Why Does Your Regulator Free-flow?

In our most recent mid-month email, we commented about regulators that were working perfectly when they were put away but go out-of-tune during storage. This usually manifests itself in a gentle hiss of air from the second stage when the regulator has been attached to a full tank. The writer of July's Undercurrent travel article encountered this after a long lay-off and assumed it was a problem with his second-stage, which it was not.

Many divers use their regulators without giving a second thought to how they work, or misunderstand what the controls provided on the second-stage actually do. Here's an explanation.

The Inter-stage Pressure and Why It's Important

Modern regulators are of a two-stage design. With its pressure-sensing diaphragm or piston, the first stage senses the water pressure and reduces the air pressure from the tank (around 3000 psi at the beginning of a dive) supplied to the second stage to about 120 psi (eight bar) more than ambient pressure. This is the inter-stage pressure. The correct inter-stage pressure is set by your service technician at the surface.

The Breathing Resistance Adjustment Knob

On many top-of-the-range regulators, an adjustment knob allows a diver to increase the valve spring tension and the inhaling effort to crack open the valve. While it affects the work of breathing, it does not affect the volume of the air supplied. Often dive shops sell them as gas flow adjustments, which is wrong.

When a regulator has been in storage, engraving may occur between the valve seat and the first-stage poppet, requiring servicing. Sometimes, when a regulator has been serviced recently, its new O-rings need time to bed in with use. Both these occurrences can mean that the pressure in the hose between the two stages can become greater than it should be, or even creep upward once the regulator is attached to a full tank.

When this inter-stage pressure is too great, the second-stage valve spring is not strong enough to close the valve completely, and it weeps air. Tightening down the breathing adjustment knob can bring a temporary solution, but this weeping of air grows less as tank pressure lessens during a dive.

A second-stage weeping air in this way is usually symptomatic of a problem at first-stage, allowing the inter-stage pressure to be too high.

What Does the Venturi minus/plus or Pre-dive/Dive Switch Do?

The second stage also has a pressure-sensing diaphragm (it doubles as the purge button).

The more the surrounding water pressure compresses it, the more it allows the second-stage valve to open when you inhale, giving effortless breathing.

Today's regulators deliver air with an ease only dreamt about a couple of decades ago. The total work of breathing of less than one joule/liter at a depth of 165 feet is now normal compared to those regulators of the recent past.

To achieve this, regulator designers have created a very clean flow of air, consistently achieving a Venturi effect within the second stage. The Venturi effect means that with constant mechanical energy, as the velocity of air passing through a constricted area increases, its static pressure decreases.

The clean flow of air behind the pressure-sensing diaphragm of the second stage is rather like the flow of air over an airplane's wing. The drop in pressure behind the diaphragm allows it to be drawn in more than it should, causing the valve to supply a greater airflow. The effect is exponential. You may have noticed your regulator tends to free-flow if you drop it from your mouth into the water at the surface.

To remedy this annoyance, a small wing-like device is introduced into that air flow within the body of the second-stage to disrupt that otherwise clean airflow. It's activated by that Venturi +/- or Predive/Dive switch.

Some divers leave their regulator in the Venturi-minus or Predive setting and never notice the difference. It takes a scientific ANSTI breathing machine, used for CE certification, to tell the difference!

Mares regulators avoid the Venturi side-effect with a bypass tube that keeps the main airflow away from the back of the second-stage diaphragm. Atomic regulators do it with an automatic ambient-pressure-sensitive Venturi adjustment, which comes into play once the regulator is immersed.

Know What Your Regulator's Doing

Like many mechanical things, a frequently used regulator seems to offer fewer problems than one that rarely sees water. If you've just had your regulator serviced, go diving locally before you go away on an expensive dive trip only to find your regulator needs re-adjustment!

As a side note, divers in cold freshwater (less than 50°F) should be aware that a freezing first-stage may jam and allow the inter-stage pressure to increase dramatically, causing a dramatic and life-threatening free-flow at the second-stage any time during a dive. 50°F may seem a long way from freezing, but the cooling effect of air being depressurized as it passes through the first-stage causes a temperature drop often far past the freezing-point of water. Thankfully, the sea rarely gets that cold, but if you are going to dive in Antarctica, you should first be properly trained in cold water techniques.