How to Calculate Pressure When Diving

In scuba diving, it is important to know the depth pressure and the partial pressure limits of your breathing gas. Scuba buoyancy compensators are equipped with pressure gauges that measure the water pressure, eliminating the need for divers to calculate the pressure. The limit for partial oxygen pressure (ppo2) is 1.6 ppo2, which is reached when the diver nears 100 feet. Exceeding this limit increases the risk of oxygen toxicity, nitrogen narcosis or other diving injuries. For this reason, recreational diving is usually limited to 99 feet or lower, as deeper dives cannot be completed without special equipment.

Instructions

    • 1

      Become familiar with Dalton’s law and how pressure affects the human body in water. Dalton’s law states that the pressure of gas is equal to the sum of the pressure of each gas in the mixture. The number is represented in decimals, with 1.0 being equivalent to 100%. One atmosphere is equal to 33 feet of sea water, plus the air above the water's surface. Each atmosphere is an additional 33 feet, so 66 feet is 2 atmospheres, and so on.

    • 2

      Convert the numbers to decimal form. At sea level, the air humans breathe is one atmosphere absolute (ATA; the ambient pressure including the air above the water) of pressure. This air is made up of 1% trace gas, 21% oxygen and 78% nitrogen. These numbers are represented in decimal form. When added, these numbers amount to .01 + .21 + .78, which is equal to 1.0.

    • 3

      Find the depth of the dive you will be making. The partial pressure of oxygen changes as a person dives, so in order to find the partial pressure of the oxygen used, you need to know the percentage of oxygen in the breathing gas, and the depth of the dive. The depth of the dive must be in atmospheres absolute, so convert the depth by dividing by 33 for salt water, or 34 for fresh water. Add 1 ATA to account for the air pressure at the surface. A dive made to 99 feet would have this calculation: 99/33 = 3 + 1 = 4, so the ATA depth of your dive will be 4.

    • 4

      Use Dalton’s Law to complete the calculation by multiplying the amount of oxygen in the breathing mix by the ATA depth. A dive to 99 feet with a nitrox breathing mixture with 32% oxygen would be: .32 x 4 = 1.28.