Introduction to Psychrometrics, Part 3 of 3

Oct 8, 2010

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.

If we increase the temperature of the system, we are increasing the kinetic energy. Warmer air + water can free up more molecules with that extra kinetic energy. Colder air + water has less kinetic energy and therefore can free up fewer water molecules. Relative humidity is therefore relative to temperature.

If we want to graphically represent relative humidity, we could show the kinetic energy in a system that has already been used to free water molecules compared to the total amount available (Figure 1). When we increase the temperature, we give the system an even greater capacity to free water molecules. Holding all else constant, increasing the temperature reduces the relative humidity (RH dropped from 50% to 33% in Figure 1). A key point is that the relative humidity can be changed without adding or removing moisture to the system.

As an aside… People will commonly describe relative humidity by saying, “The air is holding 40% of its capacity.” I dislike this terminology because the air isn’t “holding” the water vapor. Remember water molecules are like billiard balls bouncing around a fairly open table.

I’ll admit, describing relative humidity in terms of kinetic energy instead of vapor pressure is a little unorthodox. But when you boil it all down (pun intented), kinetic energy is the overarching scientific concept. Don’t believe me? Dig a little deeper into kinetic theory.

Besides relative humidity, the other three variables that measure water vapor are humidity ratio, absolute humidity and specific humidity. All of these measure the mass of water vapor in the air. You don’t think of those invisible water molecules as having much mass, but they do!

Humidity ratio is the mass of water vapor per mass of dry air. Common expressions include grams/kilograms and grains/pound. Absolute humidity is the mass of water vapor per volume of air. Common expressions include grams/cubic meter and grains/cubic foot. Specific humidity is the mass of water vapor per mass of humid air. Common expressions include grams/kilograms and grains/pound. Of the three, I personally use humidity ratio the most.

Well, there you have it… A brief introduction to psychrometrics. In later posts I’ll be discussing the psychrometric chart and applying all this knowledge to conducting an indoor air quality assessment.