Pressure increases by 1 bar (1 atm) for every 10m of seawater depth, plus the 1 bar of air already pressing down at the surface.
So at 10m you have 2 bar absolute, at 20m you have 3 bar absolute, at 30m you have 4 bar absolute.
In fresh water the figure is slightly different because fresh water is less dense: roughly 10.3m per bar instead of 10m.
Absolute pressure (ATA) always includes the surface atmosphere; gauge pressure does not.
Boyle's Law states that as pressure increases, gas volume decreases proportionally, and vice versa, at constant temperature.
This is why the biggest percentage change in a gas-filled space happens near the surface, not at depth.
Going from the surface to 10m halves the volume of a gas space, but it takes a further 20m (to 30m) to halve it again.
This explains ear and sinus squeezes, mask squeeze, and why buoyancy control gets twitchy in the shallows.
Archimedes' Principle: an object immersed in fluid is buoyed up by a force equal to the weight of fluid it displaces.
Positive buoyancy means the diver floats, negative means they sink, neutral means they hover with no tendency either way.
Salt water is denser than fresh water, so divers need more weight in the sea than in a lake or pool for the same exposure suit.
As a diver ascends, expanding air in the BCD and wetsuit increases buoyancy, so air must be vented to control ascent rate.
Dalton's Law: the total pressure of a gas mixture equals the sum of the partial pressures of each gas in it.
Partial pressure of a gas rises with depth even though the percentage stays fixed - this drives oxygen toxicity and nitrogen narcosis risk.
Henry's Law: the amount of gas that dissolves into a liquid is proportional to the partial pressure of that gas above it - this is the basis of nitrogen absorption and decompression theory.
Charles's Law links gas volume and temperature at constant pressure - relevant to cylinder fills and why a cold tank reads a lower pressure.
Don't confuse depth in metres with bar directly - always add the 1 bar for the surface to get absolute pressure.
Don't assume buoyancy changes are linear with depth - the shallow end of the dive is where volume (and buoyancy) changes fastest.
Don't mix up partial pressure with percentage - a gas mix percentage never changes, but its partial pressure does with depth.
Remember narcosis and oxygen toxicity risk are about partial pressure, not simply 'how deep you are' in isolation from the gas mix.
Decompression sickness (DCS) happens when a diver ascends and dissolved inert gas (mostly nitrogen) comes out of solution too fast, forming bubbles in blood and tissues. The deeper and longer the dive, the more nitrogen is absorbed under Boyle's and Henry's Law - pressure rises, gas dissolves into tissues, and it must be released slowly on the way up.
Dehydration, fatigue, poor fitness, obesity, older age, cold water, hard exercise underwater or right after diving, flying too soon after diving, and repetitive/multi-day diving all raise DCS risk. A PFO (hole in the heart) is a known but non-modifiable risk factor.
Students often confuse DCS (bubbles from ascent, a decompression illness) with nitrogen narcosis (impairment at depth, not bubble-related) and arterial gas embolism/AGE (caused by lung over-expansion on ascent, distinct mechanism, but also grouped under decompression illness with DCS). Also don't confuse the 18m/min ascent rate with the old 9m/min rate from older training standards - PADI's current rate is 18m per minute.
The Recreational Dive Planner (table or eRDPml/dive computer) turns depth and time into a pressure group, then tracks residual nitrogen across a dive day. As a Divemaster you must be able to teach it from scratch, work the table version by hand, and explain why the numbers exist - not just push buttons.
Every dive assigns a pressure group (A-Z) reflecting residual nitrogen. During the surface interval that nitrogen off-gasses, so the diver moves to a new, more favourable letter before the next dive. The Residual Nitrogen Time (RNT) from the previous dive gets added to the planned bottom time of the next dive to give the Total Bottom Time (TBT) - this is what you actually plan around, not the clock time you intend to spend down there.
1. Always use the next greater number for depth or time if your actual figures fall between two values on the table (round up, never down).
2. Adjusted no-decompression limits apply for repetitive dives - use the ANDL from the Repetitive Dive Timetable.
3. Wait at least one hour before the first dive of a repetitive dive series so tissues stabilise.
You will supervise divers using both tables and computers, and you need to catch planning errors before they become DCS risk. Teaching the logic (not just the lookup) is what separates a Divemaster explanation from a student memorising numbers.
A regulator should be serviced annually or per the manufacturer's schedule, even with light use, because internal parts (O-rings, seats, springs) degrade with time not just dives. As a Divemaster you don't service gear yourself but must be able to explain why servicing matters and spot obvious faults during equipment checks.
Rinse all gear in fresh water after every dive, especially BCD bladders (inflate and rinse internally to stop mould and salt crystals damaging the inflator valve), regulator second stages (never depress the purge button while rinsing unless the dust cap is on the first stage, to keep water out), and dive computers.
Scuba cylinders need a visual inspection every 12 months and a hydrostatic test typically every 2 to 5 years depending on local regulation (commonly every 5 years in the UK, every 5 years for steel and often shorter intervals elsewhere - always check local law). Never fill a cylinder past its test date or rated working pressure. Look for a current test stamp before any fill.
Check the low-pressure inflator, dump valves, and overpressure relief valve function correctly. Weight-integrated systems must release weights cleanly and quickly - a jammed weight pocket is a serious safety fault to flag immediately.
A pre-dive check should confirm easy breathing on both first and second stage, no free-flow, a working alternate air source, and an accurate SPG. Icing (regulator free-flow from cold) is a real risk in water below about 10°C (50°F) - brief divers on cold-water breathing technique.
Wetsuits and drysuits should be rinsed and dried away from direct sun (UV degrades neoprene and seals). Drysuit zips need waxing and seals need regular inspection for tears. Fins, masks and snorkels are low-maintenance but check strap buckles and mask skirts for damage before every trip.
As a Divemaster you conduct equipment checks (like a buddy check but for a group), help divers troubleshoot minor issues (leaking mask, sticky inflator), and know when to say a piece of kit is unsafe to dive and must go to a technician. You are not expected to repair regulators or cylinders yourself - that is always a certified technician's job.
As a Divemaster you supervise certified divers, run activities, and are often the first responder when something goes wrong. PADI Standards set clear limits on what you can supervise and how far your training goes before you must call in a professional or emergency services.
Remember: the exam tests exact numbers, ratios, and the correct order of actions - learn these precisely, not just the general idea.
As a Divemaster you are expected to brief divers on the local environment and adapt dives to conditions. Examiners test whether you understand how water properties differ from air and how that changes diver behaviour, buoyancy and hazards.