Basic Gas Law's:
GAS LAWS: Why do we need physics? Any diver has to understand these in order to keep his dive safe. The human body needs oxygen to survive. This has to be supplied as gas. Gases are interacting in certain rules of pressure and temperature to the fluids in our body. Therefore, we need gas laws. The physics are understood, however, some physiological interactions with the human body are still to be examined and are not fully understood.
Make it easy, start with the pressure.
Pressure = Force P= F (unit)= kg
Area A cm² Area (unit) = cm²
P (unit) = Atmosphere The whole atmosphere of air is creating the atmospheric pressure. The atmospheric pressure at sea level is referred to "one atmosphere" under standard conditions. In diving we are speaking about "Atmosphere Absolute" (ATA). At sea level or on the water surface at sea level the pressure is 1 ATA 10 metres of water, what is much denser than air, exert an additional pressure of 1 ATA (hydrostatic pressure) to the diver.
Understand this:
You don't feel it, however, at sea level there is an "atmospheric absolute" pressure of 1kg/cm² body surface. This is 1 ATA Every additional 10 meters the diver descends the pressure increases at 1 ATA In 10 meters a diver will face 2 ATA (Atmospheric Absolute Pressure)|
Absolute Pressure |
Indicated Gauge Pressure |
Depth of seawater |
| 1 ATA | 0 ATG | Sea level |
| 2 ATA | 1 ATG | 10 meters |
| 3 ATA | 2 ATG | 20 meters |
| 4 ATA | 3 ATG | 30 meters |
e.g. ATG indicates 2. The ATA is 2+1 = 3. The diver is at 20 meters depth and is receiving an absolute pressure of 3 atmospheres.
Boyle's Law: back top home It describes the relation between pressure and volume of a gas. It is a very important law for divers!! Definition: Boyle's Law states that, the product of the volume and pressure of a fixed quantity of an ideal gas is constant, given constant temperature. Expressed mathematically, the formula for Boyle's law is:
V*P=k
where:
V is volume of the gas.
P is the pressure of the gas.
k is a constant Sound difficult, no worries it isn't. How to calculate the changes in volume (V) and pressure (P)? As V*P=k (constant) a change must happen either in volume or pressure The expression is as follows P1 * V1 = P2 * V2 A diver takes a breath at surface of 6 litres and descends to 10 meters. What will happen? P1 = 1 ATA V1 = 6 litres P2 = 2 ATA (10 metres water = 1 ATA hydrostatic pressure + 1 ATA atmospheric pressure
- you remember?) V2 = ?? P1 * V1 = P2 * V2 1 * 6 = 2 * ?? 1 * 6 = V2 2 V2 = 3 litres In 10 meters water depth the volume in the divers lungs is reduced from 6 litres to 3 litres! That's on descend, on ascend the same thing will happen vice-versa. A diver takes a breath of 6 litres at 10 meters. The diver ascend rapidly to surface holding his breath. The gas volume in the lungs will expand from 6 litres to 12 litres while the pressure decrease from 2 ATA to 1 ATA !!
That will burst the lungs - pulmonary barotrauma!
Charles' Law: back top home Charles' Law is one of the most important laws governing the way a gas behaves. It takes temperatures changes of gases into account
The formula for the law is:
V = k T
where:
V is the volume.
T is the temperature (measured in Kelvin. O°K = -273°C).
k is a constant.
To maintain the relation between volume (V) and temperature (T) constant during heating of a gas at fixed pressure, the volume must increase. Conversely, cooling the gas decreases the volume. Combine Boyle's and Charles' Law it will create a general gas law P*V = k T
How will the pressure change related to the temperature? P1*V1 = P2*V2 T1 T2
Make it explosive! A diver fills up a scuba cylinder to 200 ATG (P1) at 17°C = 290°K. (T1) The diver places the cylinder in the sun. The cylinder heats up to 50°C = 323°K (T2) What will the pressure do? P2=? As the volume stays unchanged in a cylinder we can reduce the equation to P1 = P2 T1 T2 P2 = P1*T2 P2= 200*323 P2= 223 ATA T1 290 Imagine what will happen if the cylinder heats up to 80°C like in a car at summer. You think that up! In the inverted case the gauge pressure will drop while diving as the temperature in water is normally cooler than air temperature
Dalton's Law back top home It states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture.
Mathematically, the pressure P of a mixture of n gases can be defined as the summation
Ptotal = P1 + P2 +....Pn
where P1 + P2 +....Pn represent the partial pressure of each component. It is assumed that the gases do not react with each other. What does that mean and why is it so important to divers? The air we are breathing is a mixture of gases as well. It contains mainly: Nitrogen (N2) 79,02 % by volume Oxygen (O2) 20,94 % by volume Carbon Dioxide (CO2) 0,04 % by volume and traces of Neon, Argon, Xenon and Hydrogen. Therefore, the mixture of all gases (100%) with there specific partial pressure - what is exactly the percentage of each gas - will create 1 ATA. The pressure of one specific gas - the partial pressure - is e.g. for oxygen at sea level: 21 * 1 ATA = 0,21 ATA O2 100 At 1 ATA we breath normal air without any toxic effect. With increasing depth during a dive pressure will also increase Boyle's Law and therefore, the partial pressure of each gas in the air the diver is breathing. Given a scuba cylinder is filled with normal air, in 10 metres depth the partial pressure of oxygen and nitrogen will be 21 * 2 ATA = 0,42 ATA O2 and nitrogen 79 * 2 ATA = 1,58 ATA 100 100 Table of partial pressure of oxygen and nitrogen in certain depth given in ATA and metres
| 1 ATA 0 metres | 0,21 ATA O2 | 0,79 ATA N2 |
| 2 ATA 10 metres | 0,42 ATA O2 | 1,58 ATA N2 |
| 3 ATA 20 metres | 0,63 ATA O2 | 2,37 ATA N2 |
| 4 ATA 30 metres | 0,84 ATA O2 | 3,16 ATA N2 = NARCOTIC |
| 5 ATA 40 metres | 1,05 ATA O2 | 3,95 ATA N2 |
| 10 ATA 90 metres | 2,10 ATA O2 = TOXIC | 7,90 ATA N2 = LETHAL |
Nitrogen is narcotic at a depth of 30 metres as it has a 3.16 ATA. Martini's Law Recreational Scuba divers are, therefore, well advised not to dive deeper than 30 metres. The reasons why the the human body is reacting as described above is not fully understood so far.
Henry's Law: back top home It states that, at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
In other words, the amount of dissolved gas in a fluid increases as the pressure increases and vice-versa. As a human body contains at sea level (1 ATA) approximately 1 litre nitrogen (N2) the same body as a diver will bring an increased amount nitrogen in solution at 10 metres (2 ATA) until saturated given an appropriate time of duration in this depth and normal air in scuba cylinder. As gases can go into solution (human body) it also may come out of solution when pressure is reduced. This phenomena is to observe when you open a soda bottle as carbon dioxide was brought into solution under high pressure and comes out of solution as the pressure drops rapidly while open the bottle at normal atmospheric pressure.
To a diver this may have fatal consequences as the bubbles comes out of solution in his tissues and blood vessels causing decompression sickness (illness) DCS (DCI) or so called 'bends'
So called Martini's Law: back top home It states that, the effect of nitrogen to the human body is like drinking a Martini for every 10 metres depth (Stirred or shaken is irrelevant)
|
Depth in water in metres |
Effect to diver |
| 20 - 30m | Mild impairment or performance and with mild euphoria |
| 30 - 50m | Laughter, loquacity (trained diver might overcome this by self control) Overconfidence, poor response to danger Narrowed perception, fixation to certain functions or exercise Poor diving control |
| 50 m | Risk of hallucination, tiredness, judgment lost |
| 50 - 70 m | Sometimes dizziness, hysteria |
| 70 m | Leak to respond on signals or instructions and reasoning power |
| 70 - 90 m | Decreased concentration, memory loss, confusion |
| 90 m + | Hallucination, unconsciousness |