A psychrometric chart maps every important property of moist air onto a single diagram, letting you find temperature, humidity, dew point, and energy content all at once. The chart looks intimidating at first because it layers seven or more variables on top of each other, but every reading starts the same way: find two properties you already know, locate where their lines intersect, and read everything else from that single point. Most standard charts are drawn for sea-level atmospheric pressure (14.7 psi or 101.3 kPa), so if you’re working at a significantly higher elevation, you’ll need an altitude-corrected version.
The Two Main Axes
The horizontal axis (x-axis) shows dry-bulb temperature, which is simply the air temperature you’d read on a regular thermometer. Vertical lines run straight up from each temperature value. On most charts used in the U.S., this scale runs from about 20°F to 120°F. Metric charts typically span roughly −10°C to 55°C.
The vertical axis (y-axis) shows the humidity ratio, sometimes labeled “moisture content.” This tells you the actual weight of water vapor mixed into each pound (or kilogram) of dry air. On an imperial chart, the units are pounds of moisture per pound of dry air, and the lines of constant humidity ratio run horizontally from left to right. A typical indoor reading might sit around 0.0104 lb of moisture per lb of dry air. This axis gives you an absolute measure of moisture, unlike relative humidity, which changes with temperature.
The Saturation Curve
The curved line forming the upper-left boundary of the chart is the saturation curve, representing air at 100% relative humidity. At any point along this curve, the air holds the maximum amount of water vapor it can at that temperature. Air conditions to the right and below the curve are unsaturated (relative humidity below 100%). You cannot plot a point above or to the left of the curve because that would represent air holding more moisture than is physically possible at that temperature.
On the saturation curve, three temperatures converge into one: dry-bulb temperature, wet-bulb temperature, and dew point temperature all equal each other. As you move away from the curve toward drier conditions (lower right), those three values spread apart.
The Seven Properties You Can Read
Dry-Bulb Temperature
Read directly from the bottom axis. Follow a vertical line up from any temperature value. Every point along that vertical line shares the same dry-bulb temperature regardless of how much moisture the air contains.
Humidity Ratio
Read from the right-hand vertical axis. Follow a horizontal line from any point to the scale on the right. This value stays constant as long as no moisture is added to or removed from the air.
Relative Humidity
The gently curving lines that sweep across the chart from lower left to upper right represent constant relative humidity. The saturation curve itself is the 100% line. Below it you’ll see curves labeled 90%, 80%, 70%, and so on down to 10%. Relative humidity tells you how close the air is to being fully saturated at its current temperature. The same amount of moisture in cooler air produces a higher relative humidity than in warmer air, which is why these curves aren’t straight.
Wet-Bulb Temperature
Wet-bulb lines run diagonally from the upper left toward the lower right. Their values are marked along the saturation curve. To read wet-bulb temperature for any state point, follow the diagonal line that passes through your point back up to the saturation curve and read the temperature there. Wet-bulb temperature reflects how much the air can be cooled by evaporation, which is why it’s always equal to or lower than the dry-bulb reading.
Dew Point Temperature
From your state point, draw a horizontal line to the left until it hits the saturation curve. The temperature at that intersection is the dew point. This makes intuitive sense: if you cool air without changing its moisture content (a horizontal move to the left), the dew point is the temperature at which the air becomes fully saturated and condensation begins.
Enthalpy
Enthalpy represents the total energy content of the air-water mixture per unit of dry air, measured in BTU/lb or kJ/kg. On most charts, enthalpy values are printed along the saturation curve or on a scale just outside it. The lines of constant enthalpy run almost parallel to wet-bulb lines, slanting from upper left to lower right. On some charts they’re treated as identical to wet-bulb lines for simplicity, but on more detailed ASHRAE charts, enthalpy lines diverge slightly from wet-bulb lines at higher moisture levels. Enthalpy is what HVAC engineers use to calculate heating and cooling loads.
Specific Volume
Lines of constant specific volume are the steeply angled lines slanting from lower left to upper right. Specific volume tells you how much space a pound of dry air occupies (in cubic feet per pound, or cubic meters per kilogram). Warmer, more humid air takes up more volume, so specific volume increases as you move to the right or upward on the chart. This is the inverse of density: higher specific volume means lighter air.
How to Plot a State Point
You need exactly two independent properties to pin down a state point. “Independent” means they define different aspects of the air. Dry-bulb temperature and relative humidity are the most common pair because they’re easy to measure. Dry-bulb and wet-bulb is another standard combination. Once you know any two, every other property is fixed.
Here’s the process. Say you measure 75°F dry-bulb and 50% relative humidity. Find 75°F on the bottom axis and trace the vertical line upward. Then find the 50% relative humidity curve sweeping across the chart. Where the vertical 75°F line crosses the 50% curve is your state point. From that point, you can now read horizontally right to get the humidity ratio (roughly 0.0093 lb/lb in this example), horizontally left to the saturation curve to get the dew point (about 55°F), and diagonally up-left along the wet-bulb line to get the wet-bulb temperature (about 63°F).
Reading Air Processes on the Chart
The real power of the psychrometric chart is tracking what happens when you heat, cool, humidify, or dehumidify air. Every HVAC process traces a specific path across the chart.
Horizontal movement to the right represents sensible heating: the air gets warmer, but its moisture content doesn’t change. The humidity ratio stays the same, but relative humidity drops because warmer air can hold more moisture. A furnace or electric heater creates this kind of movement. Horizontal movement to the left is sensible cooling: temperature drops while moisture stays constant, and relative humidity rises.
Vertical movement upward represents adding moisture without changing the temperature, which is a purely latent process. A steam humidifier does this. Vertical movement downward means removing moisture at a constant temperature.
Diagonal movement combines both. This is what most real-world processes look like. For example, when air passes over a cold cooling coil, it moves diagonally down and to the left because both temperature and moisture content decrease. If the coil is cold enough to bring the air below its dew point, water condenses out of the air. That’s how air conditioning dehumidifies: it cools the air past the saturation curve, condensation drains away, and then the now-drier air can be reheated. On the chart, this shows up as a move to the saturation curve (cooling and dehumidifying), then a horizontal move to the right (reheating at constant moisture).
Using the Sensible Heat Ratio
Many printed psychrometric charts include a small protractor-shaped scale in the upper left corner. This is the sensible heat ratio (SHR) scale, and it helps you draw the correct process line when you know how much of your total heating or cooling load is sensible versus latent.
The sensible heat ratio is calculated as sensible heat divided by total heat (sensible plus latent). An SHR of 1.0 means the process is entirely sensible, a purely horizontal line. An SHR of 0.5 means half the energy change is temperature and half is moisture. To use the protractor, find your SHR value on the scale, draw a line from the reference point on the protractor through that value to establish the slope, then draw a parallel line through your starting state point. The resulting line shows the path the air will follow through the process, letting you find the end conditions.
Common Mistakes to Avoid
The most frequent error is confusing humidity ratio with relative humidity. Humidity ratio is an absolute measurement that doesn’t change with temperature. Relative humidity is a percentage that shifts every time the temperature changes, even if no moisture is added or removed. When you heat air in winter and notice it feels dry, the humidity ratio hasn’t changed, but the relative humidity has dropped because the warmer air could hold more.
Another common mistake is using a sea-level chart for a building at high altitude. Standard psychrometric charts assume atmospheric pressure of 29.92 inches of mercury. At 5,000 feet elevation, air pressure is roughly 12% lower, which shifts every property on the chart. If you’re working at altitude, use a chart corrected for your local pressure or an online psychrometric calculator where you can input the actual value.
Finally, watch the scale increments carefully. Humidity ratio values are small numbers (often in the range of 0.005 to 0.020 lb/lb), and misreading by one grid line can throw off your calculations significantly. Use a straightedge when tracing horizontal or vertical lines to your state point.