How does ambient pressure affect absolute humidity at saturation?

As ambient total pressure decreases (such as at higher altitudes or under vacuum), the saturation absolute humidity increases significantly because the denominator (P - P_sat) shrinks.

How is specific heat of humid air calculated?

The humid heat capacity Cp' is computed as Cp' = Cp_air + w * Cp_vapor, where Cp_air is approximately 1.005 kJ/kg.K, Cp_vapor is approximately 1.884 kJ/kg.K, and w is the absolute humidity in kg water / kg dry air.

Humid Air : Absolute air humidity definition and calculation

Q: What is the key difference between absolute humidity and relative humidity?

Absolute humidity is the actual mass ratio of water vapor to dry air (e.g., kg water per kg dry air) and remains constant regardless of temperature changes unless moisture is added or removed. Relative humidity (%) is the ratio of actual water vapor partial pressure to saturation vapor pressure at a given temperature.

Definition of absolute humid air and associated air psychrometric calculations

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1. Definition of air absolute humidity
2. Calculation of air absolute humidity
3. Absolute humidity at saturation
4. Interactive Online Absolute Humidity Calculator
5. Engineering Rules of Thumb & Design Limits
6. Air Humidity measurement
7. Absolute air humidity calculator Excel

1. Definition of air absolute humidity

What is absolute air humidity ?

The absolute humidity is defined as the mass of water in the air per mass of dry air. From a given state, if there is no water / air transfer, the absolute humidity is not changing. The absolute air humidity is independent of the temperature unless condensation or moisture addition occurs.

In process engineering, understanding and controlling absolute air humidity can be very important as it directly impacts the performances of the process but also of the final product in many industries such as drying, HVAC, food manufacturing, and pharmaceutical processing.

Absolute humidity vs relative humidity : What is the difference between absolute humidity and relative humidity ?

Air absolute humidity should not be confused with air relative humidity, which is representing a ratio of the partial pressure of water at the given condition vs the partial pressure of water at saturation.

2. Calculation of air absolute humidity

How to calculate air absolute humidity ?

Absolute humidity formula :

\(\omega = \frac{m_w}{m_a}\) = weight of water / weight of dry air

With :
\(\omega\) = Absolute humidity of air (kg water / kg dry air)
\(m_w\) = weight of water (kg water)
\(m_a\) = weight of dry air (kg dry air)

The absolute air humidity can also be calculated from the molar fraction of water in the gas.

\omega = \frac{M_{\text{water}}}{M_{\text{air}}} \cdot \frac{y}{1 - y}

With :
\(M_{\text{water}}\) = molar weight of water = 18 g/mol
\(M_{\text{air}}\) = molar weight of air = 29 g/mol
\(y\) = molar ratio of water in air = \(P_{\text{water}} / P_{\text{total}}\)
\(P_{\text{water}}\) = partial pressure of water

If y is small, which is quite often the case, then the absolute humidity can be approximated easily :

\omega = \frac{M_{\text{water}}}{M_{\text{air}}} \cdot y

3. Absolute humidity at saturation

How to calculate absolute air humidity at saturation ?

When the water partial pressure in the air reaches the saturation pressure at the corresponding temperature, then the air is saturated - it cannot accommodate more water.

\omega_{\text{sat}} = \frac{M_{\text{water}}}{M_{\text{air}}} \cdot \frac{P_{\text{water}}^{\text{sat}}}{P - P_{\text{water}}^{\text{sat}}}

With :
\(\omega_{\text{sat}}\) = Absolute humidity of air at saturation at temperature T (-)
\(P_{\text{water}}^{\text{sat}}\) = saturation pressure of water at temperature T (Pa)
\(P\) = total pressure (Pa)

4. Interactive Absolute Air Humidity Calculator

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Unit System: SI Metric (°C, Pa) US / Imperial (°F, psi)

Calculation Input Mode:

Air Temperature (°C):

Total System Pressure (Pa):

Water Partial Pressure (Pa):

Relative Humidity RH (%):

Calculation Results

Absolute Air Humidity (\(\omega\)): 0.009775 kg water / kg dry air

Saturation Absolute Humidity (\(\omega_{\text{sat}}\)): 0.019870 kg water / kg dry air

Humidity Percentage (\(\omega / \omega_{\text{sat}}\)): 49.2 %

Relative Humidity (\(\text{RH}\)): 50.0 %

Water Vapor Saturation Pressure (\(P_{\text{sat}}\)): 3143 Pa

Water Partial Pressure (\(P_{\text{water}}\)): 1571 Pa

Specific Volume (\(v'\)): 0.8572 m³ / kg dry air

Humid Specific Heat (\(C_p'\)): 1023.4 J / (kg dry air · K)

💡 Industrial Plant Rules of Thumb & Design Guidance

Condensation Prevention in Ducts: Keep duct air temperatures at least \(3\text{--}5^\circ\text{C}\) above the dew point temperature to prevent condensation, internal corrosion, and biological growth.
Pneumatic Conveying Limits: For dry powder pneumatic conveying, compressed air absolute humidity should be controlled below \(0.001\text{ kg water/kg dry air}\) (dew point \(< -40^\circ\text{C}\)) to prevent powder caking and pipe blockage.
HVAC Comfort & Process Range: Standard industrial comfort systems target \(40\text{--}60\%\) RH. Process drying loops typically operate with inlet absolute humidity \(< 0.008\text{ kg/kg}\) and outlet exhaust \(> 0.025\text{ kg/kg}\) for high thermal efficiency.
Altitude Correction: Standard psychrometric charts assume sea-level pressure (\(101.325\text{ kPa}\)). For high-elevation plants (e.g., \(1500\text{ m}\) altitude, \(\sim 85\text{ kPa}\)), saturation absolute humidity increases by \(\approx 20\%\); always compute at actual operating pressure.

6. Air Humidity measurement

It is very important to be able to measure (automatically if possible) air humidity. The following instruments can be used for this purpose :

Hygrometer : A hygrometer measures humidity in the air. It uses methods like hair tension or electrical resistance to provide accurate humidity readings. For example, a hair tension hygrometer has a hair strand that expands or contracts with humidity changes. This movement is then translated into a humidity value. Electrical resistance hygrometers rely on moisture-induced changes in electrical conductivity to determine humidity levels.
Psychrometer : A psychrometer assesses relative humidity. It works based on evaporative cooling, where water evaporation causes air temperature to drop. A typical psychrometer has two thermometers: a dry bulb thermometer for air temperature and a wet bulb thermometer covered in a wet wick. As the water evaporates from the wick, it cools the wet bulb thermometer. By comparing the temperatures recorded by the dry and wet bulb thermometers, engineers can calculate relative humidity. Larger temperature differences indicate drier air, while smaller differences mean higher humidity.

7. Absolute air humidity calculator Excel

MyEngineeringTools.com has developed a free Excel calculator that allows you to calculate the absolute humidity of air, knowing the pressure and the partial pressure of water, it allows also many other calculations related to humid air : Humid air enthalpy calculator Excel (click here)

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.