Pocket HVAC
What is psychrometrics?
Psychrometrics is the study of air and the water vapor mixed into it: how warm it is, how much moisture it holds and how much heat it carries. All of those properties are tied together, so at a given elevation, two readings like dry bulb and relative humidity are enough to work out the rest.
A psychrometric chart puts all of those relationships on one page. This calculator works them out without reading lines off a chart, for the things techs use it for: wet bulb for charging, dew point for condensation problems, and enthalpy for coil capacity. It also plots your reading on a small chart under the answer, so you can see where the air sits.
How it's calculated
Saturation pressure: ASHRAE eqns 5 (over ice) and 6 (over water) Humidity ratio W = 0.621945 × pw ÷ (p − pw) Wet bulb: ASHRAE eqns 33 and 35, solved by bisection Dew point: eqns 5 and 6 solved for the temperature where pws = pw Enthalpy h = 0.240 t + W (1061 + 0.444 t) Btu/lb, or 1.006 t + W (2501 + 1.86 t) kJ/kg Pressure at elevation z: p = 14.696 (1 − 6.8754×10⁻⁶ z)^5.2559 psia Specific volume v = 0.370486 (t + 459.67)(1 + 1.607858 W) ÷ p ft³/lb
Sources
- ASHRAE Handbook—Fundamentals (2017), chapter 1, Psychrometrics: equations 3, 5, 6, 20, 26, 30, 33 and 35
- Meyer & Thevenard (2019), PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air
If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.
Checked by Paul Williams · Updated
Questions
What does a psychrometric calculator do?
It does what a psychrometric chart does. Two readings fix the state of the air, and everything else follows. For example, 75°F and 50% RH at sea level has a 62.6°F wet bulb, a 55.1°F dew point and an enthalpy of 28.1 Btu/lb.
How do I read the chart under the answer?
Dry bulb runs along the bottom and humidity ratio (moisture per pound or kilogram of dry air) up the right side. The curves are relative humidity, from 10% near the bottom to 100%, the saturation curve, at the top. Your reading is the red dot. Follow the dashed line left from the dot to the saturation curve and you're at the dew point.
Does elevation matter?
Yes, for anything that depends on pressure. At 5,000 ft the same 75°F and 50% RH has a wet bulb of 61.8°F instead of 62.6°F, and holds about 78 grains of moisture per pound of dry air instead of about 65. Relative humidity, dry bulb and dew point describe the same air either way.
How accurate is it?
It uses the ASHRAE Handbook—Fundamentals (2017) equations and matches PsychroLib, an open-source implementation of the same equations, to a thousandth of a degree. Those equations treat moist air as an ideal gas; a real-gas model differs by under 0.05°F in wet bulb and dew point at normal HVAC conditions.
