Psychrometric Chart

For the most common chart-reading task, begin with dry-bulb temperature and relative humidity.

1. Confirm pressure and units: Check whether the chart is for sea level or another pressure and whether it uses I-P or SI units.
2. Find dry-bulb temperature: Locate the temperature on the bottom scale and follow its constant-temperature line upward.
3. Find relative humidity: Identify the curved relative-humidity line for the known percentage.
4. Mark the state point: The intersection of the two lines represents the air condition.
5. Read the remaining properties: Follow the appropriate line or scale for humidity ratio, dew point, wet bulb, enthalpy, and specific volume.
6. Check the result: Enter the same known properties in the Psychrometric Calculator when you need a calculated value or want to verify your visual reading.

A psychrometric chart combines the thermodynamic properties of a dry-air and water-vapor mixture on one graph. The standard chart region is bounded by the saturation curve, a lower dry-air boundary, and the selected temperature and humidity-ratio limits.

Dry-Bulb Temperature

Dry-bulb temperature is the ordinary air temperature measured by a properly shielded thermometer. Values are shown along the lower horizontal scale. Constant dry-bulb lines extend upward through the chart and may be vertical or slightly inclined depending on the chart coordinate system.

Dry-bulb temperature primarily represents the sensible-temperature condition of the air. Moving right generally means warmer air; moving left generally means cooler air.

Humidity Ratio

Humidity ratio, commonly represented by \(W\), is the mass of water vapor per unit mass of dry air. It may be expressed as lb water/lb dry air, grains water/lb dry air, kg water/kg dry air, or g water/kg dry air.

Constant humidity-ratio lines run horizontally. The scale is normally printed on the right side of the chart. A higher state point has more water vapor per unit mass of dry air than a lower state point.

Relative Humidity

Relative humidity compares the actual water-vapor pressure with the saturation vapor pressure at the same dry-bulb temperature. Constant-relative-humidity lines curve upward across the chart and are labeled as percentages.

Relative humidity does not directly measure the mass of moisture in the air. If air is sensibly heated without adding moisture, humidity ratio remains constant while relative humidity decreases.

Saturation Curve

The upper-left curved boundary is the 100% relative-humidity line, also called the saturation curve. At saturation, dry-bulb temperature, thermodynamic wet-bulb temperature, and dew-point temperature are equal.

Ordinary unsaturated air states appear below and to the right of the saturation curve. A point plotted beyond the saturation boundary indicates an input, unit, pressure, or line-reading error for a single-phase moist-air condition.

Wet-Bulb Temperature

Thermodynamic wet-bulb temperature is associated with an ideal adiabatic saturation process. Constant wet-bulb lines slope diagonally downward from left to right and are typically read where they intersect a labeled scale or the saturation boundary.

Wet-bulb temperature is lower than or equal to dry-bulb temperature for unsaturated air. The difference between dry bulb and wet bulb is sometimes called wet-bulb depression.

Dew-Point Temperature

Dew point is the temperature at which the existing water-vapor content reaches saturation when the air is cooled at constant pressure without adding or removing moisture.

To find dew point, move horizontally left from the state point along the constant humidity-ratio line until it reaches the saturation curve. Read the temperature at that intersection.

Enthalpy

Moist-air enthalpy, represented by \(h\), combines sensible and latent energy per unit mass of dry air. Enthalpy lines slope diagonally across the chart and are read from the applicable enthalpy scale.

Wet-bulb and enthalpy lines may appear nearly parallel, but they are not exactly the same property and should not automatically be treated as identical. Follow the line family and scale identified by the chart publisher.

Specific Volume

Specific volume, represented by \(v\), is the volume occupied by the moist-air mixture per unit mass of dry air. Its diagonal lines are useful when converting between volumetric airflow and dry-air mass flow.

Specific volume is the reciprocal of dry-air-basis density when the definitions and units are consistent. It changes appreciably with temperature and barometric pressure.

Use this reference to determine which axis, curve, or diagonal line should be followed from a plotted state point.

Values between printed lines must be interpolated. The realistic precision is therefore limited by the chart size, line spacing, printing quality, and the reader’s ability to estimate between lines.

The best property pair depends on the measurements or design information available. The pair must independently define the moist-air condition at the selected pressure.

Dry Bulb and Relative Humidity

This is the most familiar combination for indoor sensors, weather observations, and general HVAC troubleshooting. Locate the dry-bulb line and the correct RH curve, then mark their intersection.

Dry Bulb and Wet Bulb

This pair is useful when working with sling-psychrometer or aspirated-psychrometer measurements. Confirm that the wet-bulb reading does not exceed the dry-bulb reading.

Dry Bulb and Dew Point

This combination is valuable for condensation analysis. The dew point defines the horizontal humidity-ratio level, while the dry bulb defines the temperature location.

Dry Bulb and Humidity Ratio

This pair is common in HVAC calculations and design documents. Follow the dry-bulb line upward and the humidity-ratio line horizontally to their intersection.

Properties That May Not Be Independent

Dew point and humidity ratio both describe the moisture content at a given pressure, so using only those two values may not independently establish dry-bulb temperature. Relative humidity alone also does not define the air state because the corresponding moisture content changes with temperature.

The chart and calculator solve related tasks, but they are most useful in different situations. The chart is best for understanding relationships and process direction. A calculator is better when exact values, pressure corrections, or repeated calculations are required.

Use the Chart When You Need to Visualize

• How air properties relate to one another
• Whether a process adds or removes sensible heat
• Whether moisture content increases or decreases
• How outdoor air and return air mix
• How cooling, dehumidification, reheat, or humidification changes the air state
• Whether a calculated result appears physically reasonable

Use the Calculator When You Need a Number

• Pressure- or altitude-adjusted psychrometric properties
• More precision than a printed chart can provide
• Fast evaluation of multiple operating conditions
• Reliable unit conversions
• A numerical check of a hand-plotted state point
• Values for a downstream load, airflow, or moisture calculation

An HVAC process is represented by a line connecting an entering-air state to a leaving-air state. Its direction shows the changes in dry-bulb temperature and humidity ratio.

Sensible Heating

Sensible heating moves to the right at approximately constant humidity ratio. Dry-bulb temperature and enthalpy increase, while relative humidity generally decreases because no water vapor is added.

Sensible Cooling Above the Dew Point

Sensible cooling moves to the left at approximately constant humidity ratio. Dry-bulb temperature decreases and relative humidity increases until the state approaches saturation.

Cooling and Dehumidification

If the cooling surface is below the entering-air dew point, water vapor condenses. The process moves down and left because both dry-bulb temperature and humidity ratio decrease.

The idealized extension of this process toward the saturation curve is associated with the apparatus dew point. The actual leaving-air state depends on coil surface temperature, airflow, heat-transfer area, bypass factor, and operating conditions.

Reheat

Reheat moves horizontally to the right after cooling and dehumidification. Dry-bulb temperature rises while humidity ratio remains approximately constant. This lowers relative humidity without removing additional moisture.

Steam Humidification

Steam humidification raises humidity ratio and normally adds some energy, so the state moves upward and often to the right. The exact direction depends on steam condition, injection method, absorption, and heat losses.

Direct Evaporative Cooling

Direct evaporative cooling generally moves up and left: dry-bulb temperature decreases while humidity ratio increases. Under ideal adiabatic conditions, the process approximately follows a constant wet-bulb or constant-enthalpy direction.

The directions above are idealized. Real equipment paths may deviate because heat and mass transfer are distributed through the device rather than occurring at one uniform condition.

Outdoor air and return air commonly mix before entering an air-handling unit. Plot both state points on the same pressure-specific chart and draw a straight line between them. For adiabatic mixing without condensation, the mixed-air state lies on that connecting line.

Locating the Mixed-Air State

The location depends on the dry-air mass-flow ratio. The mixed state lies closer to the air stream with the larger dry-air mass flow.

1. Plot the outdoor-air state.
2. Plot the return-air state.
3. Draw a straight line between the two points.
4. Determine the dry-air mass-flow fraction of each stream.
5. Locate the mixed-air point along the connecting line.
6. Read or calculate the mixed-air properties.

Simple Mixing Interpretation

If a system uses 20% outdoor air and 80% return air by dry-air mass, the mixed point will be much closer to the return-air state than the outdoor-air state. It will not normally sit at the geometric midpoint of the line.

Cooling-Coil Analysis

Plot entering and leaving air to separate sensible cooling from latent moisture removal. The humidity-ratio change indicates condensate removal, while the enthalpy change supports total cooling-load calculations.

Condensation and Dew-Point Checks

Compare air dew point with the temperature of ducts, pipes, diffusers, windows, roof decks, chilled-water components, or other surfaces. Condensation can occur when a surface is below the surrounding air’s dew point.

Use the Dew Point Calculator when the main task is comparing an air condition with a known surface temperature.

Ventilation and Outdoor-Air Loads

Plot outdoor, return, mixed, and supply-air states to see how ventilation affects sensible load, latent load, coil operation, and dehumidification requirements.

Humidity-Control Design

Humidity ratio shows the actual moisture content more directly than relative humidity. This distinction is important for humidifiers, dehumidifiers, drying systems, process spaces, and buildings with high latent loads.

Evaporative Cooling Systems

The chart shows the theoretical relationship between dry-bulb reduction and humidity-ratio increase. It also shows why evaporative cooling capacity is strongly influenced by the entering wet-bulb temperature.

Air-System Energy Analysis

Enthalpy differences can be combined with dry-air mass flow to estimate the total energy transferred to or from an air stream. Final calculations require consistent units and an appropriate mass-flow basis.

Use this checklist before accepting a chart result.

• Wrong chart pressure: A sea-level chart can produce misleading properties at higher elevations.
• Wrong unit scale: Confirm °F versus °C, Btu/lb versus kJ/kg, and lb/lb versus grains/lb or g/kg.
• Relative humidity confused with moisture content: RH can change even when humidity ratio remains constant.
• Dew point traced in the wrong direction: Move horizontally to the saturation curve, not diagonally.
• Wet bulb confused with enthalpy: The lines may be close, but they represent different properties.
• Specific volume confused with density: Specific volume is volume per unit dry-air mass.
• Dependent properties selected: Two values that describe the same moisture condition may not uniquely establish the full state.
• Impossible state accepted: Wet bulb or dew point above dry bulb usually indicates inconsistent inputs.
• Excessive precision reported: A hand-read chart value should not be reported with more precision than the chart supports.
• Ideal process assumed for real equipment: Coils, humidifiers, and evaporative devices have finite effectiveness and nonuniform operating conditions.

Seven-Point Chart QA Checklist

1. Pressure: Does the chart match the site pressure or altitude?
2. Units: Are all chart scales and input units understood?
3. Inputs: Are the two known properties independent?
4. Physics: Are the inputs physically consistent?
5. Line family: Did you follow the correct axis, curve, or diagonal?
6. Process direction: Does the movement match the expected heat and moisture transfer?
7. Precision: Is the result reported as an appropriate estimate?

When any of these checks are uncertain, repeat the condition in the Psychrometric Calculator before using the result in another engineering calculation.