HVAC · Psychrometrics · Problem 4

Problem & Solution

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Problem: Psychometric's problem for, an air handling system to be used as 6,500 feet above sea level is designed for outside air at 65 degrees and 60% relat...
Given: 500 feet above sea level is designed for outside air at 65 degrees and 60% relative humidity; 000 feet above sea leve...
Approach: Well, normally this problem would be pretty easy because density is the reciprocal of specific volume, so we could just use the...
Calc: So the way this works is we'll find the density of outside air at sea level, and then we'll do an elevation correction, we'll a...
Calc: So let's jump over to the regular psychometric chart and use these conditions 65 degrees and 60% relative humidity, and we'll f...
Result: So as expected the density at the high elevation is lower than it was at sea level and that's closest to answer choice A.

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Student questions asked in live office hours about this problem

OH 15: Psychrometrics 4

Q: On psychometrics problem 4, I used the temperature and altitude corrections table from chapter 7.2 instead of the P2/P0 elevation correction equation—is that acceptable?

A: Absolutely, that's a smart approach. The table is faster and more convenient than the equation. You can interpolate linearly for in-between values, or use the equation only if the altitude significantly exceeds what's in the table. Whatever method gets you the answer quickest on the exam is the right choice.

OH 36: HVAC: Psych-4

Q: Can you skip the elevation/temperature correction calculations and directly use the air table density factors, or is that just a coincidence that it gives the correct answer?

A: It's totally valid and not a coincidence—the table values are generated from the underlying pressure ratio formula, so you can use either approach. I show the formula-based method to build foundational knowledge, but know both ways since the table is often faster on the exam.

OH 71: HVAC: Psychometrics Module #4

Q: Can this psychometrics problem be solved by simply looking up the density factor on page 369 of the reference handbook table rather than using the elevation equation?

A: Yes, that's completely fine—the table on page 369 is generated directly from the elevation equation, so you'll get the same accurate answer. However, I'd recommend knowing both methods because you might encounter an elevation not in the table, so understanding the underlying equation gives you more flexibility.

OH 109: HVAC: Psychometrics Module #4

Q: Can I just use the density value from the handbook table at 6,500 feet instead of calculating it with the elevation equation?

A: Absolutely, using the table directly is perfectly valid for that specific altitude. However, knowing the equation is important because it lets you handle any elevation (interpolate between values, extrapolate beyond 10,000 feet), and it's good to recognize that both the table and equation assume standard conditions of sea level and 70°F—which is baked into many rule-of-thumb calculations—so you should verify that assumption fits your problem.

OH 112: HVAC: Psychrometrics #4

Q: Can the elevation correction equation be used to solve for other psychrometric quantities besides density, or is it limited to pressure corrections?

A: For PE exam purposes, it's impractical and not required—stick with what's in the reference handbook. While elevation charts exist in practice, the equations to solve for other psychometric properties at different elevations aren't readily available in your exam materials, so don't spend time hunting for them.