What is a Psychrometric Calculator used for?

It is used by HVAC professionals to determine the thermodynamic properties of moist air. By inputting two known values (usually Dry Bulb and Wet Bulb), you can calculate all other properties like Enthalpy, Dew Point, and Humidity Ratio, which are essential for sizing equipment and diagnosing issues.

What is Enthalpy in HVAC?

Enthalpy is the total heat content of the air, measured in BTUs per pound of dry air. It includes both Sensible Heat (temperature) and Latent Heat (moisture). Calculating the change in enthalpy across a cooling coil tells you the total cooling capacity being delivered.

What are Grains of Moisture?

'Grains' is a unit of weight used to measure the amount of water vapor in the air. There are 7,000 grains in one pound of water. Humidity Ratio is often expressed in Grains per Pound (GPP) of dry air. This is crucial for dehumidification calculations.

How does altitude affect psychrometrics?

As altitude increases, atmospheric pressure decreases. This changes the density of the air and its ability to hold moisture. Standard psychrometric charts are at sea level (14.696 psi). At high altitudes (e.g., Denver), you must use corrected values or a calculator like this one.

Dew Point is the temperature at which moisture in the air begins to condense into liquid water. If a surface (like a window or duct) is cooler than the air's dew point, condensation will form on it.

💧 Psychrometric Calculator

Calculate Enthalpy, Dew Point, and Air Properties

Dry Bulb Temperature (°F)

Second Parameter

Wet Bulb (°F)

Altitude (ft)

Affects atmospheric pressure

📊 Understanding Psychrometrics

Psychrometrics is the study of the thermodynamic properties of moist air. By knowing just two properties (like Dry Bulb temperature and Wet Bulb temperature), you can determine the "state point" of the air and calculate all other properties.

Key Terms Defined

Dry Bulb (DB): The air temperature measured by a standard thermometer.
Wet Bulb (WB): The temperature measured by a thermometer with a wet wick. It reflects the cooling effect of evaporation.
Dew Point (DP): The temperature at which moisture begins to condense.
Relative Humidity (RH): The percentage of moisture in the air relative to the maximum amount it can hold at that temperature.
Enthalpy (h): The total energy (heat) in the air, including both sensible heat (temperature) and latent heat (moisture).
Humidity Ratio (W): The actual weight of water vapor in the air, usually measured in Grains per Pound (GPP).

💡 From David Park, HVAC Engineer (14 years):

"When diagnosing a cooling issue, don't just look at the temperature split (Delta T). Look at the Enthalpy change! A system might have a low temperature drop (e.g., 15°F) but be doing a massive amount of dehumidification (latent cooling). Calculating the change in enthalpy across the coil gives you the TRUE capacity of the unit."

⚠️ Common Applications

1. Dehumidification: To dry out a room, you must lower the air temperature below its Dew Point. The Humidity Ratio (Grains) tells you exactly how much water is being removed.

2. Mixing Air: When mixing return air with fresh outdoor air, the resulting condition lies on a straight line between the two points on the psychrometric chart. This calculator helps you predict that mixed air temperature.

3. Comfort Cooling: Human comfort relies on both temperature and humidity. 75°F at 40% RH feels very different from 75°F at 80% RH (which feels like 83°F!).

Reviewed by David Park

Professional Engineer (PE), HVAC Systems Designer, 14 years experience.

Air State Properties

📊 Understanding Psychrometrics

Key Terms Defined

⚠️ Common Applications

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