When you drive a borrowed car, it takes a few moments to figure out how to operate the windshield wipers and the headlights. But in your own car, your hand reaches for these switches without thinking.
Using the psychrometric chart is a little like driving a car. If you use the psychrometric chart every day, you don’t have to orient yourself. But if you are like me, and you only consult the psychrometric chart two or three times a year, it’s useful to refer to a cheat sheet every time you use it.
The psychometric chart (see Image #2, below) is a graph that shows (among other things) the relationships between air temperature, relative humidity, and dew points under a variety of conditions. The psychrometric chart is your friend. Like the periodic table of the elements, it’s a very useful invention. It organizes a lot of information and presents it graphically, allowing builders, HVAC contractors, and building scientists to answer certain questions quickly.
Remember: if you need an answer to a question involving dew points or relative humidity, consulting the psychrometric chart is much easier than making calculations using a formula.
A crash course in psychrometrics
Here is my list of the top five things that builders need to know about air and water vapor.
1. Cold air can’t hold as much moisture as warm air. When the outdoor air is at 10°F, it isn’t holding much moisture — even if the relative humidity (RH) of the air is 90%. (In Comment #1 below, Bill Rose notes that “Saying that cold air can hold less water is convenient shorthand.” He goes on to note that in some instances — for example, when air in a stud bay is bounded by surfaces of different temperatures — the shorthand rule doesn’t apply. For more information on this topic, see Rose’s comment.)
2. If the interior air of a house is humid (above 40% RH) during the winter, turn on a ventilation fan. Since exterior air during the winter is almost always dry, ventilation during the winter will lower the relative humidity of the indoor air. (Needless to say, high indoor humidity is often caused by gross building defects — for example, a crawl space with an exposed dirt floor — or by occupant behavior — for example, maintaining dozens of houseplants or several fish tanks — and these defects or behaviors must be corrected or adjusted in many cases.)
3. If the interior air of a house is dry (below 20% RH) during the winter, the house either has a leaky envelope or is being overventilated. If a house with this problem has a ventilation fan, reduce the ventilation rate by operating the fan less frequently. If there is no ventilation system, the house probably has a high rate of air leakage; in this case, perform air sealing work.
4. It’s rare for homeowners to complain that indoor air is too dry during the summer. During the summer, humidity control almost always refers to lowering, not raising, the indoor RH.
5. If the interior air of a house is humid during the summer, turn on an air conditioner or a dehumidifier. In hot, humid weather, operating a ventilation fan usually makes the indoor air more humid, not less humid.
The web has good resources
If you Google “using the psychrometric chart,” you’ll find links to plenty of articles that explain what you need to know to understand and use this useful graph. (For example, here are two: “The Psychrometric Chart and Its Use” and “Psychrometrics: Level 1.”) These instructional materials vary in clarity; different articles cater to different audiences.
I’m not going to try to recreate a textbook chapter about using the psychrometric chart. Instead, I’m going to share a few nuggets I have learned — nuggets that have helped my understanding. In choosing these nuggets, I’ve been guided by my prejudice in favor of explanations that use ordinary words instead of jargon and equations.
Defining our terms
First, we should review a few definitions.
In the HVAC industry, ordinary air temperature is referred to as “dry bulb temperature.” On most psychrometric charts, dry bulb temperatures are indicated by vertical lines; the dry bulb temperature scale is at the base of the chart.
The “wet bulb temperature” is determined by swinging a sling psychrometer — that is, a special thermometer equipped with a bulb wrapped in moist cloth — through the air. The evaporation of the moisture in the cloth cools the thermometer’s bulb below the dry bulb temperature. On a day when the air is dry, the wet bulb temperature will be lower than it will on a day when the air is humid. The wet-bulb temperature is the lowest temperature to which air can be cooled by evaporative cooling.
On a psychrometric chart, the wet bulb temperature scale is located on the curved line on the left side of the chart. Straight lines that slope down from the upper left to the lower right indicate wet bulb temperatures; these same lines also represent the enthalpy of the air.
The dew point temperature is the temperature at which moisture in the air begins to condense on hard surfaces. The dew point temperature scale is located along the right hand side of the chart (with lower dew point temperatures at the bottom of the chart, and higher ones at the top of the chart). Dew point temperature lines are horizontal lines.
If a surface (for example, the exterior of a glass filled with ice water) is below the dew point temperature of the air in a room, condensation will form on the surface. The source of these condensation droplets is the air around the glass. If the cold surface is above freezing and below the dew point, the surface will accumulate liquid dew; if the cold surface is below freezing and below the dew point, the surface will accumulate frost.
The relationship between dry-bulb temperature, wet-bulb temperature, and dew point temperature is explained this way in a Wikipedia article: “For a parcel of air that is less than saturated (i.e., air with less than 100 percent relative humidity), the wet-bulb temperature is lower than the dry-bulb temperature, but higher than the dew point temperature. The lower the relative humidity (the drier the air), the greater the gaps between each pair of these three temperatures. Conversely, when the relative humidity rises to 100%, the three figures coincide.”
Relative humidity is the amount of moisture in the air expressed as a percentage of the maximum moisture capacity of the air. On the psychrometric chart, curved lines representing conditions of equal relative humidity extend from the lower left to the upper right. The 100% relative humidity line is the uppermost (last) curved line on the chart. This line is also called the “saturation curve.” The condition of the air at any point along this line is fully saturated. At every point along this line, dry bulb temperature = wet bulb temperature = dew point temperature.
The four basic psychrometric properties are dry-bulb temperature, wet-bulb temperature, dew-point temperature, and relative humidity. If you know any two of these properties, the other two properties can be determined from the psychrometric chart.
Absolute humidity is the water content of the air, expressed as the weight of water vapor (in the U.S., in grains or pounds) per unit volume (in the U.S., per cubic foot) of dry air. A grain of water (about one drop) is 1/7000 of a pound. So, to convert grains of water to pounds of water, divide grains by 7,000.
Solving problems with the psychrometric chart
Let’s use some examples to illustrate how we can use the psychrometric chart. The easiest way to follow along with these examples is to print the psychrometric chart (Image #2, below) on paper. The second easiest way to follow along is to open the psychrometric chart in a separate tab or window of your web browser.
Q. “The outside air temperature is 35°F and the outdoor relative humidity (RH) is 80%. I’d like to operate my ventilation fan to lower the indoor RH (which is now at 40%), but I don’t know if bringing in outdoor air under these circumstances makes any sense. What will the RH of this outdoor air be once it is warmed up to 70°F?”
A. First, find the point on the dry bulb temperature scale at the bottom of the psychrometric chart that corresponds to 35°F. Then follow the vertical line upwards from this point until it intersects the curved line corresponding to 80% RH. To find out the RH of this air once it is heated up to 70°F, move to the right along one of the horizontal lines until you intersect the vertical line corresponding to 70°F. If you read the RH scale (the curved lines) at this point, you’ll see that the RH of this air at 70°F is about 22%. So the outdoor air under these conditions will do a good job of helping lower the indoor RH.
Q. The air temperature is 80°F on an ordinary (dry bulb thermometer) and 60°F when measured with a wet bulb thermometer. What is the relative humidity?
A. Find the point on the dry bulb temperature scale at the bottom of the psychrometric chart that corresponds to 80°F. Follow the vertical line upwards from this point until it intersects the sloped line (one of the straight lines that slopes from the upper left to the lower right) that corresponds to the 60°F wet bulb scale. (Remember: the numbers indicating this wet bulb scale are found along the curved line at the upper left of the psychrometric chart.) Once you’ve found this point, read the relative humidity at this point; it’s 30%. (The RH lines are the curved lines).
Q. An old air conditioner is on its last legs. The temperature of its cooling coil is 52°F — not very cold. The indoor air is at 80°F and 35% RH. Is the coil cold enough to remove any moisture from the air?
A. Find the point on the dry bulb temperature scale at the bottom of the psychrometric chart that corresponds to 80°F. Follow the vertical line upwards from this point until it intersects the curved 35% RH line. Then move horizontally to the right to read the dew point temperature at the scale at the right side of the chart. The dew point temperature of this air is 50°F. Since the cooling coil is warmer than the dew point of the air, no condensation will occur, and the coil won’t remove any moisture from the air.
Q. Outdoor air has a dry bulb temperature of 45°F and a relative humidity of 40%. Determine the wet bulb temperature and the dew point temperature.
A. Find the point on the dry bulb temperature scale at the bottom of the psychrometric chart that corresponds to 45°F. Follow the vertical line upwards from this point until it intersects the curved 40% RH line. You can read the dew point temperature on the scale at the right side of the chart; it is about 23°F. You can read the wet bulb temperature by looking for the straight lines that slope from upper left to lower right; at the point we’re looking at, the wet bulb temperature is about 36.5°F.
Miscellaneous information about psychrometrics
Fun fact #1. According to The Psychrometric Chart and its Use, “Air, in addition to having weight, also exerts a pressure, called barometric pressure, which is usually measured in inches of mercury above a perfect vacuum. Standard barometric pressure at sea level is 29.921 inches of mercury, which is equivalent to 14.696 pounds per square inch absolute. The pressure exerted by the air, or barometric pressure, is caused by both the dry air and the water vapor. Most of the total pressure is from the air, but some is from the water vapor. As is true of the weight, or density, the pressure from the water vapor is usually only about ½ to 1½ per cent of the total pressure.”
Fun fact #2. The volume of water vapor is a small percentage of the volume of a sample of air. For example, in a warm climate like that of Florida, the amount of water vapor in the outdoor air varies from about 1% by volume under cold dry conditions to a little more than 5% during hot, humid conditions.
Fun fact #3. When we cool dry air we remove sensible heat of about 1/4 of a BTU per pound of dry air per degree, and when we heat dry air, we add the same amount of sensible heat.
Fun fact #4. “It is essential to always keep in mind that air consists of two separate gases, dry air and water vapor, which act independently, each according to its individual properties, just as if the other were not there. … Although the water vapor is a very small part of air, as far as density and pressure are concerned, it is the part which complicates the psychrometric processes and calculations.”
Fun fact #5. According to a Carrier Corporation paper called “Psychrometrics,” “Since one pound of air at 100ºF, with all the water it can hold, contains 302.45 grains [of water] (about ½ ounce), this water does not have much bearing on the actual weight of the air.”
Fun fact #6. An instructor is teaching a course in a classroom that is 46 feet wide and 66 long. The ceiling height is 20 feet. The instructor asks the students, “How much do you think that the air in this room weighs? 0 pounds? 10 pounds? 100 pounds? 1,000 pounds?” The answer: 4,554 pounds.
Martin Holladay’s previous blog: “Revisiting an Energy Saving Handbook from 1979.”