Liftbags and Life


Larry "Harris" Taylor, Ph.D

This is an electronic reprint and expansion of an article that appeared in SOURCES (May/June. 1990, p. 40). This material is copyrighted and all rights retained by the author. This article is made available as a service to the diving community by the author and may be distributed for any non-commercial or Not-For-Profit use.  

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Many divers today in beginning search and recovery operations are being trained to use their regulators (either primary or secondary) to inflate lift bags. In other words, they are being told to take their life support and risk their ability to breathe in order to raise some inanimate object off the bottom. I believe this to be an unwise procedure, particularly since this technique is known to have caused at least one fatality in Michigan waters.

When you press the purge (on either your regulator or the octopus) you initiate rapid air movement through the first stage. This air moving from cylinder pressure to the much lower intermediate pressure undergoes what a physicist would call an adiabatic expansion. In order to conserve internal energy (one of the "basic principles" of Mother Nature we call thermodynamics) the temperature of the gas drops considerably (as much as -100 to -150  oF). This is known as the Joule-Thompson effect. This temperature drop, especially in cold water can drop the temperature of the gas far below its dew point (temperature at which water vapor condensation MUST occur). Because of the low temperature, any water condensing is rapidly frozen. The rapidly formed ice can either shut off gas flow, or "freeze" the valve in an open position. Most often, this Joule-Thompson effect scenario leads to a free-flow situation from ice keeping valves in an open position. 

The resultant free-flow and rapid air loss can prove too much for many divers and panic (followed in the worst case scenario by rapid ascent, pulmonary barotrauma and perhaps even a diver death) may follow. This is believed to be what occurred in the Michigan fatality. Incidentally, the possibility of free flow contributing to such a diver casualty is one reason why coroner's inquests frequently require a dew point analysis of victim' s breathing mix. It is also a contributor to the CGA limitation on Grade E air to a depth not to exceed 130 fsw. Divers in deep, cold water should take precautions to insure that their breathing mix has a dew point at least 100 oF below the temperature of the at-depth water 'cause Joule-Thompson related regulator problems simply cannot occur if there is not enough moisture present to condense and freeze. The substantial decrease in dew point (absence of water vapor) in one of the major differences between so-called Aviator's grade oxygen and other USP oxygen grades, as well as a significant difference in standards for breathing air. (Those diving below 130 fsw, especially if the temperature is less than 50 oF should consider using Grade J air in their breathing mix to minimize risks from Joule-Thompson-water-vapor-related scenarios.)

There is also a significant probability of getting your regulator or octopus entangled in the lifting rigging. Of particular concern here is the use of alternate air octopus/inflator systems  which do not give the diver sufficient room to adequately judge the lift bag inflation operation. (There have been reported incidents of divers catching these alternate air systems in the lifting rigging and taking an unwanted rapid ride to the surface.)

BOTTOM LINE: The use of a regulator (primary or secondary) for inflation of a rigged lifting device is an undesirable and unsafe diving practice, particularly in a training situation with Grade E (or lower quality) air and most definitely in cold Northern waters.

To minimize risk, I use a diaphragm first stage (concerned that debris might affect a piston system). There is no second stage. Instead, I use a Sea Tec air gun  connected to a hose from the low pressure port. The quick connection on the hose can be used for a variety of air-driven tools, including tire inflators. I most often use the trigger activated air gun. For convenience, a thicker hose protector assists in handling the air gun.


The Regulator Assembly

The air gun is an upstream valve (will not open in an over-pressure event) and, as such, failure of any valve between the cylinder and the air gun outlet could lead to an over-pressurization situation. To prevent this, an overpressure valve is added to the first stage. Finally, a miniature pressure gauge (to reduce entanglement by eliminating the hose to an SPG) is installed into the first stage. Since this regulator is not part of life support and only used for tools, this small gauge is sufficient.


Overpressure Relief Valve                                            Miniature Pressure Gauge

The regulator assembly is then used on any size cylinder that will accept a scuba connection. This rig is easily handled underwater. The air can be added in controlled bursts for initial lifting control and any compromise of life support, especially in novice search and recovery operations, is avoided. 

Lou Fead ( THE EASY DIVER ) said it best, " Dive with your brains, not your back!" There is no reason to risk your life on the frivolous use of a lifting bag!


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About The Author: 

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2022  by Larry "Harris" Taylor

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