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A Practical Guide for Divemaster Trainees, Instructor Candidates and Anyone Interested in Exactly How Much Air is Required to Lift an Outboard Motor off the Seabed

Alex Griffin, Diving Leisure London

How many people have ever gone into space on an aeroplane with a big bottle of Coke and then all the Coke has fizzed up and gone in your lap and your ears hurt? Well that's kind of like when we go diving. Understanding basic dive theory will help not only in your diving but in all aspects of your life, although mainly just the diving bits.

Divers are introduced to dive theory right from the off on an Open Water course. Buoyancy, pressure and life threatening lung injuries are explained on the DVD by a fat man going red from sunburn as he comically attempts to fill a cylinder with a bicycle pump. However, for most divers their first introduction to the real substance of dive theory takes place on the Divemaster course. The theory is split into five main areas: physics, physiology, decompression theory, dive skills and the environment. Instructors are also tested on their theory as part of their exams, so knowing your way around each subject is crucial in your ability to not look a bit thick next time a student diver asks you what a shallow water blackout is (the answer, as everyone knows, is unconsciousness brought on by a drysuit squeeze to the knackers).
Over the next few issues we'll be going through each of these areas in depth (By depth we mean a kind of scratching of the surface, very basic level of comprehension required to maybe pass a multiple choice exam. What do I look like, some kind of geek? Nobody answer that...).
So, open your dive theory workbooks to chapter 1. Set your boredom threshold to stratospheric and come with me now as we pass through the crotchless drysuit into the arcane world of physics...


Interestingly heat, light and sound are all things totally absent at the bottom of many inland UK dive sites. Although a sound grounding in physics will not tell you which direction the surface is, it will definitely impress all the dive chicks in the bar should you return to tell your stories.

Due to the fact that water is much denser than air you will lose body heat underwater via conduction up to twenty times faster than in air for a given temperature. That is unless you've left your drysuit zip open in February, in which case heat loss can be recorded at up to two hundred times faster and is the scuba diving equivalent of being flash frozen in liquid nitrogen. Not many people know that Walt Disney was preserved by being tricked into diving in Wraysbury in a shortie. He is kept in the freezer next to the sausages. Not many people know this… because I just made it up.

Again, due to its density, light waves change speed as they pass from air into water. They then change direction again as they pass through the glass of your mask before speeding up again as they hit the air behind it. This process is called refraction and causes a magnification effect that makes objects appear closer than they actually are by about a third. This can be quite annoying when trying to break off all the bits of coral that are obscuring the wreck or hitch a ride on the fins of whale sharks. Fortunately most divers' brains adjust quickly to the effect and soon you'll be filling that mesh bag with lumps of staghorn coral for the bathroom window sill.

Sound is transmitted four times faster through the water than in air, again due to the density and also elasticity of water (the elasticity bit is for any scienticians reading this article). Underwater this means that the symphony of air horns, quackers, tank bangers, rattles and revving boat engines appear to come from all around you. It's like Phil Spector's wall of sound, without the insanity, murder and subsequent prison beatings.


Buoyancy was discovered by Archimedes Screw who noticed the level of the bath rising as he conducted a violent morning ablution. The principle is that if you stick something into water then it will move a load of water out of its way. This is called displacement. If the amount of water displaced weighs more than the object, it will float, if it weighs less it will sink, if it weighs the same then the object is neutrally buoyant.
Ocean Visions
This is a very important principle because it allows us to work out the most essential skill in scuba diving: How much air to put in a lift bag to bring an outboard motor off the bottom. As I'm sure most of you will agree, I am tired of continually having to lift outboard motors, carelessly dropped by boat skippers, using the exact right amount of air in the lift bag to make it neutrally buoyant. As I write this, it is late February and I have already had to recover seven outboard motors, each time with the exact right amount of air to make it neutrally buoyant. An uninformed, ill educated and stupid person might say 'Why don't you just put air in the lift bag until it starts to rise?' But they would be wrong and I would mock them mercilessly for their crass, moronic question. Until skippers start looking after their outboards there will continue to be motors littered all over dive sites and this will still be a vitally important skill. And don't even get me started on sinking heavy objects whilst assisting on research projects. I've done three of them this month so far.


Understanding what happens to things as they go under the water is important in comprehending why the outcome of a mediastinal-whoops-a–daisy is as unpleasant as it is or, a genuine question, whether farting on the deck of the Titanic causes death.

If you cast your mind back to your Open Water course you will remember a chart showing the effect of depth on a balloon. Basically, as you take it deeper under the water the pressure causes the volume of the air in the balloon to decrease. At the same time the density increases, a bit like a diver's brain as the narcosis descends. To determine how pressure and volume change with depth you can use a very useful equation by a man called Robert Boyle:
Pressure1 x Volume1 = Pressure2 x Volume2

Sadly despite coming up with an equation to describe one of the foundations of modern physics, Boyle was to become more famous for being the first celebrity to have his name shortened to a couple of vowel sounds by the cretins at Heat magazine. This was in the olden days when there weren't programs like 'Big Brother' and 'I'm a Fuckwit Please Shoot Me Dead' to entertain the mouth breathing masses. In these times inventing a gas law was like sleeping with your team mate's wife and the press couldn't get enough.

In the end 'Robo' couldn't take the pressure and he was forced to leave the band. Charles and Dalton carried on without him for a bit, there were two more albums and limited success with a new equation that brought temperature into the mix but an ill-advised rap rock collaboration with the already unfashionable Fred Durst from Limp Bizkit sealed their fate and the gas scientists were dropped from their recording contract. A recent VH1 program brought the surviving members of the group back together. Dalton is now a successful record producer whilst 'Robo' went to work for British Gas and is a devoted family man. The sordid end of Charles in a hostel in Rotterdam needs no repetition here.


SAC rates are a way of working out how much air a diver will use depending on his/her depth and have absolutely nothing to do with the frequency of pendulous movement of a ball sack in a pair of loose fitting shorts. That'll be the second mention of men's genitalia in this article. Oh dear.

Working out SAC rates is easy. If a diver uses a certain amount of air on the surface then multiplying that amount by the pressure of the depth he's at will give you his new air consumption rate.


All the individual gases within a mixture exert their own pressure distinct from the other gases, a bit like the current political parties and their share of the vote. Nitrogen, for example, could represent the Tories, oxygen represent Labour and the few bits of a percent that make up all the rare gases like argon and neon representing the braying morons of the minority parties like UKIP and the BNP. Partial pressure is expressed as a fraction, as in UKIP exert a partial pressure of 0.03 or alternatively this is the current fraction of the British public that, despite appearances to the contrary, are effectively lobotomised.

As you head under the water the amount of gas you have to breathe into your lungs to fill the same space increases. This means it becomes possible to breathe gases at a partial pressure far greater than you would ever be exposed to on the surface. This is important in understanding how the percentage of oxygen in a mix can become toxic at a certain depth and how having only a very small percentage of carbon monoxide in a cylinder may be of no consequence at the surface, but at depth can become a serious problem.


Blimey, this is heavy going isn't it? This bit is all about how gases always want to reach an equilibrium, but I've kind of had enough now and we'll talk about this in physiology in the next issue. So I could just head straight to the summary. Hang on a sec, if I'm going to get top marks for this presentation I'd better sell a bit of dive kit: How about that new Uemis dive computer eh? Now that's what I call a dive computer. It has pretty much every feature you could want in a computer and it's going to be upgraded to full trimix later this year for free! [Go to your summary – Ed]


So is physics really, really boring? Well, the only empirical way to look at this question is to examine the relationship between science and music, a case perfectly illustrated by the BBC's attempts to sex physics up by getting the professor who used to be the keyboardist in D:Ream to front its science programs.
Aquamarine Silver
Science and music rarely crossover, in fact the last recorded time was when Brian May left science to play guitar, an uncool to cool equation only slightly unbalanced by the sheer dreadfulness of his hair and his marriage to Anita 'Anyone Can Fall in Love' Dobson.

The case of Professor Brian Cox leaving D:Ream for science upsets the balance somewhat by dint of the fact that there is nothing remotely good whatsoever about D:Ream (interestingly in 2003, when it was beginning to become apparent that things really weren't going to get any better and were in fact getting worse, D: Ream were safely sealed inside an iron ball and fired at the sun. Cox's apparent talent for science and his potential use on the Large Hadron Collider project is what saved him).

Cox also has pretty cool hair and seems like a nice guy. He does however take on a dreamy look of semi arousal when discussing solar flares which probably does scare women. Talking of dreamy looks of semi-arousal, did I mention the new Uemis dive computer?

Anyway that's about 2000 words so I'll stop. Next issue we'll tackle the subject of physiology. Embolism anyone?
Denney Diving

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