Open Water divers · Lesson 04
Dive Planning: Breathing Gas, Computers and Buddy Limits
A dive ends when the first relevant limit is reached. Gas, decompression status, conditions and personal comfort are separate limits; one reassuring number cannot cancel the others.
Copperband learning sheet
The first limit ends the dive
Four independent constraints. Every one matters.
Logic sheet: connect the idea with the decision
Use these prompts to check your understanding before discussing practical decisions with your instructor.
Remember: No-stop time is not a promise of remaining dive time. Gas, people and conditions can end the dive earlier.
Educational information only. This sheet does not replace official training, practical instruction or certification through a recognised scuba diving organisation such as PADI. It is not an emergency protocol, personal medical assessment or technical dive plan.
Agree the plan before entering
Discuss the intended route, depth, time, gas checks, turnaround decisions, ascent and separation procedure. Check equipment together using your training and the operator’s briefing. Tell the guide about recent experience, unfamiliar equipment and anything affecting your comfort.
A plan should reflect the people actually diving. Two buddies may have different cylinder capacities, consumption, computer settings and experience. The agreed route must remain suitable for both, with enough gas for the ascent and foreseeable problems.
Pressure readings are not the whole gas supply
A gauge shows cylinder pressure. How much usable breathing gas that represents also depends on cylinder size and the available supply. A larger cylinder and a smaller cylinder at the same pressure do not contain the same quantity of gas.
Gas use rises with ambient pressure and can rise with exertion or stress. A reserve is not extra sightseeing time. Ask your instructor or guide to explain the gas plan for the specific depth, cylinder, team and route; a universal pressure copied from an article is not adequate planning.
What a computer can and cannot tell you
A dive computer uses depth and time, breathing-gas settings and a mathematical model to estimate decompression status. Learn your own model’s display, ascent warnings, no-stop information and settings before the dive. Air integration, if present, is an additional feature; many computers do not measure cylinder gas.
No-stop time is not the time you can necessarily stay underwater. You may need to finish earlier for gas, conditions, cold or the buddy’s limit. A computer also does not measure every physiological factor and cannot guarantee freedom from decompression illness.
Go a little deeper
two independent clocks
Imagine that your computer still shows no-stop time but your planned gas turnaround has been reached. The gas limit governs the decision to return or ascend as planned. Conversely, a generous gas supply does not justify exceeding the agreed decompression or qualification limits. Treat the plan as a set of independent constraints, then follow the first one reached. After the dive, logging depth, conditions and consumption helps your instructor identify useful improvements.
Check your understanding
Can I follow my buddy’s computer instead of using my own?
Your decompression information must reflect your own dive history and the procedures taught in your training. Discuss missing or unfamiliar equipment before diving; a buddy’s different profile is not automatically yours.
Connect theory with training
For a Pattaya trip, tell the team your qualification, last-dive date and learning goals so the practical programme can be matched to you.
Sources and further reading
This is an independently written educational guide, not an official PADI course manual or certification assessment. Ask your instructor about unclear concepts before applying them in the water.