Introduction to Psychrometrics, Part 1 of 3

Oct 1, 2010

Psycho-metrics is a measure of how psycho you are.  That can be a helpful measurement when dealing with clients who are driving you crazy.  Today, I want to give a brief introduction to something different, called psychrometrics (notice the “r”).  Psychrometrics is the study of the physical and energy related (thermodynamic) properties of air-water vapor mixtures.

The first response I get when teaching psychrometrics is, “Why do I need to know this stuff?”  With an understanding of psychrometrics, we can better predict where condensation may form, causing water damage and leading to indoor air quality concerns.  By understand these concepts, you’ll be better able to look at a wall assembly and identify common problems.

Some of the key variables of psychrometrics include temperature, relative humidity, humidity ratio, and dew point temperature.  Before we get too deep, we need to answer the question, “What is air?” Some of the most accomplished IAQ consultants have a hard time describing the stuff.  Briefly, air is mostly nothingness.  It’s mostly empty space also known as a vacuum or void.  I like to use the illustration of an empty billiards or pool table.

Air isn’t complete nothingness, because we have molecules in the air.  Think of these molecules as balls on that billiard table.  The molecules of dry air include nitrogen (78%), oxygen (21%), and trace amounts of argon, carbon dioxide and hydrogen which make up the remaining 1%.  These balls have not filled up the pool table, leaving a considerable amount of free space.

When humidity is introduced to the air, it’s like adding a few more balls to the billiard table.  Humidity usually represents around 1% of air by mass so it isn’t a huge contributor.  You may hear people say the air is “holding” a lot of humidity or water vapor.  This is a misnomer as billiard balls don’t hold each other, but rather bounce off each other.

How do water molecules even get into the air in the first place?  Evaporation is the simple answer.  Technically it has to do with the thermal energy that breaks free water molecules from the attractive forces of a liquid surface.  This explains why we commonly find elevated humidity levels in warmer air.  It also explains why dishes washed in warm water dry a lot faster!  Warm air and water have elevated amounts of thermal energy that can free more water molecules, sending them into the air (evaporation).

In the next installation, I’ll need to cover concepts such as condensation and kinetic energy.  I’ll get into the juicy stuff later (vapor barriers, crawl spaces & attics, etc.), but we first need to cover some of the basic science.  I must give credit to Steve Horstmeyer, a meteorologist who set me straight on a lot of these concepts.