Psychrometric Calculator

About the Psychrometric Calculator

The **Psychrometric Calculator** computes moist-air properties from dry-bulb and wet-bulb temperature readings, the common pair used in HVAC engineering and weather work. From those two temperatures you can derive relative humidity, humidity ratio, dew point, enthalpy, specific volume, and air density.

Psychrometrics is the study of air-water-vapor mixtures. Dry-bulb temperature is the normal air temperature, while wet-bulb temperature reflects how much evaporation can cool a wetted thermometer. The gap between the two is a practical indicator of humidity.

It gives eight psychrometric properties, a comfort assessment, and pressure adjustment for higher-altitude locations so you can compare conditions without reading a full psychrometric chart.

When This Page Helps

Moist-air calculations show up anywhere you need to balance temperature and humidity at the same time, especially in HVAC design, indoor-air-quality work, and weather analysis.

Putting the core properties on one page removes the guesswork from chart reading and makes it easier to compare two air states directly.

How to Use the Inputs

1. Enter the dry-bulb temperature (standard air temperature).
2. Enter the wet-bulb temperature (always equal to or lower than dry-bulb).
3. Select temperature unit (°C or °F).
4. Adjust barometric pressure for non-standard altitudes.
5. Use presets for common weather conditions.
6. Check the comfort zone visualization for indoor climate assessment.
7. Review the reference table for properties at different conditions.

Example Calculation

Tips & Best Practices

• Wet-bulb depression (Tdb - Twb) is a quick indicator of humidity — larger gap means drier air.
• Comfort zone: 18-27°C dry-bulb, 30-60% RH for most building codes.
• For cooling coil sizing, use enthalpy difference between entering and leaving air states.
• At high altitudes, adjust barometric pressure — Denver at 1,609 m uses ~83.4 kPa.
• Dew point below the surface temperature of walls or windows means condensation risk.
• Data centers typically target 18-27°C, 20-80% RH per ASHRAE TC 9.9 guidelines.

Psychrometric Science

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.

HVAC Design Applications

**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.

Human Comfort and Health

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.

Frequently Asked Questions

• What is the difference between dry-bulb and wet-bulb temperature?

  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.

• Can wet-bulb temperature exceed dry-bulb?

  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.

• What humidity ratio values are typical?

  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.

• Why does barometric pressure matter?

  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.

• What is enthalpy in psychrometrics?

  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.

• How does this relate to the psychrometric chart?

  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).