Reviewing Dalton’s Law

January 12, 2010 By: Duane Johnson Category: Training

Share

Duane Johnson - Tech Instructor

I was talking with a diving friend. He mentioned that during a visit to a local dive store, the employees were referring to 50% Nitrox as 50/50. They said that 50/50 was 50% oxygen and 50% air. Air is composed of 21% oxygen and 79% nitrogen. Now, my fifth grade math may be a little rusty, but 50% + 21% + 79% = 150%. Unless this dive store has found a new gas law, it looks like a quick review of Dalton’s Law is in order.

Dalton’s Law of Partial Pressures

Assuming that the gasses do not react with each other, Dalton’s Law states that the total pressure of a gas mixture is equal to the sum of the partial pressures of each individual gas in the gas mixture.



Let’s look at a quick example. If we are diving 32% Nitrox, the gas consists of 32% oxygen and 68% nitrogen. If we add 32% and 68%, we get 100%. Physics dictate that we cannot exceed 100% of something.

Dalton’s law is the basis for calculating partial pressures of gasses.

Understanding the gasses we breathe is critical to diving safety. It should have been covered in your open water and/or Nitrox courses. If you are going to be blending breathing gasses, you definately need to have a full understanding of Dalton’s law.

Dive Safe,
Duane
Precision Diving

(No Ratings Yet)

 Loading ...

Tags: Dalton's Law, gas blending, nitrogen, Nitrox, oxygen, Partial Pressure, scuba diving

Did you enjoy this article? Want to learn more? Get all the diving tips via RSS feed or via email

5 Comments to “Reviewing Dalton’s Law”

1. Nick says:

   January 13, 2010 at 5:25 am

   Your welcome for the lovely story Duane!

   1
2. Chris Sullivan says:

   January 19, 2010 at 8:37 am

   Duane, I don’t get your math, but the dive store is wrong too. 50% O2 and 50% air would be 50% O2 + 10.5% O2 + 39.5% N2 (ignoring trace gases), otherwise known as Nitrox 60.5.

   You just can’t add percentages like that to get 150%.

   Cheers,
   CHris

   2
3. Duane Johnson says:

   January 20, 2010 at 3:36 am

   Hi Chris,

   I think they were viewing it as two separate halves. One half being 50% pure O2, then the other half being air (21% O2 and 79% Nitrogen). But you are right, when you crunch the numbers, it does come out to be 60.5% Nitrox. Thanks for the catch.

   Duane

   3
4. David says:

   January 30, 2010 at 11:06 pm

   Another concern (besides a blender’s inability to calculate basic percentages) is what are these people actually breathing when using 50/50?

   If the dive shop employees are correct in the ingredients, then isn’t it dangerous to be calling it anything other than Nitrox 60.5? On the other hand, anyone breathing that much O2 better know what they’re doing without relying on shop employees.

   4
5. Duane Johnson says:

   January 30, 2010 at 11:11 pm

   Hi David,

   It is the responsibility of the diver to always analyze his/her gas just after getting it filled and just prior to the dive. It could be dangerous if the diver believed the dive store and didn’t analyze the gas.

   But certainly, it would make me consider going back to the dive store.

   Thanks for visiting.

   Duane

   5