FORTY METRES DOWN, I’ve dumped all the air from my BC and I’m upright, finning like crazy. Billows of sand roll across the seafloor like dust clouds from an atomic explosion. I’m trying to see if I can beat the lung with the Beuchat VRT90 regulator.
“Beating the lung” is an expression meaning that you need more air from your regulator than it can deliver. It leads to air starvation.
The vicious finning, while over-weighted, was meant to simulate the demand a panicked diver might place on this budget regulator. The VRT90 breathed superbly.
It’s perhaps not that surprising that the VRT90 performed so well. The first stage is an over-balanced diaphragm design. Over-balancing is a modification that first appeared in the early 1990s to mitigate a problem with breathing air under pressure.
The pre-dive/dive switch.
As we dive deeper, we need more gas to fill our lungs – around five times as much at 40m as we require at the surface. Regulators are made to provide high flow-rates of gas on demand, so they can deliver these large volumes of air, but as it becomes increasingly pressurised, it also becomes denser and more viscous.
This can be felt from around the 30m mark in my experience, so we breathe a little harder and air seems to take a little longer to fill the lungs.
Most first stages match the increasing water pressure as you descend exactly. In an over-balanced design it delivers slightly higher pressure than the surrounding water pressure to the second stage – effectively giving the air an extra push to make it flow faster and be easier to breathe.
The second stage is a pneumatically balanced design. In an unbalanced second stage, a fixed-strength spring is needed to keep the second-stage valve closed against incoming air pressure from the first stage, until you inhale.
The spring needs to be quite strong or, on deep dives, it might be unable to hold back the air from your tank, and you’d get a freeflow.
When you want air, you have to use lung power to open or “crack” the valve against the closing force of the spring, which takes effort.
In the VRT90 balanced second stage a fixed-strength spring is also used, but it can be fairly weak, because air pressure is used to back it up.
Changes in depth and pressure automatically result in changes to the air pressure acting with the spring to seal the second stage. This matches the changing pressure of the air being delivered from the first stage, balancing it.
In effect, the changing air pressure in the VRT90’s balanced second stage acts like a variable-strength spring, always working at the optimum strength for the depth of the dive.
This reduces the effort you need to make to inhale and get the air flowing. It’s a selling point on premium-priced second stages as a rule.
So, by combining an over-balanced first stage and pneumatically balanced second stage, Beuchat has engineered a very easy-breathing regulator for a low price.
Crack the valve and the venturi effect kicks in. It maintains the air-flow by creating a vacuum that keeps the diaphragm depressed and the valve open for you throughout your inhalation.