Pressure

Under pressure.  The laws of physics applied to diving.

For you to understand the ins and outs of diving you must first understand the concept of pressure, how it varies according to the depth that you dive and what these variations mean to you.  You may well be thinking that laws of physics discovered a couple of hundred years ago are pretty irrelevant or at least unimportant or uninteresting when it comes to modern day diving.  But please read on and then make your decision.

Were going to look at pressure. Pressure is a force or weight per unit area. Everything weighs something – even air.  Therefore everything and everyone on the surface of the earth is exposed to air pressure. This pressure is called ‘Atmospheric Pressure’.

You don’t have to be a rocket scientist to know that water is heavier than air.  Therefore, as pressure is related directly to weight, water pressure must be far greater than air pressure.

Gravity keeps the air that keeps us alive, the atmosphere, held next to the earth.  (No gravity would basically mean that the air would be weightless and would  float away into space. ) The force of gravity is strongest the nearer you get to the centre of the earth.  Therefore, at sea level the force is greater than on the top of Mount Everest. This is why mountaineers must also carry air to breathe.  9,000 metres above sea level the air is about one third as dense as it is  at sea level, and therefore weighs less.  In fact everything weighs less the further you get from a centre of gravity, humans don’t notice the difference at the top of a mountain, but if you keep heading away from earth you soon become weightless as any astronaut will tell you.

 Air pressure can be specified in several ways – the most popular term used in scuba diving is “Pounds per Square Inch” or “PSI.” At sea level the pressure exerted by the atmosphere is 14.7 PSI. 

Another way to get your head around the idea of almost 15 pounds of air per inch is to remember that we’re talking about a column of air one inch square and about 50 miles high!  So that’s not really a lot of pounds for something that high.

Can you lift 300 pounds with one hand?  It might come as a shock to you but this is something that everyone from Arnold Schwarzenegger to your Grandma can do.  Here’s how . . . open your hand palm upward, now lift your hand quickly upward with the palm flat out.  Phew! Take a rest, shake it out!  Hit the showers!

What was all that about?  The average open adult hand with fingers closed  has an area of about 25 square inches.  Assuming you are at sea level, and your hand is average-sized, you are lifting 25 x 14.7 or 368 pounds of air!

So why is it so effortless? It feels effortless because air pressure is evenly distributed around your hand, and the molecules of air are easily movable. At sea level, air pressure is 14.7 pounds per square inch on top of your hand, underneath your hand, and on all sides. Therefore, you don’t really ‘lift’ 368 pounds, though that is the weight of air on top of your hand.  As you move your hand you move some air molecules out of the way and other molecules immediately come under and around your hand. The pressure surrounding your hand stays the same: 14.7 PSI and because the pressure is evenly distributed, you don’t feel any weight in lifting your hand. 

Pressure goes by many different names.  ‘Ambient Pressure’ is the pressure of your immediate surroundings. When surrounded by air, ambient pressure = atmospheric pressure = barometric pressure. When you’re surrounded by water, ambient pressure = water pressure.

The most commonly used units of pressure are ‘bars’ and ‘atmospheres’ (atms).  The main difference is that the term ‘bar’ is more common in Europe.  One atmosphere / One bar of pressure = air pressure at sea level = 14.7 PSI. 

Remember that this is just a measurement.  If you were inside a submarine you’d find that you are surrounded by one atm. of pressure, however the hull of the vessel may well be under a pressure of 10 atm.

If you know how much sea water weighs then it’s easy to calculate how much pressure you are under at a certain depth.  As it happens, sea water weighs about 64 pounds per cubic foot. Using this value, 33 cu. feet of water weighs 33 x 64 = 2112 pounds. So if you dive 33 feet deep and lie horizontally you will have 2112 pounds of water over every square foot of your body.  2112 pounds of water per square foot = 14.7 pounds per square inch, which is the atmospheric pressure at sea level.

So at 33 feet (10metres) under the water, you are under two atmospheres of pressure.  One from water directly above you and one from the air directly above that.   The deeper you dive the more the pressure increases, an increase of one atm. for every 33 feet (10 metres) depth.

Air is a mixture of gases, mainly oxygen (21% by volume) and nitrogen (78% by volume). The other 1% of air is made up of several other gases such as carbon dioxide (CO2), argon, krypton and neon.

In any mixture of gases (e.g., air), the individual gases don’t chemically combine with each other. The percentages don’t alter inside a tank of compressed air regardless of depth. This fact takes on critical importance as water pressure increases with increasing depth because, although the percentages are unchanged, the total pressure exerted by each gas component increases proportionately. The increases in component gas pressures account for some of the major problems inherent in compressed air diving: nitrogen narcosis, decompression sickness and oxygen toxicity (see Sections G and I).

Scuba divers are interested in what happens to air under water. Air under water obeys the same laws as air in the atmosphere.  The four gas laws, Boyles’s, Charles’, Dalton’s and Henry’s, are useful because they predict changes in air pressure, volume and temperature as compressed air divers descend and ascend.

Boyle’s law states:

At constant temperature, the volume of a gas varies inversely with the pressure, while the density of a gas varies directly with pressure.

Therefore, if you increase the pressure on a fixed volume of gas, the density increases. This part of the law becomes important on deep dives.  In reality it means that the inhaled air will become denser the deeper one goes. Therefore, the deeper you go, the more difficult you will find it to breathe.

Charles’s law states:

‘At a constant volume, the pressure of gas varies directly with absolute temperature.’

Given a constant volume of gas, such as that trapped in an air tank, the higher the temperature the higher the gas pressure, and vice versa.   Charles’s law is more important for dive operators and those involved in filling air tanks – especially when there is a large difference between air and water temperatures.  A tank filled in the icy cool surroundings of an air-conditioned room, will show a different pressure reading as soon as it is put in warm sea water. 

Dalton’s law states:

‘The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied the total volume.’

In layman’s terms, the pressure of any gas mixture (e.g., air) is equal to the sum of pressures exerted by the individual gases (e.g., oxygen, nitrogen, and each of the minor gases).

With increasing altitude, for example, the partial pressure exerted by each gas in the air will decrease. With increasing depth, the partial pressure exerted by each gas in the air we breathe will increase.   As you are breathing this air into your body the effects of the increase of pressure are felt inside you.

Henry’s law states:

‘The amount of any gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas in contact with the liquid and the solubility coefficient of the gas in that particular liquid.’

To keeps things simple, this law implies  that as the pressure of any gas increases, more of that gas will dissolve into any solution with which it is in free contact.

Taken together, Henry’s and Dalton’s laws predict two very important consequences, one applicable to mountaineers, the other to divers:

1)When ambient pressure is lowered (as at altitude), the partial pressure of oxygen and nitrogen in the body must fall, and there will be less molecules of each gas dissolved in the blood and tissues.

2)When ambient pressure is raised (as when diving), the partial pressure of oxygen and nitrogen in the body must rise, and there will be more molecules of each gas dissolved in the blood and tissues.

The second statement is the physiologic basis for three important problems associated with compressed air diving: decompression sickness, nitrogen narcosis, and oxygen toxicity.

And that’s why the laws discovered by geeks hundreds of years ago are important to you as you hop off your dive boat and into the clear blue sea in some exotic destination.

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