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April 2009    Download the Entire Issue (PDF) Available to the Public Vol. 24, No. 4   RSS Feed for Undercurrent Issues
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Anatomy of a Free Flow

manual controls are a marketing gimmick

from the April, 2009 issue of Undercurrent   Subscribe Now

It’s very annoying when, just as you are about to start a dive, your regulator suddenly and uncontrollably gushes air. Those who dive in cold freshwater can experience a similar effect while underwater, and that can be life-threatening if air reserves are low. Free flows due to regulator freeze-up are caused primarily when ice forming around the mechanicals of a regulator first-stage disallows it from functioning properly. However, this doesn’t explain why your regulator goes out of control just at that moment when you are about to enter a warm tropical sea.

Assuming that the first-stage of a regulator is working properly, air is delivered to the second-stage at 120 to 150 psi greater than ambient pressure. The pressure-sensing diaphragm at the front of the regulator (it doubles as a purge button) presses down a lever and opens the valve so that the exact right amount of air at a matching pressure enters the regulator body and allows you to inhale. It should be able to give you exactly the amount of air you require, neither more nor less.

Regulator designers strive to make the flow of air through the body of the second-stage clean and uninterrupted so breathing is as effortless as possible. They try to design in a “Venturi” effect, which results in a very clean flow of air rushing past the back of the pressure-sensing diaphragm. A sudden rush of fastflowing air can cause an apparent drop in pressure behind the diaphragm. This in turn is pushed in to compensate, thereby opening the valve more and causing the flow to increase.

This effect can happen where pressure differences are dramatic, at the cusp between water and air. That’s why your regulator so often free-flows as you dip it in and out of the water, when you walk into the sea or plunge off the deck of a dive boat.

Many manufacturers get around this problem by including a Predive/Dive switch on the second-stage. It positions a simple vane in the airflow to break up the Venturi effect. There is also an initial effort to “crack” open the valve. It can often be adjusted by means of a knob that can tighten the spring tension on the second-stage valve. If you want to inhale less air, simply draw on the valve more lightly. However, these spring-tensioners can often be used to make it slightly harder for the pressuresensing diaphragm to lever the valve open -- that can have the effect of correcting a badly set-up regulator’s second-stage that might be leaking through its valve.

Many top-end regulators come with these two manual controls, the Venturi Plus/Minus or Predive/Dive switch and a spring-tensioner to make breathing less effortless. It seems the market demands it. People want added value with their purchases, and divers are no different. But you don’t need them.

Mares has designed away the need to disrupt the Venturi effect by using a patented bypass tube that feeds the air supply directly to the mouthpiece and not through the regulator’s main chamber. The company also eschewed the use of a method to increase inhalation resistance, but people want knobs. So Mares has just introduced the Prestige 32 NTT, which has such a control. It doesn’t operate a movable vane as used by other manufacturers because the designers have already designed out the need for it. It simply restricts the amount that the second-stage valve can be opened to when in the “minus” setting.

Atomic regulators use a vane that is actually adjusted by a mechanical depth sensor, meaning you never need to touch it. Some Italian manufacturers, notably Effesub, have also adopted this approach. If you just bought the regulator or had it serviced, it’s a good idea to take it on a local dive to check that the technician has set it up properly before going on a dive trip abroad.

If you have no Predive/Dive or Venturi Plus/Minus switch, how do you avoid the annoying loss of gas that can occur just as you are about to start a dive? Avoid dipping the second-stage and the octopus of your regulator in and out of the water if you are shore diving. Also be aware that should your regulator free-flow in temperate conditions, you’ll simply need to increase the air pressure inside the body of the second-stage by blocking the mouthpiece or turning the front diaphragm upward for a moment. There is never any need to smack it with your hand!

John Bantin is the technical editor of DIVER magazine in the United Kingdom. For 20 years, he has used and received virtually every piece of equipment available in the U.K. and the U.S., and makes around 300 dives per year for that purpose. He is also a professional underwater photographer.

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