UTD Student & Diver Procedures #18. Ratio Deco (UTD Student & Diver Procedures Manual) by Andrew Georgitsis

Author:Andrew Georgitsis [Georgitsis, Andrew]
Language: eng
Format: epub
Publisher: UTD International LLC
Published: 2011-07-22T00:00:00+00:00

Oxygen Windows

We introduce the use of a decompression gas, or oxygen window, to help improve the quality of decompression and provide an additional breathing gas to allow us to carry less rock bottom gas in our back-gas. The standard deco gas choice we use has an average PPO2 of 1.2 over the 5 deco stops (except the oxygen bottle, which we breath at 20’/6m with a constant PPO2 of 1.6 – however, we do toggle on and off the oxygen protecting ourselves from oxygen toxicity and pulmonary damage).

We normally start breathing the decompression bottle at a max operating depth (MOD) of a PPO2 of 1.6, then as we ascend the PPO2 slowly declines to about 0.8 before we switch to the next decompression bottle. The next bottle will return us to a high PPO2 and will reopen an oxygen window and provide us a new, independent source of gas, also effecting our rock bottom gas planning.

What Do We Mean by Oxygen Window?

Essentially an oxygen window does three things:

1. Creates an artificial gradient between the free phase gas partial pressure of inert gas in the bubble, and the ambient gas partial pressure of the inert gas in the blood stream.

2. Creates an artificial gradient between the dissolved gas partial pressure of the inert gas in the tissue and the ambient partial pressure of the inert gas in the blood stream.

3. Increases the partial pressure difference of the dissolved oxygen in the venous blood and the arterial blood.

The greater these differences, or gradients, the greater the “oxygen window.” Let’s take a deeper look at numbers 1 and 2. This gradient, or window, is created by breathing higher partial pressures of oxygen which creates lower pressures of inert gases in the bloodstream. This creates the gradient between the PPN2 in the bubble / tissue and the PPN2 in the Blood (or PPHe). This gradient is created without ascending or lowering the ambient pressure (which could create bubble growth, or the potential for bubble formation and growth in the tissue). So we still have a gradient, or window, mimicking an ascent, but without consequences or reducing pressure and risking bubble growth.

Essentially, we create the gradient of inert gas by using the PPO2 of the inspired gas to offset the partial pressure of the inert gas in the arterial blood stream. The greater the PPO2, the greater the offset, and therefore the greater the gradient, or in other words, the more open the oxygen window. The deeper we go, the greater the internal partial pressure of the bubble, and if we are still breathing O2, then the greater the window. We do have to balance oxygen toxicity risk so we restrict ourselves to a PPO2 of 1.6 max when in the water, or a 1.2 average as stated earlier. In a dry environment we can drive this up to as high as 3.0 such as we do a decompression chamber.

We also have the third point (above) at work – the partial pressure difference of the dissolved oxygen in the arterial blood and the venous blood.

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