Introduction to Psychrometrics, Part 3 of 3

In the first two installments of “Introduction to Psychrometrics” I covered concepts such as air, evaporation, temperature, condensation and dew point. I strongly encourage you to read Part 1 and Part 2 before reading this final installment where I’ll be explaining relative humidity, humidity ratio and a few other concepts.

The amount of humidity in the air will affect the indoor air quality, therefore it is important to measure it. Unfortunately, there are four common terms used to quantify humidity: relative humidity, humidity ratio, absolute humidity, and specific humidity. I’ll cover all four concepts in this post.

As a quick refresher, humidity is a measure of the water molecules in the air that have escaped the surface of liquid water. I’ll be using the term “water vapor” to describe these molecules. Water vapor is the result of evaporation (see the word “vapor” hidden in there?).

Relative humidity, or “RH”, is the most commonly used expression for humidity. It also happens to be the least understood.   Relative humidity is the ratio of water vapor in the air compared to fully saturated air at the same temperature.  In other words, there is a certain amount of kinetic energy in a system to free water molecules.  RH looks at how much of the system’s kinetic energy has been used to free molecules.  When I use the term “system” I am referring to the air + any liquid water that may be present.

If a room has a relative humidity of 40%, it still has a lot of unused energy (60%).  Put a cold glass of water in that room and the kinetic energy in all gas molecules (nitrogen, oxygen, water vapor etc.) will transfer heat to the cold water.  When the water molecules heat up, that increases their kinetic energy and ability to escape the liquid surface.

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Introduction to Psychrometrics, Part 2 of 3

In my first installment on psychrometrics, I covered the basics of air, humidity and evaporation. Here I’ll cover temperature, kinetic energy, attractive forces and condensation.  If you haven’t already, I strongly suggest you read Part 1 first.

We’re all familiar with the general concept of temperature. Temperature ends up being quite complex if you dig deeper (and deeper we shall dig).  Technically, temperature is related to the average energy of motion, known as kinetic energy. To use our illustration from the last blog post, the faster the billiard balls move on the table, the greater the temperature. Continue reading

Introduction to Psychrometrics, Part 1 of 3

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?” Continue reading