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How to Use the Psychrometric Chart

The psychrometric chart provides quick answers to questions that would be take longer to calculate if you had to use a formula

Posted on Dec 5 2014 by Martin Holladay

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(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. 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 temperatureAir temperature as measured by an ordinary thermometer..” 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.

Problem #1.
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.

Problem #2.
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).

Problem #3.
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.

Problem #4.
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 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. 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.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Mooganic

1.
Dec 5, 2014 10:55 AM ET

Air holding moisture
by Bill Rose

Saying that cold air can hold less water is convenient shorthand. I sometimes use that shorthand myself. However it masks the fact that that it is the temperature of the materials that bound (or the matter contained in) the air that governs the quantity of water contained in the air. Imagine a six-sided box with five warm sides and one cold side. The upper limit of dewpoint in the air will be the temperature of the cold side, while the "air temperature" in the box will be a function of the temperatures of all the sides. This important observation pertains directly to frame wall cavities.

Also, consider snowflakes. They are formed in supersaturated air (look up Ukichiro Nakaya), and the greater the amount of water above saturation (where air purity is a factor) the prettier the snowflake.


2.
Dec 5, 2014 11:32 AM ET

Edited Dec 5, 2014 11:46 AM ET.

Response to Bill Rose
by Martin Holladay

Bill,
It's always a pleasure to have you review my work.

You wrote, "Saying that cold air can hold less water is convenient shorthand." In most cases, I'm happy to employ convenient shorthand descriptions, especially when they are useful. However, I appreciate your point about stud cavities that are enclosed by surfaces at different temperatures. Your point is well taken.


3.
Dec 6, 2014 2:20 PM ET

Edited Dec 7, 2014 9:14 AM ET.

Pilots, planes, and density
by aj builder, Upstate NY Zone 6a

Pilots, planes, and density altitude oh my

http://flighttraining.aopa.org/magazine/2007/April/200704_Features_Takeo...

Much worse than humidity.... Snow flakes just starting prior to departure.... No take off ... And the runway ends....thank fully in a grass transition area. Lots of interesting charts in a plane's operating handbook.

Walls; so cold exterior sheathing sets the in wall climate. Which sets up a differential from inside the home creating a driving force into the wall?

humidity.jpg


4.
Dec 8, 2014 12:39 PM ET

Bill Rose's box model is apt
by Dana Dorsett

Often people assume that condensation in a fiber-insulated stud bay will happen at some depth in the fiber. But since the fiber is air & vapor permeable, the dew point of that entrained air can't exceed the temperature of the cold side. All of the condensation (or adsorption) during condensation happens at the interior or exterior side surfaces, and not in the insulation.


5.
Dec 8, 2014 9:21 PM ET

Edited Dec 8, 2014 11:32 PM ET.

Martin
by Malcolm Taylor

Blogs like these that act as basic primers may not elicit the volume of responses more controversial ones do but they are immensely useful. Thanks.


6.
Dec 9, 2014 6:14 AM ET

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
Thanks for the feedback. I'm glad that you found the article useful.


7.
Dec 10, 2014 11:14 PM ET

Dew Point Calculator
by C. B.

Handy online calculator from Rochester Institute of Technology:
http://dpcalc.org/


8.
Dec 11, 2014 10:07 AM ET

Response to C.B.
by Martin Holladay

C.B.,
Thanks for providing the link to a useful calculator.


9.
Dec 11, 2014 3:35 PM ET

Another psychrometric chart use
by Rick Smith

We use the psychro chart in an entirely different way. Using the chart variation that has passive strategies and a comfort zone mapped on it, as the initial phase of passive solar NZE design we chart monthly temps and humidities on the chart, and instantly get a graphic which tells us what passive strategies are appropriate to make a comfortable NZE building. It's a really neat trick against which to check one's intuitions without any computer work.


10.
May 1, 2018 3:04 PM ET

AC with windows open?
by Elliot Grochal

Hi I'm wondering what is the threshold where it would be wiser to leave my windows open at night with the AC, versus closing windows with AC. I typically set the AC to 77 degrees, and I find that when the outdoor temp is less than 65 degrees, the indoor temp the next morning is around 74 degrees. I'm assuming that early on the AC works and sucks that cooler air indoors, thus having to work less throughout the night. When the same conditions exist and I close windows, the indoor temp the next morning stays at 77 degrees. I assume the AC is working more through the night if I keep windows closed. My big question is at what outdoor temp/humidity would it be worse to go to sleep with AC set to 77 degrees with windows open? Thanks!


11.
May 1, 2018 3:53 PM ET

Response to Elliot Grochal
by Martin Holladay

Elliot,
The answer to your question isn't simple. For more information, see this Q&A thread: Ramblings on thermal mass, AC and window fans.

Relevant comments from that thread:

James Morgan: "As a general point, just in case you're tempted, under no circumstances should you open windows at night and run a/c later in the day. Yes, it's that latent heat of vaporization thing. Whenever humidity is a factor in your cooling concerns air sealing is key."

Dana Dorsett: "I usually look at the outdoor dew point readings from a few nearby weather stations (they're often way off, especially the amateurs) before deciding if it's dry enough to make opening the windows worthwhile. If the outdoor dew points are 13C/55F or less it's fine to open the windows. But when the outdoor dew points are running 15C/59F and higher it becomes a real latent load if your goal is to keep it under 50% RH @ 23-25C.

"According to Wunderground weather station data the current dew points at this hour in Toronto are running in the 20C/68F range: https://www.wunderground.com/ca/on/toronto (Try checking a few stations using their "Change Station" pull down.)

"It will change over the course of the day, but if it's not under 60F, keep the windows closed even if it's cooler outside than indoors."

Charlie Sullivan: "Alan, your initial indoor conditions at 7 pm had a dew point of 23.4 C, while the outside dew point was 19 C. That means you could have used open windows and/or a whole-house fan to do the initial work, for the first few hours, and then had less work for the A/C to do. Dana's suggestion, to open windows when the outside dew point is lower than the inside dew point, works well to maximize your opportunity to reduce the load on the A/C through the use of outside air. Understanding why that works is harder than doing it, so it's understandable that it's not immediately obvious.

"To find inside dew point, you can either:

* Read a chart
* Enter the temperature and humidity readings into a psychrometric calcuator app on your smart phone (there are some free ones that work just fine).
* Enter the temperature and humidity readings into a psychrometric calcuator on the web
* Get a fancier indoor weather station that displays the dew point reading directly.

"As for how it works, the dew point also corresponds to the "humidity ratio" which is sometimes called "absolute humidity" to distinguish it from relative humidity. A 23.4 C dew point corresponds to 1.8 kg of water vapor per 100 kg of dry air, whereas a 19 C dew point corresponds to 1.4 kg of water vapor per 100 kg of dry air. So flushing out 23.4 C dew point air and replacing it with 19 C dew point air will mean you are removing some of the moisture from the air inside. Then when the A/C later cools the space it will not need to do as much dehumidification.

"One of the complications in understanding how fast you can cool and dehumidify a space is that there is moisture storage in the interior finish materials in your house, and in the furniture, books, etc., and on the surfaces of them. So at first your A/C is just dehumidifying the air. But then moisture starts to come out of and off of the the stuff in the house. It's the moisture equivalent of thermal mass--your A/C is also cooling all those materials and objects, not just cooling the air. But that's probably part of why it appears that your relative humidity inside gets down to 60% and sticks there even while you continue to remove substantial moisture each hour.

"Another factor is infiltration of outside air, even with the windows closed, which will tend to raise humidity in the house whenever the outside dew point is higher than the inside dew point."


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