# How does total pressure affect relative humidity and how do I correct my hygrometer for pressure?

Obviously pressurized air can hold more water than regular air, so an increase in overall pressure would result in an inversely proportional decrease in relative humidity. However, I can't seem to find any equations which reflect this.

Basically my issue is that my thermohygrometer calculates the relative humidity assuming that atmospheric pressure is 101.3kPa but where I am measuring it the pressure is about 140kPa. This can be fixed by multiplying the reading by (101.3kPa/140kPa), but I am having a hard time expressing that scientifically and mathematically.

So, what is the mathematical relationship between pressure and relative humidity?

• What is the model of your instrument? Is it an electronic/scientific instrument? May 30, 2019 at 19:13
• @EtienneGodin believe it is this one or a similar model, I can double check tomorrow: omega.com/en-us/sensors-and-sensing-equipment/humidity/… May 30, 2019 at 19:31
• ok for the model of your sensor - also can you explain how the pressure can be 140 kPa in your setting ? May 30, 2019 at 22:38
• @EtienneGodin The sensor is basically in a container that's under pressure with humid air flowing through it. May 30, 2019 at 23:19
• I live a mile above sea level (pressure about 20% lower than at sea level) and it never occurred to me to adjust the humidity reading. I'd start with the Ideal Gas Law and the Clausius-Clapeyron equation (en.wikipedia.org/wiki/Clausius%E2%80%93Clapeyron_relation). My wild guess: since relative humidity is a ratio, and both water vapor and nitrogen/oxygen change in the same way, pressure doesn't affect relative humidity.
– user967
Jun 5, 2019 at 2:55