Eli5: My head hurts from watching so many youtube videos about gas laws etc and I still dont understand what partial pressure means.


Can someone please explain to me the definition of partial pressure in a very simple simple way and how it applies to us?

Like the partial pressures of different gases in our bodies.. And why people get decompression sickness when they scuba dive.. And why having helium lessens the risk. Etc. Why are partial pressures important?


In: 3

It might be easiest to examine this from another perspective.

Think about an electronic scale. Let’s say I have three items: a toy car, a banana, and a dollar bill. I put them all on the scale. They weigh 2 pounds combined.

Now, the “partial mass” (note that this is a made up term) of an object would be the amount that it contributes to this total mass. If the car weighed a pound, the “partial mass” of the car would be a pound, as that’s what it contributes to this total weight.

This is largely what partial pressure is. If the pressure in a balloon is 5 psi, and there’s three different gases in there, they all contribute to that psi reading. If I (for example) determine that the partial pressure of the helium is 2 psi, I’m saying that if I had only the helium in the balloon, the balloon would have an air pressure of 2 psi.

Decompression sickness is semi-related. It’s known that as pressure increases, the solubility of a gas increases. This means that the farther underwater you go, the more of a specific gas can dissolve in your bloodstream. This is concerning, because that gas will become less soluble when you try to get shallower and will need to exit the bloodstream. If you go too quickly, this causes issues as the nitrogen exits the bloodstream.

Helium is less soluble in general, which means less gas will be in your bloodstream. This makes the decompression less dangerous as there’s less gas that needs to exit the bloodstream.

Okay, say you have a room with 3 gases in it.

Say the total pressure is 100 PSI.

Each of those gasses are contributing to that total pressure based on how many molecules of each gas is present.

So it could be that one gas, say nitrogen, is contributing 70 PSI out of the total 100PSI, so it has a partial pressure of 70psi.

Another gas could be oxygen, it contributing a pressure of 20 psi out of the total PSI, so it has a partial pressure of 20psi

And the third gas could be helium, contributing 10 PSI out of the total 100PSI, having a partial pressure of 10 psi.


Another way to think about it is imagine you had a perfectly sealed container, again with three gases. Let’s use the same example gases as above.

Your sealed container is at 100 psi, if you were able to remove all the oxygen out of it perfectly without taking the other two gases, the pressure in the container would now be 80psi. Because all the pressure from the oxygen is gone since you took the oxygen out.

Partial pressure is how much of the pressure a certain substance is making up. So for example in sea level air you typically have a pressure of 100kPa. And since 20% of the air is oxygen the partial pressure of oxygen in the atmosphere is 20kPa. Partial pressure of nitrogen is 78kPa, and so on.

The reason we talk about partial pressure so often is that this is the effective concentration of that substance. If you think of the atmosphere as a bunch of molecules floating around with little interaction to each other. Then you put up a plate and count the number of oxygen molecules which hits it. You do not care about the nitrogen, water or anything else in the atmosphere. Then the rate at which oxygen molecules hit this plate corresponds to the partial pressure. If you for example add double the amount of nitrogen to the air making the pressure 180kPa then the number of oxygen molecules is still the same so the number of molecules that hit the plate is still the same. You can even remove all the nitrogen and make the pressure as low as 20kPa, close to vacuum, and there will still be just as many oxygen molecules hitting the plate at the same rate as before. The nitrogen or any other thing in the atmosphere just does not matter. Only the amount of oxygen matter and this is what partial pressure measures.

So say for example that the plate is made of a material which reacts with oxygen. The rate at which it reacts is only dependent on the partial pressure of the oxygen. This is important in for example diving and space travel because humans need a certain amount of oxygen to survive but too much oxygen is toxic to us. So we would like to keep a partial pressure of 20kPa at all times. However our lungs can handle much higher and lower pressures. So we use other gasses then oxygen to adjust the pressure and keep the partial pressure of oxygen the same. For saturation diving you might use nitrogen or even better helium as these are not toxic. For space travel we use nothing, or pretty close to nothing anyway, leaving only oxygen and some humidity.

DCS isn’t exactly caused by partial pressure. It’s caused by nitrogen that has been under pressure and thereafter entering into solution in the blood stream/entering tissues entering a reduced pressure environment (by surfacing too quickly) and forming bubbles in the body. Bubbles in the wrong spot can hurt you or kill you (eg you spine, your lungs, your brain) and aren’t too pleasant to have as a rule. If the pressure is released more slowly (controlled surfacing, decompression stop) then the nitrogen is less likely to form those types of bubbles and you’re less likely to get bit by DCS.

Helium doesn’t exactly lessen the risk of DCS. What it does (in tri mix or bi mix [heliox] technical diving) is it lessens the proportion of both oxygen and nitrogen in you breathing gas mixture. So, obviously, if you’re breathing less nitrogen you’re less likely to get nitrogen bubbles in your blood stream (also lessens potentially the occurrence of nitrogen narcosis). But it also reduces the risk of oxygen toxicity. Oxygen under certain partial pressures/at certain depths can become neurotoxic. To offset that, sometimes technical divers that are going (well) beyond recreational depths will dive hypoxic gas mixes that you can’t really breath at the surface but which you can breath at significant depth. Helium, as an inert gas, is used to fill up the remaining balance of your gas mix.