How to Read a Psychrometric Chart: An 8-Step Guide for HVAC Professionals

Understanding the psychrometric chart is crucial for anyone involved in HVAC engineering, building system design, or environmental sciences. While digital tools like online calculators and smartphone apps offer quick solutions, mastering the psychrometric chart provides a deeper insight into humidity measurement and control.

At PST, we are committed to helping you achieve accurate humidity measurements. This comprehensive guide will walk you through the psychrometric chart in 8 easy steps, enhancing your understanding of various humidity parameters and their interrelationships.

What Is Humidity?

The 8-Step Framework to Read a Psychrometric Chart

Click here to see the psychrometric charts

Step 1: Locate the Dry Bulb Temperature

• Definition: The dry bulb temperature is the ambient air temperature measured by a standard thermometer (it is called 'dry bulb' in relation to the bulb of a traditional liquid in glass thermometer).
• How to Find: Look along the bottom horizontal axis of the chart, marked in degrees Celsius (°C) or degrees Fahrenheit (°F).
• Action: Draw a vertical line upward from your specific dry bulb temperature.

Example: If the dry bulb temperature is (21.1°C) 70°F, draw a vertical line at that point. Next: Depending on your needs you can cross reference with your other known parameter.

Step 2: Locate the Humidity Ratio

• Definition: The humidity ratio (or mixing ratio) represents the mass of water vapor per unit mass of dry air.
• Units: Grams of moisture per kilogram of dry air or grains of moisture per pound of dry air.
• How to Find: Check the right vertical axis of the chart.
• Action: Identify the corresponding humidity ratio for your measurement.

Step 3: Locate the Saturation Curve

• Definition: The saturation curve indicates where the air is fully saturated (100% relative humidity).
• Location: The leftmost curved line on the chart.
• Properties at Saturation:
  • Relative Humidity: 100%
  • Dew Point Temperature equals Dry Bulb Temperature
  • Wet Bulb Temperature equals Dry Bulb Temperature
  • Partial Pressure of Water Vapor equals Saturation Vapor Pressure

Step 4: Locate Relative Humidity Lines

• Definition: The humidity lines represent specific levels of relative humidity in percentage.
• Location: The interior curved lines between the dry bulb temperature axis and the saturation curve.
• Action: Find the line matching your desired relative humidity.

Step 5: Locate Dew Point Lines

• Definition: Dew point is the temperature at which air becomes saturated and water vapor begins to condense.
• How to Find:
  • Locate the vertical line labeled "Dew Point" on the right side.
  • Dew point lines run horizontally across the chart.
• Action: Draw a horizontal line from your dew point temperature.

Step 6: Locate Vapor Pressure Lines

• Definition: Vapor pressure is the pressure exerted by water vapor in the air.
• Units: Millibars (mb) or inches of mercury (inHg).
• How to Find:
  • Positioned next to the dew point scale on the right.
  • Vapor pressure lines are horizontal.
• Action: Identify the vapor pressure corresponding to your conditions.

Step 7: Locate Enthalpy Lines

• Definition: Enthalpy represents the total heat content of the air-water vapor mixture.
• Units: Kilojoules per kilogram of dry air (kJ/kg) or British Thermal Units per pound of dry air (BTU/lb).
• How to Find:
  • Located on the extreme edges of the chart.
  • Enthalpy lines run diagonally upward from left to right.
• Action: Use a ruler to match enthalpy scales diagonally across the chart.

Step 8: Locate Wet Bulb Temperature Lines

• Definition: The wet bulb temperature is the lowest temperature air can reach through evaporation.
• How to Find:
  • Diagonal lines close to but not parallel with enthalpy lines.
  • Expressed in °C with °F in brackets.
• Action: Find the line corresponding to your wet bulb temperature.

Tips and Tricks for Using the Psychrometric Chart

Knowing any two parameters allows you to determine all other properties by finding where the lines intersect, known as the state point.

Example 1: Using Dry Bulb Temperature and Dew Point

• Given:
  • Dry Bulb Temperature: 21.1°C (70°F)
  • Dew Point: 10°C (50°F)
• Steps:
  • 1. Draw a vertical line at 21.1°C (70°F) (dry bulb temperature).
  • 2. Draw a horizontal line at 10°C (50°F) (dew point).
  • 3. The intersection is your state point.
  • 4. At this point, you can read:
    • Relative Humidity
    • Wet Bulb Temperature
    • Enthalpy
    • Vapor Pressure
    • Humidity Ratio

Example 2: Determining Cooling Effect with Evaporative Cooling

• Given:
  • Dry Bulb Temperature: 23.9°C (75°F)
  • Wet Bulb Temperature: 10°C (50°F)
• Steps:
  • 1. Plot the state point where the 23.9°C (75°F) vertical line and the 10°C (50°F) wet bulb line intersect.
  • 2. Follow the wet bulb temperature line to the saturation curve.
  • 3. Read the dry bulb temperature at the saturation point.
• 4. Cooling Effect:
  • The difference between the original dry bulb temperature and the saturation point temperature represents the effective cooling of the air.

Why Mastering the Psychrometric Chart Matters

Mastering the psychrometric chart enhances your understanding of air properties and their interactions. It deepens your comprehension of how different humidity parameters relate to each other, which is essential for designing and controlling HVAC systems effectively. By becoming proficient with the chart, you can make more accurate measurements, leading to better environmental control and energy efficiency in buildings. This expertise sets you apart as a professional in humidity measurement and control, giving you a significant advantage in your field.

Get Started with Our Psychrometric Charts

To help you on this journey, we offer downloadable psychrometric charts at sea level in both SI and IP units. These charts are designed for clarity and ease of use, making it simpler for you to practice and apply the steps outlined in this guide.

We also have an online calculator to check your work.

Our technical team is also available to assist you in interpreting the charts and improving your measurements.

Conclusion

Investing time to understand the psychrometric chart will help you make better measurements and informed decisions in your field. Whether you're an HVAC engineer, building system designer, or a student, mastering this tool is invaluable. It not only enhances your professional skills but also contributes to more efficient and effective environmental control solutions.

Contact Us

Reach out to PST for more information on our products and how we can assist with your humidity measurement needs.

Useful Links and Information

Download Humidity Handbook
Relative Humidity Calculator
NPL Humidity Guides