The first stage also called a "pressure regulator" is the most important component in the equipment, as it actually allows you to reduce the air pressure within the cylinder.
The air coming in from the cylinder passes through a conical filter with a high filtering surface contained upstream of the first stage, which reduces it to a pressure between 9 to 11 bar (depending on the calibration value) with respect to the ambient pressure. The reduction takes place by means of a piston or a diaphragm balanced by a pre-calibrated spring.
Inside the body of the first stage there is a chamber where the air expands reducing pressure down to the calibration value and together with the ambient and the spring pressure, the pressure acts on the piston or the diaphragm, moving them, until the delivery flow is interrupted; each subsequent pressure decrease with respect to the calibration values will reopen the circuit until the new equilibrium.
They can be balanced and unbalanced piston or balanced diaphragm and depending on the models they can be equipped with a 360° swivel turret.
They are generally made of chromed or sandblasted finished brass, but on request they can also be made of more precious alloys.
The connection to the cylinder fittings can be ensured using one of the connections specified in the following standards (EN 250):
In the piston-type first stage the balancing is achieved by opposing different surfaces at different pressures. This system is a very simple mechanism that consists of the movement of a single component (the piston). This is an unbalanced first stage. The reasons for its longevity (it is still built and used today) lie in its reliability and affordability; the lack of control of the intermediate pressure is compensated by its reliability.
In a diaphragm-type first stage the balancing is achieved by opposing the same pressure on equivalent surfaces. This type of system has a diaphragm that transmits pressure variations to the internal valve.
In an unbalanced first stage, the air delivery is permitted by the high pressure at the surface S1 of the pad. Around the piston rod is located a calibrated spring to provide about 10 bar outlet pressure when the cylinder is fully charged. When the cylinder pressure decreases, reducing the thrust exerted on S1, proportionately decreases the intermediate pressure: this effect causes a slight increase in respiratory effort, always comfortable and widely in the limit of the Norm EN250.
In addition, the ambient pressure (the water that enters the chamber under the piston head) adds up on the surface S2 with the force exerted by the calibrated spring. This increase causes an increase in pressure exerted on the upper face of the piston head S3 which remains almost unchanged the pressure differential, always around 10 bar more than ambient pressure. In fact the two surfaces S2 and S3 are slightly different and balancing with the ambient pressure is not perfect, but the difference is minimal and is not perceived as respiratory effort.
In the balance first stage, the above mentioned problems are eliminated in a simple way: by detaching the pad from the piston, in this way the breathing-in effort and the air flow are maintained almost constant throughout the dive.
In fact, the delivery is not dependent on variations in the high pressure because the pressure of the cylinder is no longer exerted on a surface but the air flows freely in the piston shank. The two surfaces S2 and S3 are almost the same, which minimises the difference between the increase in ambient pressure and the increase in intermediate pressure.
"Always carry out adequate maintenance on your equipment!"