The psychrometric calculator will assist you in estimating the properties of moist air. We can define humid or moist air conditions using the temperature and pressure parameters like wet bulb temperature, dry bulb, and dew point temperature. And for pressure, the atmospheric pressure, partial pressure of water vapor, and saturated vapor pressure. In addition, the specific humidity and relative humidity are also useful to convey the condition of air and weather. You can use the psychrometric chart courtesy of ASHRAE to determine the above properties.

ASHRAE or American Society of Heating, Refrigerating and Air-Conditioning Engineers is the association that issues guidelines and resources that contains equations and charts to calculate the parameters. The formulae are critical in different heating, ventilation, and air conditioning (HVAC) applications. This psychrometric calculator utilizes those equations to return the parameters of the air-vapor mixture. Read on to understand what is wet bulb temperature, how to calculate dew point temperature, and much more.

What is dry bulb temperature? What is wet bulb temperature?

Before we get into dry and wet bulb temperature, let's look into psychrometry. It is the science that covers the conditions of humid or moist air. The principles of psychrometry are the lynchpin in designing different industrial and residential applications such as ventilation in large buildings, mixing streams of air in air conditioning, industrial drying, and achieving human comfort using heating, cooling, or relative humidity. The atmosphere is a mixture of dry air and water vapor, often known as moist air.

The condition of moist air is defined by a set of pressure, temperature, and humidity parameters, that are:

  • Dry bulb temperature (Tdb);
  • Wet bulb temperature (Twb);
  • Dew point temperature (Tdp);
  • Atmospheric pressure (Pa);
  • Partial vapor pressure (Pv);
  • Saturated vapor pressure (Ps);
  • Relative humidity (Φ), and
  • Specific humidity (ω).

Wet bulb temperature definition: is the lowest temperature (Twb) to which air can be cooled by adding evaporating water into the air, considering constant pressure. Wondering how to calculate wet-bulb temperature? Take a wet wick and tie it around the bulb of the thermometer. The temperature shown by the thermometer is the wet-bulb temperature.

Dry bulb temperature definition: is the temperature (Tdb) measured by the thermometer at ambient pressure, i.e., the ambient temperature. The difference between dry-bulb temperature and wet bulb temperature is the humidity of the air. The humidity of the air will be lower if the difference between the two temperatures is significant.

Dew point temperature definition: the temperature (Tdp) above which the water vapor in the air condenses and turns into a liquid state. It is the temperature at which saturation occurs for any other substance without any change in vapor pressure. You can also perform the dew point temperature calculation using the equation below. For a given vapor pressure, Pv:

Tdp=243.5 lnPv6.11217.67lnPv6.112\qquad T_{dp} = \frac{243.5 \ \ln \frac{P_v}{6.112} }{17.67 - \ln \frac{P_v}{6.112}}

Atmospheric pressure (Pa): the pressure in the surrounding us or the pressure within the Earth's atmosphere. At mean sea level, the standard pressure is 101325 Pa or 1.013 bar. Otherwise known as ambient pressure, it along with density varies with the altitude or height (Z). The relationship between pressure, p and altitude, h with temperature T0 is given by the equation:

P (h)=P0(1L hT0)g MR0 L\qquad P \ (h) = P_0 (1 - \frac{L \ h}{T_0})^{\frac{g \ M}{R_0 \ L}}

where gg, MM, R0R_0, and LL are constant parameters. Such that:

  • Gravitational acceleration, g=9.8 m/s2g = 9.8 \ \text{m/s}^2;
  • Temperature lapse rate, L=0.00976 K/mL = 0.00976 \ \text{K/m};
  • Molar mass of dry air, M=0.028969 kg/molM = 0.028969 \ \text{kg/mol}; and
  • Universal gas constant, R0=8.314 J/(molK)R_0 = 8.314 \ \text{J/(mol} \cdot \text{K)}.
Variation of pressure with respect to altitude.
Variation of pressure with respect to altitude.

Partial pressure of water vapor (Pv): Since moist air is a mixture of dry air and water vapor, the water vapor contributes to the pressure component of moist air, known as partial pressure or partial pressure of water vapor. You can obtain it by multiplying the vapor pressure of the pure component by the mole fraction.

Saturated vapor pressure (Ps): The pressure of water vapor at which water vapor and ice coexist. In other words, the pressure at saturation point. The saturation point or volume is the point when the
air is saturated with respect to water vapor, i.e., the maximum possible amount of water vapor in the mixture for a given temperature.

The dry bulb and wet bulb temperatures are related to the ambient pressure using the formula:

Pv=Pw0.00066 Pa (TdbTwb)(1+0.0115 Twb)\tiny P_v = P_w - 0.00066 \ P_a \ (T_{db} - T_{wb}) ( 1+ 0.0115 \ T_{wb})

where PwP_w is the saturated vapor pressure at wet-bulb temperature.

Types of humidity – How to calculate specific humidity?

What is specific humidity and humidity ratio?
The specific humidity (ω\omega) is the ratio of the mass of water vapor to the total mass of the moist air sample. Mathematically, the specific humidity is measured in kg of water vapor/kg of moist air. The definition of specific humidity is often confused with the humidity ratio. The humidity ratio is ratio of water vapor in moist air to the mass of dry air. The formula for humidity ratio in terms of pressures is:

ω=0.6219PaPaPv\qquad \omega = 0.6219 \frac{P_a}{P_a - P_v}

Relative humidity definition: It measures the humidity ratio compared to the maximum humidity at a given temperature. In other words, it is the ratio of water vapor pressure and saturated vapor pressure at a given dry bulb temperature. So what does relative humidity indicate? It compares the amount of water vapor present in the air to the maximum amount it can hold at a given temperature. This parameter is measured as a percentage (%). The relative humidity equation in terms of vapor pressure and saturated vapor pressure is:

Φ=PvPs\qquad \Phi = \frac{P_v}{P_s}

What is degree of saturation (μ)?: It is the ratio of humidity ratio at a point to the humidity ratio at saturation point for a given dry-bulb temperature. This parameter is deemed obsolete and rarely used to define the condition of air. The equation provides the degree of saturation formula:

μ=ωωS\qquad \mu = \frac{\omega}{\omega_S}

Specific volume and enthalpy

Specific volume: The specific volume of air, s, is the reciprocal of density. It is measured in the units of m3/kg\text{m}^3/\text{kg}.

Enthalpy: The enthalpy, h, of moist air can be determined using the equation:

h=1.006Tdb+ω(2499.86+1.84Tdb)\footnotesize h = 1.006 \, T_{db} + \omega (2499.86 + 1.84 \, T_{db})

How to use the psychrometric calculator?

You can use this psychrometric chart calculator to calculate several parameters used to define the condition of moist air. To define the condition of moist air:

  1. Enter the altitude or height, Z.
  2. The calculator will return the atmospheric pressure, Pa.
  3. Enter the dry bulb or ambient temperature, Ta.
  4. Insert the relative humidity, Φ.
  5. Fill in the humidity ratio, ω.
  6. The calculator, using the above equations, will estimate:
    a. Saturated vapor pressure, Ps;
    b. Partial vapor pressure, Pv;
    c. Dew point temperature, DPT;
    d. Specific volume, s; and
    e. Enthalpy of moist air, h.
  7. Enter the wet-bulb temperature, Tw to find saturated pressure at wet bulb temp (Pw).

Example: Using the psychrometric calculator

Find the condition of the moist air at an altitude of 100 m using the following properties.

  • Ambient temperature, Tdb = 25°C;
  • Wet bulb temperature, Twb = 20°C;
  • Relative humidity, Φ = 63.61%; and
  • Humidity ratio, ω = 0.01278 kg/kg of dry air.

To find the partial vapor pressure:

  1. Enter the altitude or height, Z = 100 m.
  2. Enter the dry bulb or ambient temperature, Tdb = 25° C.
  3. Insert the relative humidity, Φ = 63.61%.
  4. Fill in the humidity ratio, ω = 0.01278 kg/kg of dry air.
  5. The partial vapor pressure is:
Pv=Pa (ω0.6221)=20.16 mbar\scriptsize \qquad P_v = P_a \ (\frac{\omega}{0.622} - 1) = 20.16 \ \text{mbar}
  1. Similarly, the saturated vapor pressure is estimated by psychrometric calculator as 31.69 mbar.
  1. Using the dew point temperature formula:
Tdp=243.5 ln20.166.11217.67ln20.166.112=17.638°C\scriptsize \qquad T_{dp} = \frac{243.5 \ \ln \frac{20.16}{6.112} }{17.67 - \ln \frac{20.16}{6.112}} = 17.638 °C

Alternatively, you can use this psychrometric calculator for dry bulb temperature, degree of saturation, and cooling effectiveness estimation. This tool can also work as a humidity ratio calculator if you have other parameters.

ASHRAE Psychrometric chart – How to find relative humidity using psychrometric charts?

A psychrometric chart is an alternative to this calculator to determine the properties and state of moist air at a point or for a process. ASHRAE publishes a psychrometric chart to assist HVAC design engineers. Such charts can contain up to 9 different parameters.

However, you can use the three independent properties to find out the rest — barometric or ambient pressure, dry bulb temperature, and either wet bulb temperature, relative humidity, or dew point temperature. The chart contains several curves and isolines based on which the properties of moist air are determined. A sample chart with the labels is shown below:

Psychrometry chart
Psychrometry chart

Now consider a point a on the chart. Trace the point along the respective isolines to obtain the properties.

FAQ

What is dew point temperature?

The dew point is the temperature at which the vapor in the air condenses into water or the temperature at which substances get saturated.

How to find dew point temperature on a psychrometric chart?

To find dew point temperature on a psychrometric chart:

  1. Mark the state point using any of the independent properties.
  2. Draw a horizontal line from the point to the saturation curve.
  3. The point where the line intersects the saturation curve is the dew point temperature.

What does relative humidity mean?

The relative humidity is the property of air that tells us how much water vapor is present in moist air with respect to the maximum amount it can hold.

How to measure relative humidity?

The relative humidity is measured using a sling psychrometer. The device consists of two thermometers – one measures dry bulb temperature, whereas the other has a wet wick on its bulb to measure the wet-bulb temperature. Note the temperature difference between the reading of two thermometers is then used to obtain relative humidity from a chart.

What is specific humidity formula?

The specific humidity formula is:

  • SH = Mwv / Mma

where Mwv and Mma are mass of water vapor and moist air, respectively.

Rahul Dhari
Altitude & pressure
Altitude (Z)
ft
Atmospheric pressure (Pa)
in Hg
Saturated vapour pressure at Tdb (Ps)
in Hg
Partial vapour pressure (Pv)
in Hg
Saturated pressure at wet bulb temp (Pw)
in Hg
Temperature
Dry bulb temperature (Tdb)
°F
Wet bulb temperature (Twb)
°F
Dew point temperature (Tdp)
°F
Temperature of moist, cooled air (Tc)
°F
Humidity
Relative humidity (Φ)
%
Humidity ratio (ω)
kg
/kg
Humidity ratio, saturated (ωs)
kg
/kg
Degree of saturation (μ)
Properties
Specific volume (s)
1/
kg/m³
Enthalpy of moist air (h)
kJ
/kg
Cooling effectiveness (ε)
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