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Humidity Control & the history of the Psychrometric Chart

Updated: Mar 13

If you've ever wondered about the science behind air conditioning and humidity control, you're in the right place.

We're going to shed some light on why dry bulb-bulb isolines on the psychrometric chart are nearly vertical.

To uncover the truth, we embarked on a journey through history.

Dr. Willis Carrier, the father of modern air conditioning, developed the psychrometric chart in 1904. Carrier's pioneering work laid the foundation for our understanding of air conditioning and psychrometrics.

Humidity control & history of the Psychrometric chart
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When he created his psychrometric chart, Dr. Carrier used dry-bulb temperature and water vapor density as rectangular plotting coordinates.

In Carrier's original chart, the dry-bulb temperature isolines were all vertical and evenly spaced. This fundamental design choice provided a simple and intuitive way to analyze and manipulate air conditioning and humidity data.

The vertical orientation of dry bulb isolines played a significant role in shaping the field of psychrometrics.

As the field of psychrometrics evolved, so did the psychrometric chart.

Notable contributors like William Grosvenor in 1908 and Richard Mollier in 1923 expanded upon Carrier's work. They added auxiliary lines and introduced slight variations in the process orientation, enriching the chart's capabilities.

Psychrometric chart for Innovative Air Technologies, an industrial desiccant dehumidifier vendor in Covington, GA. Specialling is dehumidification systems for applications such as cannabis farming, aerospace engineering, and marine drilling.
Innovative Air Technologies Psychrometric Chart

However, it was Richard Mollier's significant contribution that truly reshaped the psychrometric chart. He introduced the use of a skewed enthalpy coordinate, replacing the vertical temperature coordinate.

This change marked a pivotal moment in the evolution of psychrometrics, providing a more accurate representation of air properties.

In 1961, ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) adopted the skewed enthalpy coordinate system in their psychrometric chart.

At first glance, the ASHRAE chart might appear identical to Carrier's original chart, but upon closer examination, subtle differences become apparent.

Condenstion can be prevented with the right desiccant dehumidification system from Innovative Air Technologies, a desiccant dehumidifier vendor in Covingont, GA. Specializing in dehumidification systems for applications such as cannabis grow rooms, environmental rooms, and surgical suites.
Condensation can be prevented with the right desiccant dehumidification system.

In the ASHRAE chart, only one dry-bulb isoline is vertical, with all other isolines being straight but diverging slightly as they extend into regions of higher humidity ratios.

The distance between dry bulb isolines in the upper portion of the chart, where humidity ratio values are high, is greater than the distance at the bottom of the chart, where humidity ratio is zero. As humidity levels rise, the dry bulb temperature lines fan out or diverge, reflecting the changing properties of air.

There you have it! The mystery of the nearly vertical dry bulb isolines on the psychrometric chart has been unveiled.

This unique chart, with its rich history and evolution, serves as a fundamental tool for engineers, HVAC professionals, and anyone working with air conditioning and humidity control.

Humidity Control & history of Psychrometric chart
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To delve even deeper into this fascinating field, we recommend "Understanding Psychrometrics" by Donald Gatley, the reference source for this blog post.

Watch this video to learn more about how desiccant dehumidification works.

Need to speak with someone about a desiccant dehumidifier? Email us at

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At Innovative Air Technologies, we strive to provide informative and engaging content through our blog. It's important to note that while our blogs may involve assistance from artificial intelligence (AI), they are not fully written by AI systems. Our team of experts actively contributes to the creation, curation, and editing of the content to ensure accuracy, relevance, and a human touch.

The integration of AI technology is used to enhance the efficiency and effectiveness of our content creation process. It assists in generating ideas, optimizing language, and improving overall readability. However, the final output reflects a collaborative effort between our knowledgeable professionals and advanced AI tools.

We value transparency and authenticity in our communication, and this disclaimer is intended to provide clarity regarding the role of AI in our blog creation. If you have any questions or require further clarification, please feel free to reach out to us. Thank you for your understanding and continued support.

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