Architectural Theory, Climate Analysis, Design Guide, Design Theory

How to Read Psychrometric Chart for Architecture

We know that HVAC plays a significant role in the comfort of a built environment. We also know that it’s quite tricky to get it just right. That’s where a psychrometric chart proves to be most helpful, as it helps solve real-time HVAC problems in a building. 

With a single chart, we will be able to find multiple air properties such as the temperature and humidity of a space. 

Why Use Psychrometric Charts in Architecture?

Using psychrometric charts, we can assess the physical and thermal properties of air in the environment. This helps us find solutions to commonly occurring HVAC problems that can be fixed beforehand when taken into consideration in the early stages of design. 

The following can be assessed with the help of a psychrometric chart:

  • Thermal human comfort.
  • Energy Requirements of a space.
  • Required design strategies to achieve comfort. 

What All Can We Read in a Psychrometric Chart?

A Psychrometric Chart helps us read the following information:

  1. Dry Bulb Temperature
  2. Wet Bulb Temperature
  3. Dew Point Temperature
  4. Relative Humidity
  5. Specific Humidity

1.Dry Bulb Temperature

This can be measured using a thermometer. When we refer to the air temperature, we often refer to the dry-bulb temperature. It is called “dry bulb temperature” because it is not affected by moisture or humidity. 

Measuring unit – Degrees Celsius (°C) or Degrees Fahrenheit (°F).

2. Wet Bulb Temperature

This refers to the lowest temperature to which the air can be cooled through the process of evaporation of water into the air. We can measure the wet-bulb temperature of a space by wrapping a wet wick around the bulb of a thermometer. 

Measuring unit – Kelvin (K)

3. Dew Point Temperature

It is the temperature that the air is cooled to get achieve 100% relative humidity. If you ever get below this temperature, the air will change its state and begin condensing. Dew point temperature is measured in a saturation curve. 

Measuring unit – °C Td

4. Relative Humidity

Relative humidity helps us measure the amount of moisture present in the air in relation to the maximum amount of moisture the air can actually hold. 

  • Hot air – holds more moisture.
  • Cold air – holds less moisture.
  • The relative humidity is 100% during rainy seasons. 

There is no particular unit in which relative humidity is measured. It is instead measured in percentage. 

5. Specific Humidity

It is the weight of water vapour present in a unit weight of moist air. It is known to be the most reliable measurement of humidity in a space. 

Measuring unit – g/kg (grams of water vapour per kilogram of air)

How to Read Psychrometric Chart Step-by-Step:

  1. To read a psychrometric chart we first find the dry bulb temperature and locate it on the graph on the horizontal axis. 
  2. From the dry bulb temperature point, draw a 45 degree line towards the saturation curve, that line represents the wet bulb temperature.
  3. The vertical axis shows the humidity ratio or specific humidity.
  4. Locate the left most curved line for the dew point temperature or the saturation temperature where the relative humidity is 100%.
  5. The curved lines present in between two ends show the relative humidity.

If you know any 2 of these parameters for a space, you will be able to plot the rest of it on the chart by yourself. 

Psychrometric Chart helps understand thermal comfort and energy requirements of a space
Psychrometric Chart

Climate Consultant Software and Psychrometric Chart

By downloading the EPW file for the location you can studying and attaching the file to the Climate Consultant Software, you can automatically get a psychrometric chart plotted for that specific site location. 

Design strategies are represented by specific zones on this chart. The chart itself is divided into 16 design strategy zones and the percentage of hours that fall into each zone are also shown. This helps us derive the most effective passive and active design strategies for that particular location. 

The software then derives detailed design guidelines for each specific zone and comfort level. 

Although there are instruments that help us derive the measurements in the blink of an eye today, it is important we know and fully understand the correlation of each of these parameters with each other. A psychrometric chart is a visual representation of their interrelation that helps us best understand these parameters and their significance. 

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