Why does pressure increase underwater?
Before explaining why pressure increases the deeper we go underwater, I think it is important to emphasize the fact that we are already subjected to a considerable amount of pressure on the surface. We don't notice it because we evolved with this ambient pressure being present at all times but it is there! And why exactly is there any pressure on the surface you may ask? Because although it may seem light, air does carry its weight, and that creates pressure all around us.
While it is hard to define an exact limit between the atmosphere and outer space, a common cutoff line is the Karman line, situated at an altitude of 100km (62 miles). Within that imaginary boundary lies more than 99% of our atmosphere. All of that atmosphere "pressing down" on us creates a pressure of 1 atm (also 1.02325 bar or 14.7 PSI) at sea level.
The higher we go, the less air we have above us and the less pressure we are subjected to. For example, at an altitude of 300m, the pressure already drops down from 1 atm to 0.965 atm. But more about altitude diving in a later section.
Now, to the interesting part. If you clicked on this article, it was to understand why pressure increases as we go deeper underwater. So let's get to explaining that!
How does pressure vary underwater?
In the diving industry, we are taught that every 10m (~33 feet) of depth adds 1 bar to the ambient pressure. With the ~1 bar already present on the surface, that means we are subjected to 2 bars at 10m under the sea (~1 bar from the atmosphere + 1 bar from the water). While that is mostly true, I want to go over some subtilities that are usually not mentioned.
Delta P
The way to find the difference in pressure between two points is through the use of ΔP, or the "difference in pressure".
A little warning, this does include a little bit of math/physics to explain the theory behind the increase in pressure. If you don't want to see any formulas, you can just skip this part!
The formula for it is:
ΔP is the difference in pressure between two points in a fluid at rest (in this case water).
ρ is the density of the fluid.
g is the acceleration of gravity.
Δh is the "difference of height" in the fluid, or in our case, the difference in depth.
To make the calculations, we will need to use the metric system: pressure in Pascal (Pa), density in kilogram per meter cubed (kg/m³), gravity in m per s squared (m/s²), and difference of depth in meters (m). When we find the resulting pressure, we will be able to convert it to Bar or PSI.
On Earth, the gravity field remains constant-ish, but we will use the widely accepted value of 9.81m/s²
If we search the density of salt water, we get a value of around 1027kg/m³.
let us take a difference in depth of 10m.
We can now calculate:
ΔP = 9.81m/s² x 1027kg/m³ x 10m = 100 749 Pa
Converting units (you can simply use Google for that!), we find that that equals 1 bar or 14.6 PSI.
This is the basic reasoning behind why we gain 1 bar every 10m of depth (or 14.6 PSI every 33 feet)
Different conditions
We can now imagine how that variation in pressure would change if we modify some factors:
- If we increase the density of the liquid, for 10m of depth difference we would also get a greater increase in pressure.
The opposite is true as well: the density of fresh water is 1000kg/m³, and if we use this value to calculate the difference in pressure in a depth difference of 10m, we would get a ΔP of 98 100 Pascals (0.981 bar or 14.2 PSI)
- If we increase/decrease the gravitational field, the difference would then also vary accordingly. Of course, this isn't possible on Earth (although there are some slight variations on the poles and on the equator), but if we imagine that we were diving on a planet with a gravitational acceleration half of that of the Earth's, we would need to go to a depth of 20m to experience an increase in pressure of 1 bar.
Diving at altitude
The reason why altitude diving is in a category by itself is because of the reduced ambient pressure. SSI and PADI both consider any dive done above 300 meters, or 1000 feet an "altitude dive". Those are mainly dives done in lakes.
I won't go into detail in this article regarding all the risks relating to diving in altitude and decompression sickness, but the main idea is that the surface pressure is lower than 1 bar, and that may create a greater differential with the potential partial pressure of nitrogen present in the tissues relative to the outside environment.
As I mentioned earlier, at 300m, you have an atmospheric pressure of 0.965 atm.
Lake Titicaca, found in the Andes, in South America, is considered the highest lake in the world, at an altitude of 3 812m (or 12 500 feet). This lake is divable and the ambient pressure at that altitude is 0.646 bar.
How does this increase in pressure affect us?
Of course, this increase in pressure is one of the big factors to take into account while diving. But why is it such an important factor and how does it affect us?
There are -amongst others- 2 main ways the increased pressure affects us: Squeezes, and the changing properties of breathing gases while under pressure.
Squeezes
Generally, a squeeze is when an air pocket gets compressed by increasing outside pressure.
The 60% of our body that is made up of water isn't too affected by any increase in pressure, but some air pockets are present throughout our body that are subject to squeezes. Those air pockets include:
- The lungs
- The inner ear
- The sinuses
- The air pocket between our mask and our eyes
- The stomach
- etc...
What does a squeeze feel like? If you've ever been on an airplane, you might have felt a variation of pressure in your ears resulting from cabin pressurization. This stems from the necessity of keeping the pressure on planes above a certain threshold.
To relieve the pressure on the plane, people might swallow saliva or move their jaw around, but this isn't enough during a dive. As pressure increases rapidly as we go deeper, our eardrums get bent from the outside requiring us to equalize to add pressure to our inner ear. To do that, we use the Valsalva method: we pinch our nose and gently exhale air through it: this will send the air through the Eustachian tubes to our inner ears and allow the pressure to equalize.
Breathing gases
Before talking about the effects of each breathing gas, it is important to note that the second stage of your regulator feeds you air/gas that is always at the same pressure as the environment around you. At 30m underwater, you will breathe air at 4 bar. This leads to (physiological effects aside) increased air consumption. and will make you run out of air faster.
Now, talking about the effects of each gas under pressure: We normally breathe air made up of 79% Nitrogen and 21% Oxygen. At 1 bar, that equates to a partial pressure of 0.79 Bar of N₂ and 0.21 Bar of O₂ (on the surface).
Nitrogen (or N₂) at increased pressures is not toxic, but will give you narcosis. The effects start around 30m (or 3.16 Bar partial pressure of N₂) but can be observed in some rare cases shallower than that. Below 45m, the effects of Nitrogen narcosis become more severe.
Oxygen (or O₂) is toxic at a partial pressure greater than 1.6 Bar. (Although we would try to stay below 1.4 Bar for recreational diving). On air (21% Oxygen), a partial pressure of 1.4 Bar would equate to a depth of 57m (187 feet), and at 1.6 PPO₂, we get a maximum depth of 66m (217 feet). Below 66m, divers need to use a hypoxic mixture, meaning a gas blend that has less than 21% Oxygen.