You learned as a toddler to avoid sticking bad things in your mouth. That's still true now that you're a diver.
People drown underwater if they don't get lungfuls of air on a timely schedule. That's all the reason any diver needs to explore the purchase of a high-quality second stage regulator when buying SCUBA gear.
What is a Second Stage Regulator?
SCUBA regulators mitigate the highly pressurized air in your tank so you can breathe it without expanding like a pufferfish. Regulators are either first stage or second stages; the first stage attaches directly to the valve on the tank, and the second stage attaches directly to the diver's mouth. The first stage delivers air pressure continually to the second stage, but the second stage should only release airflow when the diver inhales.
Three Types of Second Stage Regulators
Second stages come in a variety of shapes and sizes, but are manufactured using only three basic valve designs: Upstream, Downstream, and Pilot.
• Upstream Valves place the airflow stopper before the breathing orifice (on the up-airstream side), and are useful when the first stage regulator delivers a varying air pressure. The cracking pressure, or pressure required to open the valve and allow air to flow to the diver, is less able to be exactly adjusted in Upstream valves than in Downstream valves. Upstream valves are less expensive and don't require much tweaking before dives.
If the first stage fails, the Upstream valve will be pressed closed by the increased air pressure, so these second stages must be equipped with an additional pressure relief valve to avoid rupturing the air hose in the event of a failure.
• Downstream Valves place the airflow stopper after the breathing orifice (on the down-airstream side), and are the most popular type of second stage regulators on the market. They are useful when the first stage regulator delivers a constant air pressure, and the cracking pressure can be set very precisely to suit the diver's preference. Downstream valves are more expensive to manufacture and require more maintenance than Upstream valves.
If the first stage fails, a Downstream valve will automatically open and freeflow air.
• Pilot Valves incorporate an additional small valve (called, coincidentally, the Pilot valve) which is opened by the lowered internal pressure when the diver inhales. Some divers swear by this design, whereas others find it overly complicated and temperamental. On inhalation, the diver may experience a delay before getting air as the pilot valve opens.
My take: there's reasons why Downstream valves are the most widely used second stage regs today. Contemporary first stages are reliable, and most deliver a consistent hose pressure, making the additional mechanisms of Upstream valves unnecessary. Also, the Downstream design is mechanically more apt to freeflow in the event of a failure - a huge plus on my tally sheet.
An Additional Consideration: Exhaust Ports
Most second stage regs have twin exhalation ports placed symmetrically on the right and left sides. Another design is to have a single exhaust port placed asymmetrically on either the right or left side of the mouthpiece.
Most divers use the symmetrical style; they exhaust air to the "up" side (and thus, more easily avoid bubbles crossing the diver's mask and blocking vision), are easy to clear, and simple to repair. Asymmetrical regulators are generally smaller (thus lighter and with less swimming resistance), and can be used over either right or left shoulder, but are generally more expensive, will exhaust bubbles across the mask, and must have the exhaust port in a downward attitude to clear, which can be problematic in practice (or when buddy breathing).
What to Look for When Comparing Second Stages
Some manufacturers have priced their regulators so that there is a markup when buying first and second stages separately, so if you're concerned about price, look to buy a complete set up: first and second stage, secondary/octopus, hoses and attachments, and, quite probably if you have the means, an integrated computer console with depth and pressure gauges. A look at the entirety of regulator systems is beyond the scope of what I've attempted here. Even if you're buying an integrated system, take a close look at the included second stage(s) and consider the following:
Comfort. Most divers should only think about their second stage in a couple of instances:
1. When they want to take a breath, but the regulator isn't in their piehole for any of a number of bothersome reasons (having purchased the best regulator on the market doesn't help someone in this situation), or
2. When sideways hose pull, or jaw/mouth discomfort focus their attentions on the bothersome regulator and away from all the pretty fishes. This situation can be avoided by being sure to buy a reg with a flexible hose, hose of correct length, a "hinged" attachment that allows the reg to swivel around, and a mouthpiece that's neither too stiff, sharp, or large. Also, consider a mouthpiece that can be fitted precisely to your bite by heating and biting, the way football mouthpieces are.
Construction. How hard are you on your equipment? Plastic second stages can be lighter than metal ones but less durable. Or, one can go all-out and buy a super-light and bombproof carbon fiber reg.
Is the cracking pressure easily tuned? Cracking pressure should be adjustable to as low a threshold as possible, to make breathing as effortless as possible. If you have to suck on the regulator with each inhalation, that's unnecessarily tiring.
Does the reg deliver positive pressure? On the other hand, if the reg is pushing air into you, that's positive pressure, and it's actually a negative. If you test dive a regulator and feel like it's blowing you up like a balloon, that's an indicator that the regulator may be unreliable.
Also, some regulators are simply touchy - either under pressure, or lack of pressure - does this model have a reputation to freeflow periodically at either depth or near the surface? For example, a reg unit that likes to freeflow near the surface can empty a lot of your tank as you pass through the surf zone - before your shore dive really begins.
Is there any cracking delay? Pilot valves can unsettle a diver with an unnatural hesitation before delivering air - a delay before cracking open. Cracking delay makes for an unnatural breathing rhythm, and sure-fire way to cultivate claustrophobia at depth.
How big are the exhaust port(s)? The larger the holes, the more easily air can be exhaled through them. This is especially significant for those who go deep, where the air is more dense.
Does the rig include an additional second stage (an "octopus") for backup and buddy-emergencies? If so, is it of the same design and quality of the primary? Many setups include an octopus as a feature, but shave cost by choosing one of inferior quality.
What's the big deal, you ask? Well, while the chance of you ever really having to use it are (hopefully) slim, you surely want that octopus to function when you need it. Or consider this scenario: your cheap octopus begins leaking a steady stream of bubbles from your tank to the ocean. What a great way to shorten your dive! Avoid air loss and gain peace of mind by looking for the same quality in your octopus as your primary.
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