Deep Stops and Gradient Factors May 16, 2018 by lukesheppard

So I’ve been reading up on advanced decompression theories and current understandings to further my diving.  I was watching a great lecture by Dr Simon Mitchell, linked at the bottom, who explained decompression stops really well and thought I’d share with you guys.  This is an ever changing subject that does affect us and I’m interested in your thoughts and any studies and evidence you may have come across.

Deep Stops

My first dive computer had a setting to disable Deep Stops, which I never really learned about in my diving courses so I figured it was like a safety stop but deeper and somehow better so I left it on.  Now I understand that there are two perceived methods for in water decompression.  Beyond this point I’m assuming you have a pretty fundamental understanding of what happens to your body under pressure along with saturation of gases in tissues and that different tissues have different speeds.

As you descend the ambient pressure increases and the more gases you breathe in get absorbed into your tissues until they reach equilibrium or saturation.  The deeper you are the faster the rate as the gradient is greater.  As you ascend the ambient pressure decreases and the adsorbed gases in your tissues make them supersaturated and the excess gas needs to escape.  If they escape too quickly then a DCS occurs so years ago decompression algorithms were drawn up so you can ascend safely.  Ascending is ideally as fast and as safe as possible and the verdict is still out on the best way to do it.  The best way to ascend would be so slowly that you’re tissues never go above the saturation line but that isn’t practical.

To put it really simply you have to stay under a certain line of absorbed gas to ascend safely and there are a couple ways to do that.  The first method sticks pretty closely to the super saturation line while staying below it.  Drawn up by Haldane this method dramatically reduced decompression problems but divers where still found with bubbles in their systems occasionally, so a new method was researched to try to minimise bubbles forming.

Time isn’t shown too well in these graphs but if you watch the video at the bottom they’ll make sense.

Because these bubbles were being found in some divers this led to Deep Stops as it was believed that the first ascent to a shallower stop will cause bubbles to form and deeper stops would be better to prevent bubbles from forming. Deep stops also had the added benefit of being shorter as you don’t get so close to the supersaturation limit and can exit the water faster.  Sounds good right? Bubbles don’t form and you have shorter stops so you can ascend earlier.

Deep stops are great for fast tissues, that absorb and expel gases quickly, but for slow tissues they are still absorbing gases during a deep stop which is made to get rid of dissolved gases getting you into more trouble later on in the dive.  If you look at the algorithm of your dive computer it will be something like a ZHL-16 ADT MB with the 16 meaning that it accounts for 16 different tissues.  Some tissues saturate and desaturate really fast whereas others take forever to desaturate.

So the traditional or Gas Content Model can cause bubbles to form and Bubble Models leave your slow tissues supersaturated, which is best?  This is where gradient factors can come in.

Gradient Factors

 Gradient Factors allow you to control your decompression algorithm and change how you ascend to make your Bühlmann model act like a Bubble model.  A gradient factor is two numbers which draws a new line that your computer will keep you under to get you to the surface.

A sensible gradient factor should help prevent bubbles forming at your first stops as you’re not ascending too far too fast but you’re not as deep that your slow tissues are taking on gas. You always stay under your super saturation limit and you spend less time on stops.

Here are a couple random gradient factors so you can better see what effect they have on your stops.

So this all mainly applies to deep diving beyond recreational limits but it’s worth knowing what’s going on in your body even at shallower depths.  It’s also interesting to know that we haven’t quite figured out a right or wrong answer as to how decompression works and how best to ascend from a dive.

Granted most of this doesn’t take into account many other factors that affect decompression like body temperature, body composition, exertion and a whole host of other factors that you need to consider. There are whole studies dedicated to how your body temperature during different times parts of the dive can affect you.

So what do you think is the best decompression method is and what do you practice? Do you even know what algorithm your computer uses? Or the benefit of different algorithms?  All I usually hear is that ‘Suunto computers are too conservative’ but I doubt they know that Suunto computers have a range of 3 algorithms to choose from…