Partial pressure - please put in simple words

[Buoy_A]

I think that part of the confusion of partial pressure lies in the traditional understanding of the word "pressure". When we think of "pressure", we tend to think of a force that pushes against something. However, when speaking about partial pressure, an outward pressure has nothing to do with the pushing of anything. Instead, it relates to a concentration of molecules for a given volume.

To give you an example, when you inflate the tires on your car, you put more air in. This causes the air inside to push outward on the walls of the tire and make it more rigid. This type of pressure has nothing to do w. partial pressure. The additional gases when diving do not push outward on your cell walls. These additional gasses do not create any type of "pushing" force. If you think this is what is occurring, you're thinking of "pressure" incorrectly when it comes to gases and diving.

However, when you inflate your tires, something else happens: The concentration of air molecules per a given volume also increases. This concentration can be described as a pressure (as the concentration is greater than the ambient pressure outside of the tire) and this type of pressure has everything to do w. gases and diving.

So to better understand partial pressure, it's probably easier to think of an increase in concentration of a gas, rather than the pressure of a gas. As you descend underwater, we know that the concentration of air must increase relative to the concentration experienced at the surface. You inhale air at a 'concentration' of 1ATA at the surface. At about 100ft you're inhaling a concentration of air of about 4 times that amount. So, since oxygen is a component of air, you're receiving 4 times the concentration of oxygen. As your lungs exchange gases with your blood, your blood is receiving a much richer concentration of oxygen relative to what you were breathing at the surface, even though oxygen still only represents 21% of the air you're breathing. This higher concentration can be toxic to humans. It's important to know what levels of concentration of oxygen you are breathing. The measure of this is what they call partial pressure. It is just a measurement of what levels of concentration is represented by specific gases at a given depth.

 

[beanojones]

Got to admit, I am not totally understanding the term of "partial pressure".

Can someone in layman terms explain partial pressure.

Second, why a change in partial pressure becomes deadly?

How does lethality of other gases change with partial pressure?

Thank you.

When you get an explanation of partial pressure that works for you, make sure you say which one it is, please.

My question is to you (and others that can't get a handle on the idea) is from the instructor's point of view: Which book exposed you to the idea of partial pressure? Did it help understanding at all, or just leave you incompletely understanding things?

As far as the why in high partial pressures each gas becomes toxic (deadly is mostly an unused descriptor), the answer is not so simple, for two reasons. Firstly, each gas causes different partial pressure problems, and secondly, unfortunately, they are all just brute fact answers. There is no "why" for most answers in science, there is just a brute fact itself.

High partial pressure oxygen is toxic in two different senses. Low partial pressure is incompatible with life.

High partial pressure nitrogen is narcotic, and then at greater pressures, toxic. Low partial pressures are without effect.

High partial pressure helium is "narcotic" at extreme pressures. Low partial pressures are without effect.

High partial pressure hydrogen has not been used enough to generate reliable data. Low partial pressures are without effect.

All the toxicities are "explained" mostly by saying they happen, and describing when they happen.

 

[Texasguy]

I liked Mustard Dave's explanation because then it made clear what the 1.4 or 1.6 actually means and how to calculate that, before it was just some faceless fractions. Looking at tables in the book, I made little connection on what is going on in the formulas there. sharkbaitDAN's explanation was over my head, it kind of mimicked the confusing book's explanation. I also think that Buoy_A's post also helped.

The idea of partial pressure was introduced in the nitrox course, I believe. But it was more focused on calculations and numbers, knowing not to go over 1.6 or better 1.4.. So, during the course I got the skills to keep me alive but not detailed understanding of the background information.

 

[beanojones]

Thanks for the feedback. I never want to overwhelm people with info, but never want to starve curiosity.

Sometimes when discussing Nitrox ideas its worth thinking about breathing 100% oxygen would be like**. There is no distinction between pressure and partial pressure in that case, so it might be simpler to think about, and then reason it out to mixes of Nitrogen and Oxygen.

Oxygen is toxic at higher pressures. (1.4 or 1.6 atmospheres, depending on factors). This is true whether oxygen pressure is from the partial pressure from a Nitrox Mix, or the total pressure from 100% O2.

So the total oxygen pressure of breathing of breathing 100% O2 at 20'/6m is 1.6 atm (1 atmosphere from the air, plus 0.6 atmosphere from the weigh of the water). Because of the way pressures are additive, but the effects of each gas are independent of the total pressure, this is the exact same oxygen pressure that would happen if you dove with 40% O2 Nitrox to 99'/30m.

So:

at 20'/6m, 1.6 atmospheres total pressure times 100% Oxygen= 1.6 atmosphere pressure of Oxygen, 0 atmospheres of pressure of Nitrogen

at 99'/30m, 4.0 atmospheres total pressure times 40% Oxygen= 1.6 atmospheres pressure of Oxygen, 2.4 Atmopheres of pressure from Nitrogen.

In the second case, we usually refer to the Oxygen pressure as partial pressure of the oxygen (often written PPO2) because there is also pressure due to Nitrogen in the Nitrox. But in terms of the effects of oxygen on our bodies, 100% Oxygen at 20 feet, and 40% Oxygen at 100 feet are the exact same thing, 1.6 Atmospheres of Oxygen pressure.

Does that way of thinking about it help, or just make things more confusing?


** Kids don't breathe 100% O2 at home !! But know that the Navy O2 rebreathers use 100% O2.

 

[drbill]

The core of "partial pressure" is that gases can exist in a mixture (like air, being 21% oxygen and 79% nitrogen) but the gases behave as though they don't know the others are there.

And we behave as if there are no other gasses in the atmosphere. Argon, Neon, Helium, etc., just don't get no respect. Of course maybe it is just because they are inert. Yes, I'm teasing.

 

[EFX]

... The first stage of your regulator is adjusting the pressure of the gas you are breathing to what is the equivalent of surface pressure,....

This is off topic but the ideas can be carried over to partial pressures and the resulting flow of individual gasses into and out of our bodies. Actually, the first stage delivers gas at a pressure 100-140 psig above the ambient pressure of the water. We need a gas flow from tank all the way to our lungs. Flow is determined by the pressure drop across any two endpoints and the resistance to flow between those endpoints. What this means is that there must be a higher pressure at the output of the first stage than the output of the second stage. At 99 feet the pressure is (99/33 + 1)(14.7) or ~ 59 psia. The 2nd stage will deliver gas to our lungs at this pressure neglecting resistance in the airway. In order to get flow out of the 2nd stage we need a higher pressure coming in. Assuming 140 psi above ambient we need a pressure of 140 + 59 or 199 psi. If we started our dive with 3000 psi and consumed 100 psi getting there the pressure drop across the 1st stage is 2900 - 199 or ~ 2700 psig. Where this becomes significant is why regulators begin to breath hard at depth with low air in the tank. You can appoximate how low you can go at any depth using the example above. If we factor in some resistance to flow through the system the pressure drops will be higher to get our required flow. Let's use 160 psig. At 99 feet we can take our tank pressure down to 59 + 160 or 219 psia. What this means is at 219 psia flow stops and we get no air. So, at some higher tank pressure the reg will start breathing hard and delivering lower flow. As we ascend the ambient pressure comes down and the tank to 2nd stage output pressure drop increases providing more flow and hopefully you can make it to the surface.