In a previous post, we had discussed the different varieties of first stages in today's market. This post, as the title suggests, will be discussing the different styles of 2nd stages. As the name suggests the second stage is the 2nd stage of pressure reduction, ultimately taking the Intermediate pressure (IP, the pressure in the hoses), down to ambient pressure. These days there are 3 main types of 2nd stage, two of which are quite common, and one that is not so commonly used. All three of the main designs are whats called a downstream valve. this means in the event of a failure, the valve will fail open allowing gas to flow. Some old style regulators did use an upstream valve, but in the event of a failure in the system the valve would quite often fail closed, not allowing any gas to flow.
At least with modern 2nd stage regulators being a downstream valve, in the event of a failure in the 1st or 2nd stage regulator, the gas will flow both alerting the diver to the presence of a problem but also allowing him or her to abort the dive and still receive gas from their regulator on the ascent.
The three styles of 2nd stage are:
1) Unbalanced
2) Balanced
3) Servo Assist Valve
The Unbalanced 2nd Stage
In similar fashion to the 1st stage regulators, Unbalanced 2nd stages have the gas pressure (Air, Nitrox etc) on one side of the sealing surface but not the other. The following diagram shows the valve of an unbalanced regulator without the body(as the shapes and design can vary quite substantially between various models and manufacturers).
In an unbalanced regulator, the air pressure is held back by a spring pushing on a seat which is sitting against the knife edge of the orifice. When you inhale, you create a negative pressure in the regulator body which pulls the soft diaphragm inwards which in turn pushes on the lever. This then has the effect of pulling the seat away from the orifice allowing air to flow through the regulator.
This design of regulator is a very simple design with not many parts to cause potential failures and is a good all round regulator for divers of any skill level. In the unbalanced 2nd stage regulator however, being that the spring is the only force holding back the air pressure from the 1st stage; the work of breath can be heavier than with a Balanced 2nd stage. Due to the breath of the diver having to overcome the spring pressure to enable the air flow to begin.
If you're having trouble visualising from the diagram please see these reference photos below:
The Balanced 2nd Stage
The Balanced 2nd stage, much like its first stage counterpart, is pneumatically balanced meaning there is air pressure on both sides of the seat surface. This means that the air pressure is not exerting any opening or closing force on the action of the regulator making the spring pressure the only force that holds the seat to the sealing edge of the orifice.
Being that the spring is not required to hold back the entire air pressure as it has to in the unbalanced 2nd stage, the work of breath is able to be lower thanks to the lighter cracking effort most balanced 2nd stages are able to reach without fear of freeflow. The main benefit of a balanced 2nd stage for me is that as you descend and the IP from the first stage increases due to the increase in ambient pressure, the balance feature of the 2nd stage continues so the cracking effort remains constant. In an unbalanced 2nd stage where the spring pressure is holding back the IP, as the IP increases with depth eventually the IP can overpower the spring causing a free flow. The Balanced 2nd stage doesnt have this thanks to the pressure being equal on both sides of the seating surface.
As can be seen in the above diagram, the air enters and still flows through the orifice as with the unbalanced reg, however the seat has a small hole in the center of it allowing air to pass through the poppet to the far side which is captured by a balance chamber. The Balance chamber allows the air/gas pressure to be on both sides of the seat allowing the spring to push the poppet and seat towards the knife edge of the orifice sealing flow except for when the diver inhales pulling the diaphragm down and causing the lever to override the spring allowing gas to flow.
Some balanced second stage regs have a user adjustable cracking effort, these regs have a dial on the exterior of the reg that the user can rotate on a fine thread that pushes the plug in or out increasing the spring tension or decreasing it. This movement depending on the model of regulator is usually only a small range of motion but can have a surprising difference on the cracking effort.
The third style of Second stages is the servo assist. These are not that common in the everyday divers setup and to keep this post a bit shorter I will aim to cover them in a future post.
To summarize between the First Stages and Second Stages there are a massive amount of options out there as to brands, style, size, performance and weight. What you plan on doing with them, what your budget is and even what aesthetics you prefer will vary what regulators you end up with. Personally i prefer my regs to be balanced both first and second stages so as to retain a constant IP and cracking effort in my setup regardless of if diving at 10m or 45m. This combined with half of my dives in the past 6 months have been in a technical setting means that regardless of my depth i have comfortable equipment I know is going to be performing well above requirements. If budget doesn't allow a full set of balanced regs, as a minimum I personally would go for a high quality balanced first stage and have the 2nd stages as the unbalanced set, this keeps the work of breath somewhat constant as the line pressure will remain steady as the tank pressure decreases. Thank you for your time, I hope this has cleared up any questions you may have about your equipment and if you ever have a question, feel free to contact me and have a chat. Ross