The psychrometric calculator takes humidity ratio, saturated (ωs), atmospheric pressure (pa), saturated vapour pressure at tdb (ps), saturated pressure at wet bulb temp (pw), dry bulb temperature (tdb), temperature of moist, cooled air (tc) and wet bulb temperature (twb) and returns partial vapour pressure (pv), dew point temperature (tdp) and cooling effectiveness (ε). It is built for quick answers, for checking somebody else’s arithmetic, and for the moments when a formula book is not within reach.
What Is the Psychrometric Calculator?
This psychrometric calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns humidity ratio, saturated (ωs), atmospheric pressure (pa), saturated vapour pressure at tdb (ps), saturated pressure at wet bulb temp (pw), dry bulb temperature (tdb), temperature of moist, cooled air (tc) and wet bulb temperature (twb) into partial vapour pressure (pv), dew point temperature (tdp) and cooling effectiveness (ε).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Psychrometric Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with humidity ratio, saturated (ωs) set to 10, atmospheric pressure (pa) set to 5, saturated vapour pressure at tdb (ps) set to 2, the calculator returns partial vapour pressure (pv) of 99.97299, and altitude (z) of 23,051.52286, and relative humidity (φ) of 4,998.64964. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out partial vapour pressure (pv)?
Enter humidity ratio, saturated (ωs), atmospheric pressure (pa), saturated vapour pressure at tdb (ps), saturated pressure at wet bulb temp (pw), dry bulb temperature (tdb), temperature of moist, cooled air (tc) and wet bulb temperature (twb). The calculator then works through the relationship and reports partial vapour pressure (pv), dew point temperature (tdp) and cooling effectiveness (ε).
What does the psychrometric actually calculate?
It takes humidity ratio, saturated (ωs), atmospheric pressure (pa), saturated vapour pressure at tdb (ps), saturated pressure at wet bulb temp (pw), dry bulb temperature (tdb), temperature of moist, cooled air (tc) and wet bulb temperature (twb) and converts them into partial vapour pressure (pv), dew point temperature (tdp) and cooling effectiveness (ε) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter humidity ratio, saturated (ωs)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.