{"id":987474944,"date":"2024-09-10T15:10:59","date_gmt":"2024-09-10T15:10:59","guid":{"rendered":"https:\/\/courses.asiascubainstructors.com\/lessons\/physics2-pressure-and-volume-copy\/"},"modified":"2024-09-13T16:49:48","modified_gmt":"2024-09-13T16:49:48","slug":"physics-4-partial-pressure","status":"publish","type":"lessons","link":"https:\/\/courses.asiascubainstructors.com\/de\/lessons\/physics-4-partial-pressure\/","title":{"rendered":"Restricted content"},"content":{"rendered":"

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Physics<\/h1>\n

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4. Partial pressure<\/h2>\n

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In a mixture of gases, each gas exerts a pressure proportional to the percentage of that gas in the mixture. We call this partial pressure<\/strong>.<\/p>\n

Air is a mixture of gases, containing, 21% Oxygen and 79% Nitrogen. At the surface the pressure is 1 bar. So the partial pressure of Oxygen is 0.21 bar and the partial pressure of Nitrogen is 0.79 bar.<\/p>\n

As we go deeper the percentage of gasses remains the same but the absolute pressure increases. So the partial pressure of each individual gas increases also.<\/p>\n

To calculate the partial pressure of a gas at depth divide the percentage of the gas in the mix by 100 and multiply by the absolute pressure at that depth.<\/p>\n

For instance, when breathing air with 21% Oxygen and 79% Nitrogen at 10 meters, 2 bar, the partial pressure of Oxygen is 0.21 x 2 = 0.42 bar and the partial pressure of Nitrogen is 1.58 bar.<\/p>\n

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\ue007<\/span>
<\/strong><\/span><\/p>\n

Examples<\/strong><\/span><\/p>\n

What is the partial pressure of oxygen in air at 30m?<\/p>\n

21 \u00f7 100 = 0.21<\/p>\n

0.21 x 4 = 0.84 bar.<\/p>\n

 <\/p>\n

What is the partial pressure of nitrogen at 25m with EANx32?<\/p>\n

(EANx32 has 32% Oxygen therefore 68% Nitrogen)<\/p>\n

68 \u00f7 100 = 0.68<\/p>\n

0.68 x 3.5 = 2.38 bar.<\/p>\n

<\/span><\/p>\n

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Why is partial pressure important for divers?<\/h3>\n

Divers need to understand the partial pressure of gases in a mix because it directly affects their safety. Every gas can become toxic at high enough pressures. For instance Nitrogen can cause Nitrogen Narcosis. Even Oxygen can become toxic at high enough pressures.<\/p>\n

Simply knowing the percentage of gas in the mix isn’t enough, since the effect of the gas on a diver also increases when pressure increases with depth. The effect of breathing a gas on a diver depends on the partial pressure of the gas.<\/p>\n

For example, breathing Oxygen at a partial pressure above 1.4 bar poses a high risk of Oxygen toxicity.<\/p>\n

 <\/p>\n

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\ue007<\/span>
<\/strong><\/span><\/p>\n

Examples<\/strong><\/span><\/p>\n

How deep can you dive with Enriched air that contains 36% Oxygen without exceeding an Oxygen partial pressure of 1.4 bar?<\/p>\n

1.4 \u00f7 0.36 = 3.9 bar<\/p>\n

(3.9 – 1) x 10 = 29 meter<\/p>\n

<\/span><\/p>\n

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Contaminated air<\/h3>\n

Some questions are about the effects of breathing contaminated air. The effect of breathing a gas depends on the partial pressure. The partial pressure depends on the absolute pressure and on the percentage of gas in the mix.<\/p>\n

If a diver breathes air containing 1% carbon monoxide at 10m (2 bar) it would have the same effect as breathing 2% carbon monoxide at the surface (1 bar) since the partial pressure would be the same.<\/p>\n

 <\/p>\n

 <\/p>\n

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\ue007<\/span>
<\/strong><\/span><\/p>\n

Examples<\/strong><\/span><\/p>\n

If a diver breathes air containing 0.5% carbon monoxide (CO) at 30m it would have the same effect as breathing ___ % carbon monoxide at the surface.<\/p>\n

To calculate this simply multiply the percentage of CO in the air by the pressure:<\/p>\n

0.5% x 4 bar = 2%<\/p>\n

 <\/p>\n

However, if the question asks “What percentage of CO will the diver breathe at 30 meters?” the answer remains 0.5%, the same as at the surface. The composition of the gas does not change with depth but the partial pressure, and so the effect it has on a diver, does change.<\/p>\n

Read the question carefully to understand whether it\u2019s asking about the percentage<\/em> of gas in the mix or the effect<\/em> of the gas at depth!<\/p>\n

<\/span><\/p>\n

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