When we dive, we use a gas mixture to breathe at a pressure equal to the pressure of the environment around us. This pressure is called absolute pressure. It is the sum of the water pressure and the atmospheric pressure acting on us.
The pressure created by the atmosphere at the surface of the Earth is called atmospheric pressure. At sea level, it is 760 millimeters of mercury, or one atmosphere (one bar). However, its value is constantly changing due to processes occurring in the atmosphere. The concept of “absolute atmospheres” has been introduced to denote true pressure (ATA). In our calculations we will use the notation PATA to express absolute pressure.
By European standards, cylinder pressure is measured in atmospheres (bar), as reflected on the pressure gauge, while water pressure is measured in meters of salt water (msw)’or meters of fresh water (mfw) and shown by the depth gauge.
As you will recall, when diving, the water pressure increases by one atmosphere (1 bar) every 10 meters (msw). Therefore, every 10 meters of water column (msw) corresponds to an increase in pressure of 1 atmosphere (ATA) or 1 bar.
In order to calculate oxygen dive limits, you need to be able to determine the PATA at sea at a certain depth. To determine (PATA) you need to add the atmospheric pressure to the pressure gauge reading in equal units. For example, if the depth gauge reads 20 msw (i.e., 2 ATA or 2 bars), then PATA equals 3 atmospheres (ATA) or 3 bars.
You can also calculate this mathematically.
To do this, first determine the relative pressure at depth (D) in atmospheres (Atm) using the following formula:
PAtm = msw : 10 msw
Then let’s convert the relative pressure into absolute pressure (PATA) – To do this let’s add to this value the pressure of the atmosphere – 1 ATA.
PATA = (D msw : 10 msw) + 1 ATA
That is, at a depth of 20 meters underwater, PATA is equal to:
PATA = (20 msw : 10 msw) + 1 ATA PATA = 2 ATA + i ATA P = 3 ATA (bars)
Now let’s look at another way to determine the PATA by depth. To do this, add 10 msw, which is equal to atmospheric pressure (1 ATA), to the depth value and divide by 10 msw.
PATA = (D msw +10 msw) : 10 msw
Let’s apply it to the same example. At a depth of 20 meters, PATA is equal to:
PATA = (20 msw + 10* msw) : 10 msw PATA – 30 msw : 10 msw P = 3 ATA (bars).