Calculate psychrometric properties from dry-bulb and wet-bulb temperatures. Find RH, humidity ratio, enthalpy, dew point, and air density for HVAC design.
The **Psychrometric Calculator** computes all moist air properties from dry-bulb and wet-bulb temperature readings — the classic measurement pair used in HVAC engineering and meteorology. From these two temperatures, you can determine relative humidity, humidity ratio, dew point, enthalpy, specific volume, and air density.
Psychrometrics is the science of air-water vapor mixtures. Every HVAC system — from residential air conditioners to clean rooms — is designed using psychrometric calculations. The dry-bulb temperature measures air temperature, while the wet-bulb temperature (from a thermometer wrapped in wet cloth with air flowing over it) captures the cooling effect of evaporation, which depends on humidity.
This calculator provides 8 psychrometric properties with comfort zone assessment, visual indicators, and a reference table spanning common temperature ranges. It accounts for barometric pressure variations for high-altitude locations. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case.
Psychrometric calculations are essential for HVAC system design, indoor air quality assessment, weather analysis, and any application involving moist air. This calculator replaces manual chart reading with accurate computed values. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
Psychrometric relationships: Pw = Pws(Twb) - 0.000662 × P × (Tdb - Twb) RH = (Pw / Pws(Tdb)) × 100 W = 0.622 × Pw / (P - Pw) h = 1.006 × Tdb + W × (2501 + 1.86 × Tdb) Where: Pws = saturation vapor pressure (Magnus formula)
Result: RH = 52.3%, W = 8.6 g/kg
With dry-bulb 22°C and wet-bulb 16°C: Pws(22) = 2.643 kPa, Pws(16) = 1.818 kPa. Pw = 1.818 - 0.000662 × 101.325 × 6 = 1.416 kPa. RH = 1.416/2.643 = 53.6%. This is comfortable — within the 30-60% RH range at a pleasant temperature.
Psychrometrics was formalized in the 19th century by Willis Carrier, who developed the first scientific psychrometric chart in 1904. His work enabled the design of air conditioning systems by providing a visual tool for tracking air state changes through heating, cooling, humidification, and dehumidification processes.
The fundamental property pair — dry-bulb and wet-bulb temperatures — was chosen because they are easy to measure with simple instruments. A sling psychrometer (two thermometers, one wet, whirled through the air) was the standard humidity measurement tool for over a century and remains valuable as a calibration reference.
**Cooling Coils:** When moist air passes over a cooling coil below its dew point, both temperature and moisture decrease. The total cooling load = mass flow × enthalpy difference. The sensible heat ratio (SHR) — the fraction of total cooling that is temperature reduction vs. dehumidification — determines coil design parameters.
**Humidification:** In cold weather, heated outdoor air has very low relative humidity (often below 15%). Humidifiers add moisture to reach comfortable levels. The humidity ratio difference between supply and return air, multiplied by air mass flow, gives the required humidification rate.
ASHRAE Standard 55 defines thermal comfort as a function of temperature, humidity, air speed, radiant temperature, clothing insulation, and metabolic rate. The psychrometric comfort zone (approximately 20-26°C, 30-60% RH) represents conditions acceptable to 80% of occupants. Below 30% RH, dry skin and respiratory irritation increase; above 60% RH, mold growth and discomfort from sweating become problems.
Dry-bulb is the air temperature measured by a standard thermometer. Wet-bulb is measured by a thermometer wrapped in wet muslin with air flowing over it — evaporative cooling lowers it below dry-bulb. The drier the air, the larger the difference.
No — wet-bulb temperature is always equal to or less than dry-bulb. They are equal only at 100% relative humidity (saturated air), when no evaporation can occur.
In comfortable indoor conditions (22°C, 50% RH), the humidity ratio is about 8 g/kg. In tropical climates it can exceed 20 g/kg, while in cold winter air it may be below 2 g/kg.
At higher altitudes, lower atmospheric pressure allows more evaporation, affecting wet-bulb depression and all derived properties. At 1,500 m elevation (~85 kPa), psychrometric calculations can differ by 10-15% from sea level.
Moist air enthalpy is the total heat content per kg of dry air, including both sensible heat (from temperature) and latent heat (from moisture). It is the key property for sizing cooling and heating coils.
The psychrometric chart is a graphical representation of these calculations. Each point on the chart represents a unique air state defined by any two independent properties (e.g., dry-bulb and RH).