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.
