Dive Computers, Gauges & Equipment — PADI IDC / DM Study Notes

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Dive computers, gauges, DSMBs, lift bags and rebreathers explained — PADI IDC / DM theory, Will Welbourn, Go Pro Caribbean

Depth gauges

Capillary depth gauge

The capillary gauge is the most frequently examined depth gauge type — understand it thoroughly.

How it works A small-diameter clear plastic tube, open at one end and sealed at the other, is wrapped around a circular dial. As you descend, water enters the open end and compresses the trapped air according to Boyle's Law. You read depth by noting where the water column ends against the depth markings on the dial.

Because the markings are based on Boyle's Law, the depth increments get progressively smaller the deeper you go — the first 10 m / 33 ft takes up half the entire tube. Reading the gauge accurately becomes harder with depth.

Key exam point — capillary gauge at altitude A capillary gauge is the only depth gauge that is not a sealed unit calibrated at sea level. When diving at altitude, the air trapped inside the tube starts at a lower pressure than at sea level. This causes the water to travel further into the tube than it would at the same actual depth at sea level — the gauge overreads depth, showing you deeper than you actually are.

The practical result: the gauge is already displaying theoretical depth, so you do not need to apply altitude conversion factors when planning with a capillary gauge. Your NDL lookup on the RDP uses the depth shown directly on the gauge.
Exam trap Every other depth gauge type is a sealed unit calibrated at sea level — they show actual depth and require conversion to theoretical depth when diving at altitude. The capillary gauge is the only exception.

Other depth gauge types

Type How it works Notes
Oil-filled (Bourdon tube) A spiral Bourdon tube inside the gauge tightens under pressure and moves the needle Most common mechanical gauge; cheapest and best value
Diaphragm A flexible diaphragm connects to the needle via rods, levers, and gears Less common; more expensive but more accurate than oil-filled
Electronic (dive computer) Electronic depth sensor, usually integrated into a bottom timer or full dive computer Now the most common type; increasingly replacing standalone gauges

Exposure suits

Wet suits — what provides the insulation?

Key concept Wet suits are made of neoprene — a foam rubber containing thousands of tiny trapped air bubbles. It is the air bubbles in the neoprene that provide insulation, not the neoprene itself. A thin layer of water enters the suit, is warmed by your body, and the neoprene prevents that warm water from losing heat to the surrounding water.
Exam trap Students often say water provides the insulation, or neoprene provides the insulation. The correct answer is the air bubbles trapped inside the neoprene.

Dry suits — argon as inflation gas

Technical divers using trimix (a gas mixture containing helium) cannot inflate their dry suit from their back gas. Helium is a very poor insulator — using trimix as dry suit inflation gas provides almost no thermal protection.

Standard air cannot be used as an alternative because it creates a risk of isobaric counterdiffusion — a form of skin decompression sickness. The solution is argon: an excellent insulator, no isobaric counterdiffusion risk, and low cost.

Key concept — dry suit seals Dry suit seals (neck, wrist, ankle) require periodic replacement. When fitting a new neck seal, it must be trimmed with scissors to achieve the correct fit — a seal that is too tight can trigger the carotid sinus reflex (a sudden drop in heart rate and blood pressure).

SMBs and DSMBs

Type Design features Risk if released from depth
Open-bottom SMB No valve — air added through open base Air spills out on surfacing; may not remain inflated
Sealed SMB (one-way valve) One-way valve prevents air escaping after inflation No spill risk, but no over-pressure release — can burst or blow seams if over-inflated or released from depth
DSMB (delayed SMB) Non-spill design plus over-pressure relief valve Expanding air vents safely; air cannot escape back out — designed specifically for release from depth
DSMB — the correct choice for release from depth The two features that make a DSMB suitable for release from depth: (1) non-spill design — air stays in when the bag tips over at the surface; (2) over-pressure release valve — expanding air vents safely as the bag ascends, preventing burst seams.

SMB / DSMB safety rules

  • Always use a reel to manage the line when towing a DSMB or SMB — loose line is an entanglement hazard as you change depth
  • Never attach an inflated SMB or DSMB to your BCD or D-ring — serious entanglement and uncontrolled ascent risk

Lift bags

Rule Weight / limit
Use a lift bag rather than lifting by hand Object heavier than 7 kg / 15 lb
Maximum lift for recreational divers 45 kg / 100 lb
Bag size to select Slightly more lift capacity than the object's weight
Correct technique Add air at depth until the object becomes neutrally buoyant, then swim it up — venting small amounts of air during the ascent to maintain neutral buoyancy. Position yourself to the side of and level with the lift bag throughout: safe venting access, no risk of being pulled up if control is lost, and the object cannot fall on top of you.

Alternate air source inflators

An alternate air source inflator combines the BCD inflator hose with a second regulator second stage. In an out-of-air emergency, the procedure differs from a standard octopus share:

Out-of-air procedure — alternate air source inflator The donor gives their primary regulator to the out-of-air diver and switches to breathing from the alternate air source inflator themselves. This is the reverse of a standard octopus procedure.
Why the donor keeps the inflator, not the primary The alternate air source inflator hose is intentionally short — it must be, so the donor can still reach the inflator to control buoyancy. It is not long enough for a second diver to breathe from comfortably during an ascent. This is why the out-of-air diver receives the primary (long hose), not the inflator.

Always identify your buddy's alternate air source type during the pre-dive buddy check so you know exactly what to expect if an emergency occurs.

Dive computers

Air-integrated computers

What air integration adds An air-integrated computer reads tank pressure via a transmitter or hose and displays two additional pieces of information: (1) your current gas consumption rate; (2) an estimated remaining gas time based on your current depth and consumption rate. This number changes as you change depth — deeper means faster consumption, shallower means slower.

Nitrox and multi-gas computers

A nitrox or multi-gas computer tracks oxygen exposure based on the blend you are breathing and your current depth. It monitors your OTUs (oxygen tolerance units) using the oxygen clock, alerting you before you approach safe oxygen exposure limits. Essential for any diver using nitrox or mixed gases.

Exam trap — gas time remaining The estimated gas time remaining on an air-integrated computer is dynamic. It updates continuously as depth changes. It is not a fixed number — treat it as a real-time estimate, not a guarantee.

Closed-circuit rebreathers (CCR)

The two tanks

Key concept — gas supply A CCR contains two tanks: one tank of pure oxygen, and one tank of diluent gas — either air or trimix depending on planned depth. The rebreather uses both to manage the gas mix the diver breathes throughout the dive.

The breathing loop and CO2 scrubber

On open-circuit, exhaled gas goes into the water. On a rebreather, exhaled gas stays within a closed loop. It passes through a CO2 scrubber — a canister of chemical absorbent that removes CO2 — and is then recirculated back to the diver. The counter lungs are the flexible bags that hold this circulating gas.

Exam trap — CO2 on a rebreather CO2 is colourless, odourless, and gives no warning before incapacitation. Monitoring CO2 levels in the breathing loop is critical. A failed scrubber is one of the most serious rebreather hazards.

Buoyancy control on a rebreather

Key concept — counter lungs Because exhaled gas stays within the loop rather than leaving the diver's body, breathing has much less effect on buoyancy than on open-circuit. The counter lungs offset the volume change caused by the lungs. On open-circuit, inhaling makes you rise and exhaling makes you sink. On a rebreather this effect is largely cancelled — if anything it may be reversed.

As a result, rebreather divers control buoyancy almost entirely through BCD inflation and deflation, not breath control.

Electronics — oxygen sensor and depth sensor

The rebreather's electronics monitor two things continuously: the oxygen partial pressure in the breathing loop (via an oxygen sensor) and the current depth. Using these two values the unit always knows exactly what ppO2 the diver is being exposed to, allowing it to prevent oxygen toxicity automatically.

The set point — how the mix is adjusted

Key concept — set point The diver programs a target oxygen partial pressure — the set point. The rebreather maintains this by injecting gas from whichever tank is needed:

• ppO2 too low (e.g. after ascending) → inject oxygen into the loop
• ppO2 too high (e.g. after descending) → inject diluent (air or trimix) into the loop

Divers typically use a lower set point for the surface and descent phase, and a higher set point (commonly 1.3 atm) for the main portion of the dive.
Exam trap — open-circuit vs rebreather gas mix Open-circuit: gas ratio is fixed throughout the dive — only partial pressures change with depth.
Rebreather: gas ratio changes continuously to maintain a constant target oxygen partial pressure (the set point).
Rebreathers and dive computers explained — PADI IDC / DM theory, Will Welbourn, Go Pro Caribbean

Equipment — Topics

First Stages Second Stages Tanks & Valves Misc Equipment Practice Exams
← Equipment Hub ← Theory Hub
Book your course now — Learn from Will Welbourn in person
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