# How do i calculate the concentration of oxygen in humid air?

For a combustion engine, i have data which is measured at certain conditions, say 15C, 1013hPa, and 50% humidity. I need to correct this when the actual conditions change to stay accurate.

It's quite easy to correct for temperature and pressure with Boyle&Gay-Lussacs law, it's the humidity i'm stuck at. Since humid air contains less oxygen, less fuel should be injected. So i need to calculate how much less oxygen the air contains, effectively, to compensate for that.

So lets assume we have 50% humid air at 1030hPa and 15C, which has an oxygen concentration of 21.00%. How would one calculate the concentration of oxygen here, when the humidity increases to 75%?

EDIT: I may have found a way, but I’m not sure it’s correct. The saturated vapour pressure is known and present on the internet. The saturated vapour pressure at 15C is 1704pa. At 50% humidity that’s 852pa. If you have this humid air at 1013hPa then that means that you need 1,008 times as much air to get the same amount of oxygen: 1/(1-(852/101300)) = 1.0084 Is that right?

Cheers