Psychrometrics: The Science of Gas-Vapor Mixtures

Psychrometrics (also known as psychrometry or hygrometry) is the branch of engineering focused on the physical and thermodynamic properties of gas-vapor mixtures. Derived from the Greek words psuchron (cold) and metron (means of measurement), this field is essential for managing environments where air and water vapor interact.

The discipline evolved during the sixteenth and seventeenth centuries following the invention of the thermometer and hygrometer. In 1818, German inventor Ernst Ferdinand August patented the term "psychrometer," an instrument based on the principle that dry air accelerates evaporation while wet air slows it down.

Key Facts

Core Focus: The study of thermodynamic properties of air-water vapor mixtures.
Primary Tool: The psychrometric chart, pioneered by Willis Carrier in 1904.
Critical Application: Controlling mold and fungi by keeping relative humidity below 75%.
Measurement: Uses dry-bulb and wet-bulb temperatures to determine relative humidity.
Alternative Mapping: The Mollier diagram is a common alternative to the ASHRAE-style chart in many European countries.

Fundamental Psychrometric Properties

Temperature Measurements

In psychrometrics, temperature is categorized into three distinct types to describe the state of the air:

Dry-Bulb Temperature (DBT): The temperature measured by a standard thermometer sheltered from direct solar radiation. In most contexts, "temperature" refers to DBT.
Wet-Bulb Temperature (WBT): A thermodynamic property representing the temperature of a mixture of air and water vapor. It is measured using a thermometer with a sensing bulb covered in a wet sock. Accuracy depends on airflow; speeds up to 5,000 ft/min are ideal. Airflow at 1–2 m/s is called screen temperature, while 3.5 m/s or more is sling temperature.
Dew Point Temperature (DPT): The saturation temperature at which water vapor begins to condense into liquid (dew) or solid (hoarfrost). This temperature marks the base of clouds in the atmosphere.

Humidity and Energy Metrics

Humidity is measured in several ways depending on the required precision and application:

Relative Humidity (RH): The ratio (expressed as a percentage) of current atmospheric moisture to the amount required for saturation at the same temperature and pressure.
Specific Humidity: The mass of water vapor relative to the total mass of the moist air sample.
Absolute Humidity: Also known as water vapor density, this is the mass of water vapor per unit mass of dry air.
Humidity Ratio: The proportion of water vapor mass per unit mass of dry air, often plotted as the vertical axis on charts.
Specific Enthalpy: The total energy (heat content) of both dry air and water vapor per kilogram of dry air.
Specific Volume: The total volume of the mixture per unit mass of dry air.

The Psychrometric Chart

A psychrometric chart is a graphical equation of state that depicts thermodynamic parameters of moist air at a constant pressure. For altitudes up to 2,000 ft (600 m), a sea-level chart is typically used.

A psychrometric chart for sea-level elevation

Locating Parameters on the Chart

The chart allows engineers to determine all air parameters if any three independent parameters (including pressure) are known:

Dry-Bulb Temperature: Plotted on the horizontal axis (abscissa) as slightly inclined vertical lines.
Humidity Ratio: Plotted on the vertical axis (ordinate) as horizontal lines.
Relative Humidity: Represented by hyperbolic lines; the 100% RH line is the saturation curve.
Wet-Bulb Temperature: Oblique lines that intersect the saturation curve.
Specific Enthalpy: Diagonal lines sloping downward from left to right.
Specific Volume: A family of nearly parallel, equally spaced straight lines.

Practical Examples

To determine relative humidity, find the intersection of the vertical dry-bulb line and the diagonal wet-bulb line. For example, a dry bulb of 25°C and a wet bulb of 20°C results in a relative humidity of approximately 63.5%.

To calculate the amount of water to be added or removed to change humidity, engineers find the difference in the humidity ratio between the initial and final states and multiply it by the weight of the dry air.

The Mollier Diagram

Developed by Richard Mollier in 1923, the Mollier (Enthalpy–Humidity Mixing Ratio) diagram is an alternative to the ASHRAE chart. While it looks different, it uses the same underlying data. If an ASHRAE chart is rotated 90 degrees and mirrored, it resembles a Mollier diagram.

The Mollier diagram is widely preferred in Russia, Scandinavia, and several Central and Western European countries, including Germany, France, and the Netherlands.

Summary of Psychrometric Terms

Comparison of Key Psychrometric Properties
Property	Definition	Common Unit (SI)
Dry-Bulb Temp	Ambient air temperature	°C / K
Wet-Bulb Temp	Temperature after adiabatic saturation	°C / K
Dew Point	Temperature where condensation begins	°C / K
Relative Humidity	Ratio of current moisture to saturation	%
Humidity Ratio	Mass of vapor per mass of dry air	kgw/kga
Specific Enthalpy	Total heat energy of the mixture	J/kg

Frequently Asked Questions

What is the difference between a sling psychrometer and a powered psychrometer?

A sling psychrometer requires a person to manually whirl the device to create airflow over the wet bulb, whereas a powered psychrometer uses a built-in fan to achieve the necessary air velocity.

How does relative humidity affect mold growth?

Relative humidity is a critical factor in biological control; wood-destroying fungi generally cannot grow when relative humidity is maintained below 75%.

What is the psychrometric ratio?

The psychrometric ratio is a dimensionless property that relates absolute humidity and saturation humidity to the difference between the dry-bulb temperature and the adiabatic saturation temperature.

Why is air pressure important in psychrometrics?

Many psychrometric properties, including vapor pressure and the relationships plotted on psychrometric charts, are dependent on the atmospheric pressure at the sample's location.

What happens when air reaches its dew point?

When air reaches its dew point, it becomes saturated. Any further removal of heat causes the water vapor to condense into liquid water (fog) or, if the temperature is below freezing, into solid hoarfrost.