Best Practices – Dive Planning Part 3

In the previous articles, we discussed some strategies for better dive planning. In this article, we will discuss forming an ascent strategy so that we can safely put into practice what we discussed previously.

First, let us briefly discuss a couple common decompression models. In order to safely utilize the strategies I have discussed previously, you must have a basic understanding of decompression theory. There are two models that most decompression software and tables use. The first model is the dissolved gas phase. This model has it’s roots with Haldane and Buhlman. In a nut shell, this model assumes that a great reduction in ambient pressure is required to drive out the dissolved inert gas in our body. This model does this by ascending to a shallow stop depth and staying there for a long period of time. Thus the radical reduction in ambient pressure on our bodies causes the inert gas to move out of it’s dissolved state, into our blood stream, into our lungs and out of our bodies through exhalation. The second model I mentioned is the free phase model. The theory behind this model is that through a great reduction in ambient pressure, bubbles come out of our tissues and move freely through out bodies. To control the size of the bubble growth, the model starts our safety stops deeper. Thus limiting the size of the bubble growth. Then we can control our ascent by doing a series of short stops till we get to the surface.

By now you are probably asking yourself “This is great, but what does this got to do with recreational dives?” Simply put, you need to understand what is going on inside your body while you dive. When you start to think about this stuff, you can start to realize the consequences of your actions. That is, if you want to push the limits of no decompression limits, you need to be able to control bubble growth and off-gas efficiently to make it to the surface safely.

In order to end our dives safely, we need to be able to 1) control bubble growth and 2) off-gas efficiently. In order to do this, we can create an ascent strategy based on both models mentioned above. To create this strategy, we must know when our bodies start to off gas. That is, when we start our ascent, when will bubbles come out of our tissues. The general rule of thumb is that our bodies start to off-gas at 75% of your average depth, in feet. So if you do a dive to 100 feet, your body will start off-gassing at around 75 feet. So if our goal is to control bubble growth, we need to start our stops deeper. I’m not suggesting that we start our safety stops at 75 feet from my example. However, we need to start slowing down our ascent at that 75% depth. To slow down our ascent, we change to an ascent rate of 30 feet per minute and introduce a deep stop. In recreational diving, a deep stop is a pause at 50% of our average depth. The deep stop is a pause, that is it should be between 30-60 seconds in duration. After we complete the deep stop, we then ascend 10 feet at a time till our first safety stop. Each stop at 10 foot intervals is the same duration as the deep stop.

If you are still paying attention at this point, you read that I mentioned our first safety stop. That’s right, our ascent strategy to incorporate the best of both decompression models mentioned previously requires us to perform multiple safety stops. In traditional open water classes, the safety stop is defined as a stop at 15 feet for 3 minutes. This is based on the Buhlman decompression model. You ascend, stay at a shallow depth to off gas. However, if our goal is to control bubble growth, we need to start the safety stops deeper. To do this, let’s spread our three minutes over multiple depths. That is we spend one minute at a deeper depth, then one minute a little shallower, then finally one more minute at our shallowest safety stop. So, we start our first safety stop at 30 feet for one minute. Then move to 20 feet for another minute. Finally we do our last safety stop at 10 feet for one minute. If we average this out, we get 3 minutes at 20 feet. Pretty darn close to the safety stop taught in traditional open water classes.

So our ascent strategy should incorporate deep stop methodology and the safety stop procedures mentioned above. Let’s do an example. If we do a dive to 100 feet for 25 minutes on air (yes, yes, I know). This dive is pushing the limits of our no decompression time. When we leave the bottom, we can travel at a rate of 60 feet per minute till we get to 75 feet. We can do this because our bodies won’t start off gassing until we reach that 75% depth area. Once we reach 75 feet, we slow down to 30 feet per minute until we reach our first deep stop. This deep stop will be at 50 feet. Once we hit 50 feet, we hang out there for 30 seconds and then move up to 40 feet, then 30 feet. At 30 feet, we stay a full minute. Move to 20 feet and 10 feet for one minute respectively. Now, once we finish our last safety stop at 10 feet, we can’t just go rushing to the surface. Our ascent from 10 feet to the surface should take about a minute. The reason for this is that the shallower we are, the easier/quicker bubbles form. If surface conditions make a 10 foot uncomfortable or impossible, due to wave conditions, surge, etc. we can can stay at 20 feet and do two minutes there. Then the ascent to the surface should be between two and three minutes.

So now you can see how we can create an ascent strategy to 1) control the growth of bubbles, and 2) off-gas those bubbles efficiently. One thing you must remember, is that when you start to incorporate ascent strategies like this, you MUST take into account that you have to reserve enough gas in your tank(s) to be able to perform this ascent, in addition to reserving enough gas for you and your buddy to perform this ascent while on an air share.

I welcome your questions and comments.

Dive Safe,
Duane
www.precsiondiving.net

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