Build Your Own Psych Chart – In Memory of Bill Coad

I can’t really remember which of my HVAC mentors was the first to introduce me to psychrometrics and the psychrometric chart.  They all generously shared their knowledge and time with me as I tried to take the engineering basics I had learned while pursuing my Aeronautics/Aircraft Maintenance Engineering degree and adapt them to my newly adopted career in HVAC. 

But one interaction that has always stood out to me occurred when I asked Bill Coad a question after coming back into the office from a troubleshooting trip out in the field.  My recollection is that as we started the discussion, Bill asked me if I had brought along my psych chart.  When I said that I had not and started to leave to return to my office and procure it, Bill stopped me and said it was not a problem;  that we would just make one.

So, Bill pulled out a pad of the green engineering quad-ruled paper that everyone in the office used for doing their calculations, and over the course of the discussion that followed, constructed a psychrometric chart, deriving the various relationships and parameters required from thermodynamic fundamental principles and the steam tables contained in his copy of Thermodynamic Properties of Steam, by Keenan and Keyes. 

Since that time, I have always thought it was an extremely cool engineering trick, and on occasion, sought to do it myself with varying degrees of success.   My efforts were supported by witnessing occasional repeat performances of the feat by Bill and my other mentors and eventually, by consulting the thermodynamics text book that inspired him to develop the exercised in the first place1 along with Bill’s written documentation of the process that he provides on his website2.

Sadly, Bill recently suffered a stroke and passed on;  this link will take you to his obituary if you are unfamiliar with his accomplishments and the impact he had on the industry.  As a friend of mine commented upon hearing the news, for many of us who knew and admired him, it seems like “the end of an era”. 

Because he inspired me so much, over the past week or so, I have found myself wanting to do something that somehow honored him and the knowledge he so generously shared with me and others like me.  And in doing that, I kept coming back to the recollection of his Construct Your Own Psychrometric Chart trick which he documents in one of the discussions on his web site

While the written discussion Bill provides in the article on his web site is certainly complete in providing the key guidance you would need to learn to perform the “trick”, reading his description of how to do it is somewhat different from watching him do it in an interactive work session.   That’s because when he was working through it with you in person, you could ask clarifying questions when you were not quite following the line of reasoning. 

At first, it was hard for me to do that, not wanting to expose my ignorance.  But Bill considered dialog to be a part of the learning process and always took the time to thoroughly answer my questions, never treating them like they were silly or obvious.  And he was always careful to document key points we were discussing on his note pad, handing the pages to me for reference as I left.

So, I have decided that in his honor, I will do a string of blog posts that will (hopefully) complement his discussion of the topic and share the insights he shared with me.  Specifically, I will walk you through how to do the “draw your own chart” trick, using Excel to do the math and draw the lines instead of a green engineering pad. 

Incidentally, there is nothing wrong with doing this on a green engineering pad.  I still enjoy picking up a pencil and a using green quad-ruled pad;  I always have one with me in my computer bag. I also have a slide rule in there, mostly for sentiment but I occasionally get that out too.  In fact, the first time Bill did this with me, he did some of the math on slide rule.

In any case, if you decide to follow along and try your hand at this, the result should be something along the lines of the image below, which is the chart I created to support the string of posts.


In doing that, you will be exposed to the fundamental principles behind the lines on the chart and maybe learn a few Excel tricks too. 

Despite having done this before on paper, I learned a few things by going through it again using Excel, including some additional insights into the fundamental principles when I decided to add the constant enthalpy lines and a few Excel techniques that allowed me to get the chart looking the way I wanted it to look.

One of the reasons I think Bill liked walking his students through the Construct Your Own Psychrometric Chart exercise was that the chart represents a graphical tool for applying the fundamental principles behind HVAC processes to design, troubleshooting, and operating challenges.  As such, it is possible to use a psychrometric chart with out fully understanding and appreciating the underlying principles behind it. I certainly did that for a long time and in some ways, probably still do.  

By answering my psychrometric question by first using the fundamental psychrometric principles to build a psych chart,  Bill exposed me to the principles in addition to answering my question.  As a result, I was more grounded in my understanding.  For instance, I realized that there were assumptions behind the chart that meant there were limitations to using it on a scale broader than a typical HVAC process.

That doesn’t mean that by the time Bill and I finished that first discussion, I had a complete and through understanding of the science of psychrometrics.   In fact, what I probably had was a complete and thorough understanding of how much I didn’t know. 

But, as Kahlil Gibran said, perplexity is the beginning of knowledge and my curiosity had been aroused.  In addition, witnessing Bill’s mastery of the subject and realizing that it was based on an understanding of the fundamental principles inspired me.  Those things lead to self study and discovery, some of which happened as the result of field experiences that demonstrated principles that I was struggling to understand.   Meanwhile, I had a useful tool in the form of a psych chart that I could use to solve my practical day to day problems.

So hopefully, what follows will in some small way provide similar insights and inspiration for others and while honoring Bill’s leadership and mentoring skills.    If’ I’m successful, soon some of you will be amazing friends and family with your psychrometric prowess.  And if I’m really lucky, maybe I will inspire a young and curious engineer and expand their horizons, just like Bill did mine.


David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for a recent index to previous posts


1.     Bill frequently referenced Engineering Thermodynamics by Herman J. Stoever.  I eventually bought a copy of my own and have found it to present thermodynamics in a very approachable manner, in particular as they the science applies to building systems and processes.  This was helpful to me since my thermodynamics course in college was (appropriately) focused on applying the principles to aeronautics, my course of study at the time.  You can still find used copies around, including on the Powell’s Books web site and

2.  Bill’s website is at this link and contains numerous resources of interest if you are involved with HVAC, including a version of his ASHRAE paper on energy conservation as an ethic, copies of all of the Fundamentals to Frontiers articles he wrote for Heating, Piping, and Air Conditioning magazine, a .pdf copy of his book Energy Engineering and Management for Building Systems and the narrative on the process for building your own psychrometric chart, which I am discussing in this string of blog posts. 

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