As a diver considering the purchase of their own scuba regulator system, one of the first questions that might come up is, “Do you want a diaphragm or piston first stage?”
Before getting into the explanation of differences between the two styles, let’s briefly review the purpose of a regulator system.
Like the name implies, a scuba regulator controls (or regulates) the air pressure from your scuba cylinder, and distributes that air at an intermediate pressure (IP) that each piece of attached gear can function at.
Impressively, the regulator delivers this consistent IP even while a diver is changing depth throughout the dive (resulting in constantly varied conditions of temperature, pressure, and breathing needs).
In other words, the mechanics of the first stage are continuously adjusting with the ambient conditions to deliver a steady flow of IP to the diver’s gear throughout the dive.
When discussing a diaphragm versus a piston style regulator, you are talking about an internal mechanical difference in how the first stage of a regulator is built to control this air flow using different pressure reactions.
Both styles react to ambient hydrostatic pressure and use the pressure changes resulting from the pull of a breath to trigger the mechanisms that move high pressure air from the tank into the intermediate pressure chamber of the first stage.
So is a diaphragm or a piston style regulator better?
Like anything else gear-related, there is no clear-cut winner. There are benefits and downsides to each style, and the decision between the two often comes down to diver preference and the type of diving the regulator is being used for. To narrow down your choice, let’s discuss the difference between the two.
Main Benefits of Each
- Diaphragm regulators are inherently balanced. When a regulator is balanced, it means that even when a diver gets low on air near the end of the dive, the ease of breathing remains the same as when the diver started with a full tank. Pistons come balanced or unbalanced (balanced is more expensive).
- Diaphragm regulators are inherently environmentally sealed, meaning none of the internal parts are directly exposed to the water during a dive (reducing the chances for freezing/free-flow). It costs extra to make a piston regulator environmentally sealed.
- Piston regulators allow more air into the IP chambers, resulting in more air with each breath, and some divers swear that this makes it easier to breathe.
- Piston regulators have a simpler design, making them less expensive to service.
- Piston regulators are generally less bulky in shape, sometimes making it easier to streamline the regulator assembly with swivels and better hose attachment organization.
Piston regulators are generally more common, as they are cheaper to maintain and simpler in design.
As the name implies, a piston regulator essentially uses a sliding metallic piston against a tensioned spring to control the flow of pressurized air through the first stage.
The tail end of the piston is pressed against a hard plastic seat, called the high pressure seat, and that plastic separates the high pressure chamber (air coming from the tank) from the intermediate pressure chamber of the first stage (which then flows into the second stage we breathe from).
When the air tank is turned on, high pressure air flows through the first stage’s intake valve and builds up against the tail of the piston in the high pressure (HP) chamber.
At the same time, the tensioned spring around the piston (called a bias spring), is “measuring” the ambient pressure around the intermediate pressure chamber through small parts that are exposed to the water.
When a diver pulls a breath, this pressure difference between the two chambers is what triggers the piston tail to release from the high pressure seat, relaxing the bias spring to allow air to flow between the HP and IP chambers through the piston shaft.
Once the IP chamber is filled with enough air to reach an “acceptable” intermediate pressure level (usually between 120-150 psi) or the diver exhales (isn’t demanding air from the IP chamber), air pushes back down the piston shaft against the high pressure seat, closing the IP chamber off from the HP chamber and allowing the IP chamber’s air to flow into the low pressure hoses (second stages and LPI).
When the IP chamber’s pressure gets too low for breathable air pressure (or a diver inhales), the piston is triggered open again to restore the IP chamber air level. This chain reaction is occurring continuously as a diver breathes, as shown by this video:
While diaphragm regulators have the same function of controlling air via pressure triggers, the first stage of a diaphragm regulator uses more parts (in a more complex design) to carry out this function.
The main distinction is the part used to separate the HP and IP chambers: instead of a metallic piston moving with a bias spring to separate the chambers with a high pressure seat, a diaphragm first stage uses lifter-poppet valve assembly with a thick piece of rubber (diaphragm) sandwiched against a bias spring to perform the same function.
When high pressure air flows from the tank into the HP chamber of the diaphragm first stage, the lifter-poppet valve is pushed up against the diaphragm, which opens a path between the chambers and allows air to flow into the IP chamber (and then out into the low pressure hoses for second stages and LPI).
When the IP reaches the maximum breathable pressure (or diver exhales), the bias spring pushes the diaphragm back down to seal against the air from the HP chamber.
Just as it does in the piston style first stage, the bias spring is also using ambient pressure from the diver’s depth to regulate the IP chamber levels (along with the reduced pressure that occurs when a diver inhales, demanding air).
While these explanations are simplified and gloss over additional assembly parts that work together with the parts named to perform the regulation of HP air flow to the IP chamber, it is clear that the main mechanism used to separate the HP and IP chambers in each regulator is the reason for distinction between the styles.