Psychrometrics Made Simple


United Trades Exclusive
Psychrometrics Made Simple

By Dan Spector


Psychrometrics. No, it is not an offshoot of Scientology—thank goodness. Are you a heating and air conditioning professional? Do you install ventilation systems with the skill and precision of Julia Child baking a chicken potpie? If you do, you should probably know something about psychrometrics. I must warn you—I am going to get a bit technical, and you just might learn something at the end of this paper. But, I can guarantee that possessing sound, theoretical knowledge of psychrometrics will help you understand more about how ventilation systems function, as well enable you to simplify the technical workings of the systems when talking to clients.


Psychrometrics, at its core, is an area of engineering investigating the moist properties of air. (Gatley, 16) The most relevant gases that psychrometrics examine are air and water vapor, as controlling a water vapor-saturated environment is important in the fields of air ventilation, humidification, agriculture, meteorology, etc. Human comfort in large part depends on air humidification—the amount of gaseous water vapor may be the difference between a dry, parched throat in the winter and a sweaty, muggy environment in the summer! The goal of an experienced ventilation professional is to use psychrometrics to ensure that the air is comfortably humidified so clients can live in comfort, as well as breathe easier.

History of the Psychrometric Chart

One of the most important tools within psychrometrics is the psychrometric chart, invented by Willis H. Carrier in 1904. (Gatley, 18) Carrier is considered the inventor of modern air conditioning, although like any landmark engineer, he had some help from previous research. The original Carrier Psychrometric Chart was compiled from empirical data from meteorologist William Ferrel as well as a Professor Marvin. (Gatley, 17) Carrier compiled this data to create a descendant of the Psychrometric Chart, called the Hygrometric Chart:

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This chart possesses some features that would become a mainstay of modern psychrometric charts, such as dry bulb temperature on the horizontal axis, more commonly referred to as air temperature. (Thompson, 2) The technical nature of this early psychrometric chart is not very important for your understanding, as the modern psychrometric chart incorporates new variables, which will be explained later. What is important to note, however, is that psychrometrics has a long, storied history of over one hundred years, and has been a vital mainstay of the air conditioning trade. While printed psychometric charts are now being replaced with psychrometric software based on ideal gas algorithms with 99% accuracy (Gatley, 17), the psychrometric chart is still a secondary feature on these software programs. But more significantly, the psychrometric chart still serves as an important educational tool to professionals who want to learn more about how air humidification works. Want to teach an old HVAC dog some new tricks? The technical aspects of the modern psychrometric chart will be discussed below.

How does a Psychrometric Chart Work?

The psychrometric chart might seem daunting at first, but with enough practice, it couldn’t be easier! Here is a modern version of the psychrometric chart:

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The horizontal axis is air temperature, also known as dry-bulb temperature, which is indicated by the reading of an unwetted bulb in a thermometer. (Thompson, 1) The vertical axis is humidity ratio, which indicates how much water vapor is present in the air (although this is a ratio, think of it as a number value!) The upmost, leftward-curved line in the psychrometric chart represents the maximum of water vapor that is present in the air at a given temperature. This is also called the 100% relative humidity line. According to Joseph Thompson, “the maximum amount of water that the air can hold doubles for every 20 degrees F increase in temperature.” (Thompson, 1) The other leftward curves, called relative humidity lines, represent the proportion of humidity in the air at a given temperature relative to its maximum. For example, at 100 degrees F, 40% relative humidity would yield a humidity ratio of approximately .015. Note the curvature of the graph: high relative humidity can be achieved at lower temperatures with a low humidity ratio, whereas it takes a higher humidity ratio to maintain higher relative humidity at warmer air temperatures. Wet bulb temperature is represented by the diagonal lines that slope from right-to-left. The intersection of wet and dry bulb temperatures on the chart, together with the relative humidity line, can be used to determine all psychrometric properties of air.

If you are like me, you probably had to read that paragraph a few dozen times before you actually began to conceptualize what the chart is conveying. Still confused? Not to worry! Let’s stretch our psychrometric psyche and do some exercises.


1. A dry bulb (air temperature) thermometer reads 100 degrees Fahrenheit and a wet bulb thermometer reads 68 degrees Fahrenheit. What is the relative humidity?

Find the vertical line on the graph marked 100 – this is the 100 degrees dry-bulb line. Now find the diagonal right-to-left line marked 70 degrees, and envision an imaginary line to the 100 degrees vertical line. Notice where they meet on the graph. The relative humidity line intersects at this point—this relative humidity line is marked 20%. Therefore, the air has 20% relative humidity at a dry bulb temperature of 100 and a wet bulb temperature of 68.

2. What is the humidity ratio at this point?

The humidity ratio is the located on the vertical axis. Therefore, imagine a horizontal line running from the intersection of the 100 degrees dry bulb / 68 degrees wet bulbs wet bulb point to the vertical axis located on the right side of the graph. The humidity ratio of this point is approximately .0075.

3. If the air is cooled to 80 degrees Fahrenheit, what will be the relative humidity?

A cooling of air temperature represents a shift to the left. Find the vertical line marked 80. Now, refer to the diagonal wet-bulb line of 68 degrees in the previous problem. You will see that this intersection is located a little below the 60% relative humidity line. Therefore, we can estimate that the relative humidity of the cooled air is approximately 58%.

See, it’s not so bad! Let me introduce another variable into the mix: dew point. If you’ve ever examined your lawn in the early morning, you will know that water droplets often form on blades of grass. This is because the grass cools rapidly, and moisture in the surrounding air condenses at a rate greater than it can evaporate. ( Therefore, water droplets form on the grass—much to the squirrels delight!

In the context of the psychrometric chart, the dew point temperature of air is the point at which the given temperature is at 100% relative humidity. Refer to this example:

4. What is the dewpoint temperature of the air at 90 degrees Fahrenheit wet-bulb temperature and 80 degrees dry-bulb temperature?

Find the point where the vertical dry-bulb line and the diagonal wet-bulb line intersect. Now, imagine an imaginary horizontal line going to the left-most of the graph, at the 100% relative humidity sloped line. Mark a point at that place on the 100% relative humidity slope. Now, imagine a vertical line descending from the point to the dry-bulb temperature x-axis. You will notice this line intersects the x-axis at approximately 75 degrees Fahrenheit (this is the dry-bulb/air temperature). Therefore, the dewpoint temperature of the air at 90 degrees Fahrenheit wet-bulb temperature and 80 degrees dry-bulb temperature is 75 degrees Fahrenheit.


Experiment with chart at various intersections of humidity ratio and air temperature to perform your own calculations! Once you are able to understand the mathematics of the chart, it will make measuring the humidity of your systems quite an easy process. There is a recap of the technical terms introduced in this paper below. Good luck!


Psychrometrics: The area of engineering investigating the moist properties of air.

Psychrometric chart: A tool used by engineers and ventilation professionals to determine psychrometric values such as the relative humidity, humidity ratio, and relative humidity.

Dry-bulb temperature: Also known as air temperature, the reading of an unwetted bulb in a thermometer. Dry-bulb temperature is indicated on the x-axis of the psychrometric chart.

Wet-bulb temperature: Indicated on the diagonal lines sloping from right-to-left on the psychrometric chart, wet-bulb temperature represents the reading of a thermometer with a wetted cotton wick around the bulb. (Thompson, 3)

Humidity ratio: Simplified, the amount of water vapor in the air.

Relative humidity: The proportion of humidity in the air at a given temperature relative to its maximum. Represented by the sloping left-to-right lines in the chart possessing percent values.

Dew point: The dew point is the temperature below which the water vapor in air condenses into liquid water at the same rate at which it evaporates. (100% relative humidity) (

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