Equipment terminology made simple
- May 12
- 5 min read
During your Divemaster and IDC courses, you will have many new terminologies and technical jargon thrown at you that can be somewhat confusing when you start. But understanding how your equipment works and what all those fancy-sounding parts actually do not only will help pass your exams, but more importantly, will make you become a safer, more confident diver.
Regulators
Your regulator is your lifeline underwater. Very simply put, its job is to take the high-pressure air from your tank (via the 1st stage) and then deliver that air (via the 2nd stage) at the right pressure so you can breathe easily. Here are some key terms related to how this all works:
Upstream vs Downstream
These terms describe the direction air flows in relation to the valve’s sealing mechanism.
Downstream valve: The air pushes with the direction the valve opens. This is the most common design in scuba regulators because it’s fail-safe — if something goes wrong, the valve will only allow air to flow freely rather than cutting it off. In case of failure, you’ll get a free-flow, but you’ll still have air!
As opposed to Upstream valve in which air would push against the valve. If it fails, it would block airflow completely, not what you want underwater and therefore these are rarely used in modern recreational diving gear.
Demand valve
The demand valve is in the second stage of your regulator. It gives you air on demand which means only when you inhale. That’s why it’s called a “demand” valve — it only supplies air when you need it, not constantly flowing.
Fail-safe
As mentioned above, a fail-safe system means that, in case of regulator failure, it will still allow air to flow rather than stopping it. That’s why downstream valves are used — they prioritize air flow.
Environmental seal
In very cold water, your regulator’s first stage has the potential to freeze. An environmentally sealed regulator protects the internal parts from exposure to water by using a barrier, like a membrane or oil which sits between the water and the mechanism. This helps prevent freezing and corrosion, and extends the regulator’s life.
Piston vs Diaphragm
These are two different mechanisms for the first stage of a regulator (the part that first reduces the tank’s high pressure to an intermediate pressure)
Piston type: Simple, fewer moving parts, more rugged and reliable, great for warm water. They are usually more affordable, however, they’re more exposed to the environment and not ideal for silty or cold conditions unless sealed.
Diaphragm type: More complex and more moving parts, more sensitive to pressure changes and better sealed; generally, the internal parts do not come in to contact with the surrounding water, all of which, makes them a good choice for cold or dirty water.
Both work perfectly well — it’s mostly about what conditions you dive in.
Balanced vs unbalanced
This has to do with the breathing effort of the regulator and how it changes with added pressure (so at depth):
Underbalanced: Breathing effort increases as you go deeper or as your tank empties
Balanced: Consistent airflow no matter the depth and as tank pressure drops giving you smoother breathing no matter the depth.
Overbalanced: Increases airflow at depths so breathing gets easier as your depth increases
Venturi valves or pre dive switch
Many second stages use the Venturi effect to make breathing easier, it helps direct airflow smoothly. The Dive/Pre-Dive switch (sometimes labelled “+ / -”) helps control that, preventing free-flow at the surface. If you turn the switch to + it will mean the regulator will be easier to breath and more likely to fee-flow. If you turn the switch to – it will mean harder to breath and less likely to free-low – deciding which way to turn depends entirely on your personal preferences.
Cylinders
These hold the compressed air so we can breathe on the dive... obviously, but there are various terminologies to understand regarding the tanks.
Burst disc
A burst disk is a small, usually copper disc that sits in the tank valve and acts as a safety device. Tanks are tested for their working pressure as well as their bursting pressure. The burst disc will be set to a pressure between the working pressure and the bursting pressure of the tank. If the tank becomes over-pressurized, beyond the working pressure, the burst disk will rupture and allow the safe release air via small holes, to prevent the tank from exploding. Without it, if the tank valve were to reach the bursting pressure, the tank could be propelled forward with huge and dangerous force. However, the presence of the burst disc, means the tank would instead, safely release this pressure before the risk of explosion or becoming a potential missile!
J-valve and K-valve
These are types of tank valves; the part that controls air coming out of your cylinder.
K-Valve: The modern standard. Simple open/close valve: you turn it on, and air flows; turn it off, and it stops.
J-Valve: An old design used before pressure gauges were common. It had a built-in “reserve” system for when your air was low, resistance increased, warning you to pull a lever to access your last bit of air. These are now mostly historical.
Open, Closed, and Semi-Closed Circuits
These terms describe different breathing systems:
Open Circuit: What recreational divers use. You inhale from the tank, exhale into the water, and bubbles go out — simple!
Closed Circuit (CCR): Used in technical diving. Your exhaled gas is recycled — CO₂ is removed, oxygen added — and there are no bubbles. Very efficient, and great for getting closer to nature (the ninja divers), but complex and very expensive.
Semi-Closed Circuit: A mix between the two — some gas is recycled, but not all and bubbles are still exhaled.
Depth and pressure gauges
There are 2 things we need to know when we dive; how deep you are and how much air you have in your cylinder. Before dive computers, there were 2 types of mechanical pressure gauges:
Capillary Depth Gauge: This is a very simple design and was the original way to measure depth (in pressure). It is a small transparent tube of air with a plug on one end. As you descend in the water and pressure increases, naturally, the air in the tube compresses as the water enters the open end; how much the air compresses tells you how deep you are. For example at 10 meters, the air in the tube would be condensed into half the space. These are inexpensive but only really accurate at shallow depths.
Bourdon Tube: This is the most common design used in SPGs to measure the tank pressure. It consists of a curved metal tube, which has gears connected to a pointer. As the pressure increases in the curved tube, it will try to straighten. The pointer, which is attached to the links and gears, will move in correlation to how much the tube tries to straighten. As the pressure decreases, the tube will relax, and the pointer will follow.
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