Table of Contents

  1. Understanding Psychrometric Charts
  2. Dry Bulb Temperature (Tdb)
  3. Wet Bulb Temperature (Twb)
  4. Dew Point Temperature (Tdp)
  5. Humidity Ratio or Moisture Content
  6. Specific Air Volume
  7. Sensible Heat Ratio (SHF)
  8. Relative Humidity (RH)
  9. Enthalpy
  10. Combination of Properties

Welcome, HVAC enthusiasts! Understanding psychrometric charts might seem daunting, but fear not. We’re here to make it as easy as A.B.C., well in this case H.V.A.C! In our guide on psychrometric charts, we will reveal the mystery of the powerful tools that drive the world of heating, ventilation, and air conditioning (HVAC). While these charts may sound complex, we’re here to break them down into simple, relatable terms, so everyone, from the average person on the street to seasoned HVAC professionals, can grasp their importance.

Think of psychrometric charts as the key to indoor comfort. They’re like the wizards behind the curtain, ensuring the air in your home, office, or any indoor space is just right. Whether it’s keeping you cool on a scorching summer day or toasty warm during the winter chill, psychrometric charts are the unsung heroes of HVAC.

In this guide, we’ll explore the ins and outs of psychrometric charts, making them accessible and understandable. So, let’s embark on this journey to demystify HVAC’s secret sauce and discover the top 10 things that every HVAC engineer and technician must know. By the end of it, you’ll have a newfound appreciation for the role these charts play in our daily comfort.

Psychrometric Chart: Visualizing Air Properties
Psychrometric Chart: Visualizing Air Properties. Source: Researchgate

If you’re looking to learn the basics of HVAC, we recommend the following top 3 books:

These are the top 3 best books to learn the basics of HVAC, and they will provide you with valuable knowledge on your HVAC adventure.

1. Understanding Psychrometric Charts

To understand what a psychromteric chart is and how it used I would recommend that you first read  Introduction to Psychrometry: Understanding the Properties of Moist Air.  In addition to that, a practical understanding of where you will find examples of the use of psychrometric charts in your everyday life can be found by reading Practical Applications of Psychrometry in Various Industries and Environments.  Now that that you understand that the psychrometric is graphical tool used by HVAC professionals to analyze and control the air’s temperature, humidity, and other properties for efficient heating, ventilation, and air conditioning systems, we can get into the thick of things!  Let’s learn about all the properties of air.

To understand what a psychrometric chart is and how it’s used, we recommend diving into the fascinating world of psychrometry. For students and engineers eager to master the art of psychrometrics, there’s no better guide than “A Guide in Practical Psychrometrics for Students and Engineers.

This comprehensive resource takes you on a journey through the intricacies of psychrometric charts, making the complex seem simple. Whether you’re a student embarking on your HVAC studies or an experienced engineer fine-tuning air conditioning systems, this guide is a must-have. It’s your key to unlocking the mysteries of psychrometrics.

Learn more about “A Guide in Practical Psychrometrics for Students and Engineers” on Amazon:

This guide equips you with the knowledge to analyze and control air temperature, humidity, and other properties efficiently. It’s not just a book; it’s your companion in the realm of psychrometrics, ensuring you’re well-prepared to tackle the challenges of heating, ventilation, and air conditioning systems. Take your understanding of psychrometric charts to the next level with this invaluable resource.


2. Dry Bulb Temperature (Tdb)

Dry Bulb Temperature, often referred to as “ambient air temperature,” is essentially the temperature of the air in your immediate surroundings. To measure it, you simply place a thermometer in the air where you are. This reading accurately reflects the Dry Bulb Temperature (Tdb), provided that the thermometer is shielded from direct sunlight, or any other radiation, to prevent interference from other heat sources. Additionally, it’s crucial that the measurement is taken in a dry environment, as excessive moisture in the air can distort the readings. Tdb is the metric we rely on when we simply want to know how hot or cold it is outside.

Tdb is measured with a standard thermometer that isn’t exposed to moisture or radiation. Tdb is usually expressed in degrees Celsius (°C) or Fahrenheit (°F). Kelvin (K) is also used, where zero Kelvin means absolutely zero heat and is equivalent to -273.15 °C. To determine the dry bulb temperature from a psychrometric chart, you take the temperature at a vertical line on the X-axis. It’s the reference point, representing the heat content, and the constant dry bulb temperatures appear as vertical lines in the psychrometric chart. So, whether you’re a newbie exploring the HVAC realm or a seasoned professional fine-tuning an air system, remember Tdb. It’s the unsung hero of temperature measurement, straightforward and reliable.

These are the top 3 psychrometers that we suggest for taking dry bulb temperature readings:

Dry Bulb Temperature on Psychrometric Chart
Dry bulb Ttemperature on psychrometric chart represented as vertical lines. Source: NC State University

3. Wet Bulb Temperature (Twb)

Wet Bulb Temperature, often abbreviated as Twb, is a crucial parameter in psychrometry. It represents the temperature of air undergoing adiabatic saturation, a process where air becomes saturated with moisture without any heat exchange.

Measuring Twb involves using a thermometer with its bulb covered by a wet muslin cloth. As the moisture on the cloth evaporates into the air, it has a cooling effect on the thermometer bulb, causing the temperature reading to drop. This temperature, the wet bulb temperature, is typically lower than the dry bulb temperature (Tdb) of the surrounding air.

The rate of evaporation from the wet cloth and the temperature difference between the dry bulb and wet bulb are influenced by the humidity of the air. In more humid conditions, evaporation is slower.

It’s important to note that Twb will always be lower than the dry bulb temperature (Tdb), except under one specific circumstance – when the air is completely saturated with moisture, reaching 100% relative humidity (RH). In that unique case, the wet bulb and dry bulb temperatures are identical.

By plotting the dry bulb and wet bulb temperatures on a psychrometric chart or Mollier chart, you can gain insights into the state of the humid air. Look for lines of constant wet bulb temperatures on the chart, which run diagonally from the upper left to the lower right. These lines are invaluable tools for understanding and working with psychrometric data.

Wet bulb temperature represented on a psychrometric chart as constant diagonal lines.
Wet bulb temperature represented on a psychrometric chart as constant diagonal lines. Source: NC State University.

4. Dew Point Temperature (Tdp)

Ever wondered about the magic temperature at which air starts to create dew, and everything feels a bit… well, saturated? It’s called the Dew Point, and it’s like the air’s own personal saturation point. Think of it as the moment when air just can’t hold its moisture any longer, and voila, you get dew!

So, what’s the deal with the Dew Point Temperature (Tdp)? When it’s close to the ambient air or dry bulb temperature (Tdb), you know you’re in a high humidity situation, and things might feel a tad muggy. But when the Dew Point is way lower than the air temperature, you’ve got low humidity on your hands, which can feel pretty crisp.

For a fun experiment, think about that cold soda bottle in your fridge. When it’s super chilly, you’ll see moisture droplets forming on the outside. Well, that’s the Dew Point of the air being higher than the temperature inside the fridge.  Now, to measure the Dew Point Temperature, all you need is a metal can, some ice cubes, and a trusty thermometer. Mix the ice and water in the can, give it a good stir, and watch what happens. When the air’s vapor decides to turn into droplets on the can’s surface, you’re pretty close to the Dew Point of the actual air.

On a psychrometric chart, the lines that represent the Dew Point Temperature are the horizontal lines running from the left to the  right. These lines display how the Dew Point Temperature changes with variations in the Dry Bulb Temperature and can be a handy reference for HVAC engineers and technicians. When working with the chart, follow these diagonal lines to identify the Dew Point Temperature, a crucial factor in assessing moisture levels and potential condensation in air.

So, when you’re consulting those psychrometric charts or geeking out on all things HVAC, remember the Dew Point – it’s the air’s way of saying, “I’m feeling a bit saturated today!”

Dew point temperature represented on a psychrometric chart as constant vertical lines.
Wet bulb temperature represented on a psychrometric chart as constant horizontal lines.  Source: NC State University

5. Humidity Ratio or Moisture Content

Specific Humidity is like the “water content” of the air. It’s measured in grams of water vapor per kilogram of dry air. Think of it as the amount of moisture that the air is carrying. Now, air can be a bit picky – it can only support a certain amount of moisture at a given temperature. This limit is what we call saturation humidity.

On the psychrometric chart, we find humidity ratio represented by lines that run horizontally. You’ll spot these lines on the right-hand side (Y-axis) of the chart. They start at the bottom and rise to the top, indicating how the humidity ratio changes with varying conditions. So, if you ever wondered how much moisture your air can handle, this is where you’ll find your answers!

Humidity Ration represented on a psychrometric chart as constant vertical lines read off the right hand side.  Source: NC State University

6. Specific Air Volume

Specific Volume is like the personal space of air – it’s all about how much room a certain amount of air takes up under specific conditions. In simple terms, it’s the opposite of air density. When the air gets warmer, it’s like it’s had a few extra cups of coffee; the molecules get excited and start jiggling around, making them spread out more. This makes warm air less dense than cool air – and that’s why it rises, kind of like a helium balloon at a birthday party. So, remember, warm air has more specific volume and is a bit of a lightweight.

Now, things get interesting when we throw humidity and atmospheric pressure into the mix. The more moisture vapor the air holds, the more spacious it becomes. And when the overall atmospheric pressure is cranked up, the air gets a bit shy and huddles closer, reducing its specific volume. You’ll find specific volume marked on the Psychrometric Chart as lines that slant from the lower right-hand corner to the upper left-hand corner. They’re like the rebels of the chart, always going against the flow.

Specific volume represented on a psychrometric chart as constant diagonal lines from the top left to the bottom right.
Specific volume represented on a psychrometric chart as constant diagonal lines from the top left to the bottom right.  Source: NC State University

7. Sensible Heat Ratio (SHF)

The Sensible Heat Ratio is a critical parameter used to determine the proportion of sensible heat and latent heat contributing to the overall cooling load. On the ASHRAE psychrometric chart, a protractor is employed to precisely measure and interpret this ratio by plotting the slope of the corresponding line. This information is valuable for optimizing cooling systems and ensuring efficient operation.

The ASHRAE psychrometric chart provides us with a handy tool, a protractor in the top left corner, to plot the slope of the line representing the Sensible Heat Ratio.  To get the SHR, take a ruler and put it along the slope slope ot the line showing the psychrometric process being studied(long red line on Figure[…].  Then move the ruler towards the protractor in the top let, while still keeping the ruler parallel to the line of the of psychrometric process.  Pleace the ruler at the center point of the protractor and the point where the arc of the protractor meets the the psychrometric process line will give the SHF(short red line on the psychrometric chart protractor.)

The Sensible Heat Ratio (SHF) is calculated using the following formula:

SHF = Sensible Heat (Qs) / (Sensible Heat (Qs) + Latent Heat (QL))


  • SHF = Sensible Heat Ratio
  • Qs = Sensible Heat
  • QL = Latent Heat

This equation is instrumental in determining the distribution of sensible and latent heat in cooling processes, providing valuable insights for efficient HVAC system design and operation.


Sensible heat ratio shown on a psychrometric chart protractor.
Sensible heat ratio shown on a psychrometric chart protractor. Source: Facility Dynamics Engineering.

Section 8. Relative Humidity (RH)

Relative Humidity (RH) serves as a yardstick for quantifying the moisture content air can retain at a specific temperature. It’s no secret that the warmth of the air plays a pivotal role; as temperatures rise, so does the air’s moisture-carrying capacity. Within the realm of psychrometric charts, the lines that denote constant relative humidity take shape as curves, originating at the lower left and gracefully sweeping their way up to the upper right of the chart. Notably, the boundary of absolute saturation, which marks 100 percent relative humidity, in the chart’s upper-left corner.

Understanding Relative Humidity provides HVAC professionals with critical insights into air moisture levels, an invaluable asset when optimizing heating, ventilation, and air conditioning systems.

Relative humidty curves on a psychrometric chart.
Relative humidty curves on a psychrometric chart.  Source: NC State University

9. Enthalpy

Enthalpy, a fundamental metric in the world of thermodynamics, quantifies the heat energy contained within the air. It consists of two distinct sources: sensible heat, originating from the air’s temperature, and latent heat, rooted in the air’s moisture content.

The unison of these two forms of energy gives birth to what we term ‘air enthalpy.’ This essential value is usually expressed in Btu per pound (Btu/lb.) of dry air or kilojoules per kilogram (kJ/kg).

Enthalpy plays a pivotal role in the realm of air heating and cooling. It’s the magic that determines whether you’re dealing with dry, scorching hot air, loaded with sensible heat, or the pleasantly cool, moist variety carrying an abundance of latent heat. On the psychrometric chart, the enthalpy scale can be found to the top left of the chart’s saturation boundary, marked with lines of constant enthalpy, flowing diagonally from left to right. These lines follow a path quite similar to the constant wet bulb temperature line, providing valuable insights into the air’s energy content.

When calculating the enthalpy of moist air, you’ll want to use the formula:

h = (1.007 * t – 0.026) + g * (2501 + 1.84 * t)

Where ‘g’ symbolizes the water content in kg/kg of dry air and ‘t’ represents the dry bulb temperature in degrees Celsius.

Understanding enthalpy is a must for HVAC engineers and technicians who aim to master the art of optimizing air conditions.

Enthalpy lines represented on a psychrometric chart.
Enthalpy lines represented on a psychrometric chart.  Source: NC State University

Section 10: Combination of Properties

You now know the different areas of a psychrometric chart and how to find the various properties of air under specific conditions, but that is just the begining! The most important part of understanding psychrometric is putting it all together, better referred to as combination of properties.  Combining properties will allow you to intepret the chart and determine exactly which process is taking place based on the air conditions given.  The chart below is the complete chart combining most of the lines and other parameters so far

Complete psychrometric chart combining most of the lines and other parameters
Complete psychrometric chart combining most of the lines and other parameters. Source: Testbook.


In wrapping up our exploration into the world of psychrometric charts, it’s essential to highlight how they benefit HVAC engineers and technicians in real-world scenarios. These charts may seem complex, but in plain terms, they are like maps guiding professionals in making indoor spaces comfortable and efficient.  Think of it this way: just as a GPS helps you find your way on the road, psychrometric charts help HVAC experts navigate the world of air conditioning and heating. They provide insights on how to control temperature and humidity, ensuring your home or office stays cozy and healthy.

Whether you’re a seasoned HVAC pro or someone starting out, these charts are your secret weapon for creating the perfect indoor environment. They bridge the gap between theory and practice, offering practical solutions for better indoor air quality and energy-efficient systems. So, next time you step into a comfortably cooled or heated space, you’ll know there’s a bit of science behind it, making your surroundings just right.