Regulator Free Flows

(This document is copyright 4.9.2010 by Walter Bolton and may not be reproduced in any form without the author’s written consent.)

The term “Free Flow” in SCUBA regulator jargon is sometimes misinterpreted. A free flow in a second stage does not automatically mean that the regulator is malfunctioning or maladjusted. In fact, a finely tuned second stage can be triggered to free flow by simply dropping it in the water. In fact, as a technician, we are trained to test a properly adjusted regulator by initiating a free flow and then turning the “Venturi Lever” and stopping the free flow to be sure everything is working correctly. Hopefully these notes will clarify some things about “Free Flows.”

There are two primary types of second stage free flows which might be labeled as “The Slow Bubbler, or ”The Massive.“ This second type, “The Massive” can be further broken down into two types — one caused by designed venturi effect and the second by adiabatic cooling.


This type of free Flow is marked by a very slow bubbling of air out of a second stage. The bubbles may only “bloop” out every second or so like: Bloop —————– Bloop etc. or they might be a little closer together like: Bloop —– Bloop —– Bloop, etc. This type of free flow can suggest a possible elevated medium air pressure coming from the first stage which is over-powering the spring in the second stage OR it can indicate a maladjustment of the second stage — possible lever height, spring tension, or low pressure crown adjustment. It could possibly also indicate a worn or damaged low pressure seat or crown orifice. This symptom suggests a need for adjustment or service.

THE MASSIVE (Sometimes called a “GUSHER”)

The characteristics of a “Massive” free flow is when the second stage valve is opened and then stays open and air blasts out of the mouthpiece to the horror of every inexperienced diver within 10 yards of the event! Actually, this type of free flow is often less insidious than the “Slow Bubbler” — it just looks and sounds worse. Often all it takes to stop the free flow is to put one’s finger or thumb into the mouthpiece to interrupt the venturi effect. Let me explain this wonderful characteristic of gasses:


“The Venturi Effect is a special case of the Bernoulli effect, in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli effect. It is named after the Italian physicist Giovanni Battista Venturi.” (

So what modern regulator manufacturers have done is design their second stage “boxes” to maximize this venturi effect and increase the performance of our regulators. Basically what Aqua Lung has done is designed the port from the valve in the second stage to aim at the mouthpiece opening in such a way that it creates a venturi effect. The partial vacuum thus created when the diver inhales causes the diaphragm to be drawn in and hold the valve open, thus enhancing the flow of air to the diver. One only needs to initiate the airflow and then the enhanced performing regulator “delivers” air to the diver until he or she stops breathing — in other words, interrupts the venturi effectt — eliminates the partial vacuum — and the enhanced air flow ends.

This type of free flow can be triggered by simply dropping the second stage into the water with the mouthpiece upwards. The diaphragm is popped upwards by the water, it hits the lever, which opens the valve for just a split second and triggers the venturi effect.

If one looks at their second stages there is often some type of lever or device to interrupt this type of free flow and make it less likely that it will occur when the regulator is dropped out of one’s mouth at the surface, hits the water with the diaphragm down, and triggers the venturi effect. This lever is often labeled with a “+” and a “-.” The “+” means the air passage is uninterrupted and the venturi effect is ON. The “-” means the air passage is slightly obstructed and the venturi effect is OFF. SO — If triggering a free flow by dropping one’s regulator into the water is a problem, set this little device in the “-” position. It does not change the basic adjustment of the second stage at all — it only interrupts the venturi effect.


One possibly unfortunate characteristic of gasses which contributes to “Massive” free flows is called “adiabatic cooling.” This can be a little more troublesome especially when combined with cold water and a regulator that utilizes the venturi effect to enhance performance. So here is Adiabatic Cooling:

“Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings.” (

This may help explain this physical behavior: As most divers know, when one fills or pressurizes a SCUBA tank, the temperature of the air in the tank rises as the gas molecules in the tank become more dense. Now the opposite is true when the gas in a SCUBA tank is released and expands — it gets cold. One sometimes sees the effect of this cooling with the visible formation of frost on the valve of a SCUBA tank (or propane tank) that is being rapidly emptied. This adiabatic cooling occurs at every stage of depressurization of diving gasses. In other words, when a diver breathes gas from his regulator he is causing the gasses to expand and thus results in adiabatic cooling at that point in the system (usually most noticeable at the valves).

So here is what can happen:

  1. Cold conditions — air temps at, near, or below freezing — Water temps near freezing. (I know, I know, if it is frozen we can’t swim through the stuff.)
  2. Something triggers a rapid airflow through a valve (like the second stage is dropped into the water) which results in adiabatic cooling — which further lowers the temperature at the valve.
  3. Which can then cause a valve in a moist environment to actually FREEZE open! The only solution in this case is to turn the air off, allow the valve to re-warm to above freezing — then slowly turn the air back on and only actuate the valve in an environment above freezing while avoiding rapid air flows. This is why one can dive in 36˚ F water without a problem as long as no rapid airflow is triggered — like taking one’s regulator out of one’s mouth underwater while pointing the mouthpiece upwards (which will trigger a free flow). When training students I try to habituate them to ALWAYS point the mouthpiece down when they remove it from their mouth in the water or immediately above the water.

Several Aqua Lung regulators are designed to minimize such adiabatic cooling freeze ups. This is usually visible externally on the regs with “fins” to gather the heat from the ambient environment, environmentally sealed first stages, and some with the exhalation port directed around the second stage valve to extract heat from the diver’s breath. The Legend Supreme series, the new Titan LX Supreme, the Kronos Supreme, and the Legend Glacia (Which is really engineered for ice diving!) All of the APEKS XTX40 and above regulators are engineered for extreme cold environments.

APEKS also makes an inline (hose) device to eliminate or stop free flows.

So in actuality, not all free flows indicate a problem with the regulator — in fact, a properly adjusted regulator can easily be triggered to free flow which indicates that the design of the regulator is such to enhance the venturi effect and actually improve the breathing performance of the regulator!

I hope this encourages some questions! I am more that happy to explain and demonstrate this stuff “hands on” at the shop.

(Submitted to and approved by Todd Anderson — Aqua Lung International Engineering Dept. : 4.9.10)