Psychrometrics is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. Most commonly, it refers to the study of moist air (a mixture of dry air and water vapor). A psychrometric chart is a graphical representation of these properties at a constant pressure.
Key Terms Explained
Dry Bulb Temperature: The temperature of air measured by a standard thermometer freely exposed to the air but shielded from radiation and moisture.
Relative Humidity (RH): The ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature).
Dew Point: The temperature at which water vapor in the air begins to condense into liquid water (dew).
Humidity Ratio (Specific Humidity): The mass of water vapor present in a unit mass of dry air. Usually expressed in grams of water per kilogram of dry air (g/kg).
Enthalpy: The total heat content of the moist air, including the sensible heat of the air and the latent heat of the water vapor.
Why Use a Psychrometric Calculator?
While the physical chart is great for visualization, digital calculators provide precision required for HVAC (Heating, Ventilation, and Air Conditioning) design, meteorological analysis, and industrial drying processes. Calculating the dew point is particularly critical for preventing mold growth or condensation on building surfaces.
Example Calculation
If you have an indoor office environment with a Dry Bulb Temperature of 22°C and a Relative Humidity of 50% at standard sea-level pressure (1013.25 hPa):
Property
Result
Dew Point
11.12°C
Humidity Ratio
8.28 g/kg
Enthalpy
43.15 kJ/kg
This means that if any surface in the room is colder than 11.12°C, condensation will likely form on that surface.
function calculatePsychrometrics() {
var T = parseFloat(document.getElementById("dryBulbTemp").value);
var rh = parseFloat(document.getElementById("relHumidity").value);
var P = parseFloat(document.getElementById("atmPressure").value);
if (isNaN(T) || isNaN(rh) || isNaN(P) || rh 100 || P <= 0) {
alert("Please enter valid positive numerical values. Relative humidity must be between 0 and 100.");
return;
}
// Saturation Vapor Pressure (Magnus formula)
// es = 6.112 * exp((17.67 * T) / (T + 243.5)) in hPa
var es = 6.112 * Math.exp((17.67 * T) / (T + 243.5));
// Actual Vapor Pressure
var e = (rh / 100) * es;
// Dew Point Temperature
// Tdp = (243.5 * ln(e/6.112)) / (17.67 – ln(e/6.112))
var lnE = Math.log(e / 6.112);
var Tdp = (243.5 * lnE) / (17.67 – lnE);
// Humidity Ratio (w) in kg/kg
// w = 0.62198 * e / (P – e)
var w = 0.62198 * e / (P – e);
var wGrams = w * 1000; // convert to g/kg
// Specific Enthalpy (h) in kJ/kg
// h = 1.006 * T + w * (2501 + 1.86 * T)
var h = 1.006 * T + w * (2501 + 1.86 * T);
// Specific Volume (v) in m3/kg
// v = Ra * T_kelvin / (P – e)
// Ra = 287.058 J/(kg·K)
var T_kelvin = T + 273.15;
var v = (287.058 * T_kelvin) / ((P – e) * 100);
// Update Display
document.getElementById("resDewPoint").innerHTML = Tdp.toFixed(2);
document.getElementById("resHumRatio").innerHTML = wGrams.toFixed(2);
document.getElementById("resEnthalpy").innerHTML = h.toFixed(2);
document.getElementById("resVapPress").innerHTML = e.toFixed(2);
document.getElementById("resVolume").innerHTML = v.toFixed(3);
document.getElementById("resSatPress").innerHTML = es.toFixed(2);
document.getElementById("psych-results").style.display = "block";
}