Like the diver’s beating heart that moves air from the lungs to the rest of the body, the scuba regulator’s first stage provides the critical connection between divers and the air in our cylinder, allowing us to work and play underwater.

The regulator first stage’s main function is to reduce the high tank pressure to an intermediate pressure that can be utilized by the second stage and provide air on demand to the diver. Modern regulator first stages are precision-made and designed to work under demanding conditions wherever divers care to explore.

We’ll be examining both piston and diaphragm first stages, and their differences and similarities to help you understand how this vital piece of equipment allows us to breath while enjoying the ocean’s depths.

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READ MORE: How a Regulator’s Second Stage Works

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1. The Parts

Both piston and diaphragm regulators have either a DIN or yoke style fitting to connect them to the scuba cylinder; an inlet filter to prevent contaminants from entering the regulator; a regulator body incorporating intermediate- and high-pressure chambers; a bias spring; medium-pressure fittings for second stages, inflator assemblies, and accessories; and high-pressure fittings for gauges and transmitters. Piston regulators have a piston-style valve assembly with a high-pressure seat separating the first stage’s high- and intermediate-pressure chambers, while diaphragm regulators have a diaphragm, lifter-poppet valve assembly and high-pressure seat performing the same function.

2. How It Works

As you inhale on the regulator’s second stage, pressure in the first stage’s intermediate chamber is reduced. The force of the bias spring and the ambient water (hydrostatic) pressure push inward on either the diaphragm or the base of the piston head, raising the valve and creating an opening between the intermediate- and high-pressure chambers. Air flows from the high-pressure chamber into the intermediate-pressure chamber and down to the regulator second stage via the connecting hose. When the diver stops inhaling, pressure inside the intermediate chamber increases until it is greater than that of the bias spring and hydrostatic pressure and the valve closes.

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WATCH: How to Care for Your Scuba Regulator

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The first stage is designed to provide air at ambient pressure, so it must adjust for the changes in pressure as depth changes. To do this, a method is needed for the valve assembly to “sense” the ambient pressure changes and adjust accordingly. Piston regulators have the bias spring, the underside of the piston and a portion of the piston shaft exposed to the water to provide the hydrostatic pressure necessary for operation. Diaphragm regulators have one side of the diaphragm and the bias spring in contact with water to provide hydrostatic pressure but the rest of the components are sealed off from the environment on the other side of the diaphragm. In both cases, the amount of pressure required to open and close the valve assemblies varies with the ambient water pressure on the exposed surfaces.

3. Balanced vs. Unbalanced

A balanced first stage, whether piston or diaphragm, is designed so that tank pressure does not impact the operation of the valve. This ensures consistent breathing effort independent of depth or tank pressure. Currently, all diaphragm regulators in production are balanced. In diaphragm systems, balancing is accomplished by routing intermediate-pressure air to both sides of the complete valve assembly and passing the valve stem through both the high-pressure and intermediate-pressure chambers. In balanced piston first stages, the incoming high-pressure air does not directly act on the piston-valve, which also passes through the intermediate and high-pressure chambers. Unbalanced regulators have tank pressure acting directly on the high-pressure seat, and incoming high-pressure air will act to close the valve as intermediate pressure rises. This works fine while tank pressures are high, but can result in heavier breathing resistance when at higher ambient air pressure (deeper water) or when tank pressure is low. The unbalanced piston will still provide adequate air supply in these cases, but it will require more effort by the diver to breath.

4. Piston vs. Diaphragm

Now to the eternal debate over which is best: a balanced piston or diaphragm first stage. In reality, it boils down to a matter of personal preference and both designs work well at providing breathable air to their users. There are some distinguishing characteristics, however, that may aid in deciding which may be best for you.

• The nature of a diaphragm first stage’s design means that it is environmentally sealed.

• There is less chance of components being affected by ice particles in cold water, or by silt, sediment or other contaminants in turbid waters.

• Diaphragm regulators tend to be complex with more parts than a piston regulator and have a smaller diameter valve, meaning lower overall volume of air to the second stage.

• Piston regulators are of simpler design and the size of the piston stem allows for a larger volume of air to be supplied to the second stage, making them popular for their ease of breathing effort.

• Piston regulators have more components exposed to the environment, making them more susceptible to contaminants affecting performance, icing in cold water and resulting free-flow from a stuck piston.

• Piston regulators may require more maintenance of exposed parts due to contamination.

5. Additional Features

Regulator first stages are closed systems and are not designed with user-adjustable features, but there may be enhancements incorporated by the manufacturer. Both types of regulators may incorporate environmental protection, to include a second diaphragm to protect the bias spring of a diaphragm first stage, or by filling the exposed section of a piston regulator with a viscous fluid and sealing it off with a flexible ring. Some regulators incorporate automatic devices to close the high-pressure inlet and prevent contaminants or water from entering the first stage when the regulator is not connected to a tank. Some regulators may incorporate a special medium-pressure port with greater airflow specifically designed for high performance second stages.