Decompression Theory: Half Times

The theory exam in the IDC/IE is the part most candidates can struggle with. Sure, you can dive and give a beautiful underwater skill demonstration, but trying to get your head around some of those tricky theory questions can sometimes be a challenge. Whether it’s the physiology, physics or equipment, that your having trouble with: we can help. We have written a series of blog posts that explain in a nice and simple way some of the harder concepts so you can ace the dive theory!

Half Times

The concept of Half-times is part of diving theory that many people get confused about, but don’t worry—it’s actually a pretty simple concept. Let’s break it down...

What's Being Timed?

Our bodies are subject to the gasses we breathe when diving. Some gasses like Oxygen, are used by our bodies. However, some gasses are inert, like Nitrogen, meaning we cannot use them. These inert gasses are still absorbed by our bodies, but since we have no use for them, all the inert gas that has been absorbed, must then be released. When we go underwater and are subject to pressure, our body starts absorbing Nitrogen. When we come back up and the pressure decreases, the Nitrogen will be released again. But it doesn’t happen instantly. This is where half-times come in.

But before that, we need to understand tissue compartments…

Our bodies are made up of various types of tissues (muscles, fat, bone etc) and each one will have a different rate of absorption, largely depending on their blood supply. Those which have a better blood supply will absorb and release gasses faster. In order to make it simple, a mathematical theory has been created, which places all our tissues into 14 theoretical categories, which are called “compartments”. Each compartment is based on their rate of absorption, so therefore has a different half time.

So, back to half times...

Currently, our tissues are in equilibrium; the amount of gas dissolved in our tissues, is equal to the surrounding pressure (1 bar). When we descend to depth, and the pressure is increased, our bodies will no longer be in equilibrium with the surrounding pressure. However, if we remain at depth, our body would continue absorbing gas, until, eventually, it would be equal to the surrounding pressure and once again, be in equilibrium. A half-time is the time (in minutes) it takes for that tissue compartment to go halfway from its beginning pressure to saturation pressure. But this time is diminishing: If you place a tea bag in a hot cup of water, to begin with it will rapidly start to colour the water and the rate of infusion will be very fast. As you leave the teabag in the water, it will continue to infuse, but at a slower rate and the water will remain the same colour. This concept is the same for half-times in tissue compartments.

At 6 halftimes, we consider the compartment as full

Each half-time gets you halfway closer to full saturation, but it never quite gets to 100%. When a compartment get to 6 halftimes, we consider the compartment to be full.

Half-times are measured in meters of seawater…

So, let’s use a super simple example we go for a dive to 40m:

• At 1 half time, we would have absorbed half the amount of Nitrogen, so 20msw of N

• After another halftime, we would have absorbed half the remaining amount, so 30msw

• At 3 halftimes, half again, so 35msw and so on.

To Sum up..

Half-times are just a way to model how your body absorbs and gets rid of nitrogen. By understanding them, dive computers and tables help keep divers safe from things like decompression sickness.