PADI Divemaster Theory key terms

The legal and ethical obligation of a divemaster to act reasonably, keep divers safe, and avoid foreseeable harm during supervised activities. It underpins every decision made on a dive boat or site.

The maximum number of divers one professional may responsibly oversee, set by training standards and adjusted for conditions, experience, and visibility. Exceeding it increases risk and breaches standards.

Overseeing already-certified divers during guided dives, including briefing, monitoring air and depth, controlling group movement, and conducting headcounts. The divemaster leads but does not teach new skills.

A pre-dive talk covering site layout, entry and exit, depth limits, hazards, marine life, emergency procedures, and the dive plan. A clear briefing reduces confusion and prevents avoidable incidents underwater.

A systematic count of divers at defined points such as before entry, after surfacing, and on the boat. Accurate counts confirm everyone is accounted for and are recorded rather than assumed.

Pairing divers so each monitors the other's air, depth, direction, and wellbeing, and can assist in an emergency. Divemasters assign and verify buddy teams before every dive.

The agreed actions when a diver loses contact with a partner: search briefly, ascend safely, reunite at the surface, and alert the divemaster. It is explained in every briefing.

A structured response when a diver is unaccounted for, including immediate headcounts, checking likely locations, notifying the captain, and initiating search and rescue. Time is critical and roles must be pre-assigned.

A written, site-specific plan listing emergency contacts, evacuation routes, nearest chamber, oxygen location, and assigned roles. It is rehearsed so responders act without hesitation.

Directing the response to an accident: securing the scene, providing first aid, delegating tasks, documenting events, and liaising with authorities. Calm leadership protects the casualty and reduces liability.

A factual written record of an accident or near miss, capturing times, conditions, actions taken, and witnesses. Completed promptly and objectively, it supports follow-up, insurance, and learning.

Evaluating a site or activity for hazards, likelihood, and severity, then deciding how to eliminate or control them. It is done before dives and revised as conditions change.

Supporting an instructor during training by setting up equipment, demonstrating skills, supervising students, and giving feedback, while never teaching or certifying independently. The instructor retains responsibility for students.

Performing a required scuba skill slowly, precisely, and to standard so students can copy it. Demonstrations must be flawless, clearly visible, and repeated if students cannot see or follow.

Training conducted in a pool or pool-like conditions where students learn and practise basic skills before open water. The divemaster helps position students, manage equipment, and keep order.

A training dive in natural water where students apply skills under instructor supervision. Divemasters assist with logistics, buddy checks, and monitoring student safety and air supply.

Judging whether a student or diver can recognise and manage problems such as panic, exhaustion, or equipment failure, and respond appropriately. Divemasters report observations to the instructor.

A pre-dive verification that each diver has air on, correct weighting, and understands the plan, often summarised as a short sequence of checks. Divemasters ensure it is done every time.

At constant temperature, gas volume is inversely proportional to absolute pressure, so volume halves when depth doubles. This explains why wetsuits compress, buoyancy changes, and divers must equalise on descent.

The principle that a fixed mass of gas at constant temperature changes volume inversely with pressure. It governs lung overexpansion injury, mask squeeze, and buoyancy shifts during ascent and descent.

Total pressure at a depth, combining atmospheric pressure at the surface with the pressure of the water column. It is used in dive calculations and expressed in atmospheres or bar.

The pressure exerted by one gas within a mixture, equal to its fraction times the total pressure. It determines how much nitrogen or oxygen enters tissues and the risk of toxicity.

A reversible impairment of judgement and coordination caused by dissolved nitrogen at depth, resembling alcohol intoxication. It resolves on ascent, and divers manage it by limiting depth and staying alert.

Illness from nitrogen bubbles forming in tissues or blood after pressure reduction, causing joint pain, numbness, or other symptoms. Treatment involves oxygen, fluids, and recompression in a chamber.

The maximum time a diver may spend at a given depth and still ascend directly to the surface without a required decompression stop. Dive computers and tables calculate it from nitrogen loading.

A voluntary pause, typically three minutes at around five metres, before surfacing to allow extra nitrogen elimination. It adds a margin of safety and is standard practice on most dives.

A device that tracks depth, time, and nitrogen absorption and displays remaining no-decompression time and ascent rates. Divemasters check that divers understand and follow their own computers.

The state in which a diver neither sinks nor rises, achieved by adjusting weighting and using the BCD and breathing. Mastery protects the reef, conserves air, and makes skills easier.

Managing BCD inflation, breathing, and body position to maintain the desired depth and trim. Good control prevents rapid ascents, reef contact, and wasted effort.

A diver's body position in the water, ideally horizontal and streamlined with legs raised. Good trim reduces drag, protects the environment, and improves air consumption.

Confirming a diver carries just enough weight to descend and hold a safety stop with an near-empty cylinder. Correct weighting improves buoyancy control and reduces effort.

Delivering breaths to a non-breathing casualty, in water or on a boat, until advanced help arrives. It is combined with chest compressions when trained and equipped to do so.

Giving high-concentration oxygen to a suspected decompression or drowning casualty using a demand valve or non-rebreather mask. It is a priority first aid step while arranging evacuation.

Damage caused by holding breath while pressure falls during ascent, forcing gas to expand and rupture lung tissue. Prevention is continuous breathing and a controlled, unhurried ascent.

An uncontrolled or assisted rise to the surface when air is lost or a diver cannot resolve a problem at depth. It is a last resort because of barotrauma and decompression risk.

Explaining a site's layout, entry and exit points, depth profile, currents, and hazards so divers can plan their dive. It is a core part of the pre-dive briefing.

A zone where human activity is regulated to conserve ecosystems and biodiversity. Divemasters brief divers on local rules and model low-impact behaviour within these areas.

Behaviours that minimise environmental impact, such as good buoyancy, no touching or feeding wildlife, no anchoring on reef, and proper waste disposal. Divemasters demonstrate and enforce them.

Loss of colour in corals when heat or stress makes them expel symbiotic algae, often leading to death. Divemasters explain local pressures and avoid practices that add further damage.