psi off the gauge[8].
Where "about 41 minutes" comes from: 15 breaths of 4.106 psi is 61.6 psi a minute at 33 feet. From a full 3,000 psi down to 500 psi[10] is 2,500 psi, and 2,500 ÷ 61.6 = 40.6 minutes. The same sum gives 81.2 minutes at the surface, 27.1 at 66 feet and 20.3 at 99 feet, where the air and water press 3 and 4 atmospheres[7][8]. The count in step 5 starts at 2,984 psi, because the breaths in steps 2 to 4 have already taken 16 psi, so it ends at 40.3 minutes and 605 breaths.
Other figures, counted other ways: DAN gives 5.3 gallons a minute (20 liters) for light to moderate activity[10][11], a little less than the Navy's 5.9 gallons (22.5 liters); at that rate a full tank would last 45.7 minutes at 33 feet. DAN's own table gives an aluminum 80 about 70 minutes at the surface, 35 at 30 feet and 17 at 100 feet at that activity, without saying how it counted them[10]. For the hose, Scubapro gives about 138 psi above the water[3] and Hollis about 140[4].
Where the numbers come from
- Catalina's S80 aluminum tank is in its 3000 psi (207 bar) series: 678 cubic inches (11.1 liters) of water inside, 7.25 inches wide, 25.9 inches long, 31.3 pounds, 1.6 pounds heavy in the water when full and 4.1 pounds light when empty: Catalina Cylinders, SCUBA specifications
- "All capacities are exact except the industry standard S80 which has an actual capacity of 77.4 cu ft": Catalina Cylinders, SCUBA specifications sheet (Rev 4/19)
- The first stage brings the tank's pressure down to "an intermediate relative pressure of about 9.5 bars (138 psi)"; the second stage "reduces pressure further to balance air with the ambient pressure": SCUBAPRO Regulators Manual
- Another maker gives "an intermediate pressure of approximately 140 psi"; its second stage opens when the diver breathes in and closes when they breathe out: Hollis Regulator Owner's Guide (12-4014 r07, 2016)
- The second stage gives air only when the diver breathes in: the breath pulls the diaphragm in, the diaphragm pushes the lever, and the lever opens the valve; breathing out opens the exhaust valve: Cressi regulator user manual
- At sea level the air presses 14.7 pounds per square inch, and every 33 feet you go down in the sea adds one atmosphere: NOAA Ocean Service, How does pressure change with ocean depth?
- 33 feet of sea water presses 14.7 psi, one atmosphere, and each further 33 feet adds one more; going from the surface to 33 feet doubles the pressure and halves a gas's volume: NOAA Diving Center, Pressure (lesson slides)
- Atmospheres absolute = (feet of sea water + 33) ÷ 33; cubic feet per psi = the tank's rated volume over its rated pressure; air used at depth = air used at the surface times the atmospheres; minutes = air available over air used a minute: NOAA Diving Program, Diving Medical Technician Reference and Formulas
- U.S. Navy breathing rates: rest is 10 liters a minute (a 1 liter breath, 10 a minute); light to moderate work is 22.5 (a 1.5 liter breath, 15 a minute): Michael F. Ward, Dive Lab, Testing for Re-Inspired Carbon Dioxide (2020)
- "Return to the surface with the gauge indicating 500 psi or greater"; light to moderate activity at the surface is 20 liters a minute; its own table gives an aluminum 80 about 70 minutes at the surface, 35 at 30 feet and 17 at 100 feet at that activity: Divers Alert Network, Smart Guide to Air Consumption
- One cubic foot is 28.316846592 liters and 7.480519 US gallons; one gallon is 231 cubic inches and exactly 3.785411784 liters: NIST Handbook 44 (2026), Appendix C, tables of units
The drawing is not to scale. The sea is drawn squashed so the sand sits at about 44 feet, and the diver is drawn about 3 times bigger than the sea around him, so his gear can be seen. The tags on the line, the reef, the fish, how fast the drawn bubbles rise, the way down being two breaths long, a breath being half in and half out, and the panel's layout are this drawing's own choices. The count plays at 242× real speed; steps 1, 3 and 6 are not timed to real life. The diver's tank is shown yellow; Catalina makes it in several finishes[1].