How to factor atmospheric pressure into relative humidity calculations

[Jonathon]

TL;DR

ASHRAE's psychrometric charts specify that they are valid at "sea level" / "atmospheric pressure: 101.325 kPa" or for example at "1500 metres" / "84.556 kPa".

Some people say atmospheric pressure is irrelevant to relative humidity, but then why do ASHRAE'S psychrometric charts include those specifications?

 

[russ_watters]

It's not irrelevant but how relevant depends on the altitude and process.. Get/try a psychometric calc app for your phone and see what the differences are.

 

[Lnewqban]

Welcome, Jonathon!

Please see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html

:)

 

[Jonathon]

Welcome, Jonathon!

Please see:
http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html

:)

Thanks. The section on "How much moisture can the air "hold"?" was very interesting (to me). I should perhaps have stated that I know how to calculate relative humidity at sea level. I'm trying to factor atmospheric pressure into that calculation. Although it's possible I missed an indirect connection, I don't think the article addressed

How to factor atmospheric pressure into relative humidity calculations?​

 

[Dullard]

100% RH occurs when the partial pressure of water vapor (in the air) equals the vapor pressure of water at that temperature.

If you're talking strictly about RH, atmospheric pressure doesn't matter. There are lots of (other) ways to describe how much water is in the air (mass/mass, mass/volume, etc.) - you need to account for atmospheric pressure with those.

 

[russ_watters]

100% RH occurs when the partial pressure of water vapor (in the air) equals the vapor pressure of water at that temperature.

If you're talking strictly about RH, atmospheric pressure doesn't matter. There are lots of (other) ways to describe how much water is in the air (mass/mass, mass/volume, etc.) - you need to account for atmospheric pressure with those.

Good point -- the OP asks about RH specifically (pressure doesn't matter) and the psych chart in general (pressure does matter to some of the other parameters).

 

[Jonathon]

Thanks - pressure does matter to some of the other parameters on a psychrometric chart, but not to relative humidity. Cool - that sufficiently answers my question. Thanks.