Psychrometric Chart (Interactive + Printable)

A psychrometric chart is a single graph that ties together every physical property of moist air — temperature, relative humidity, moisture content, dew point, wet-bulb temperature and heat content — so that any two of them locate a point that gives you all the rest. It replaces half a dozen separate formulas with one picture.

The horizontal axis is dry-bulb temperature: what an ordinary thermometer reads. The vertical axis is how much water vapor the air is carrying, shown here in grains per pound of dry air. Everything else on the chart is a family of lines connecting points that share some other property. Because the air's state is completely defined by two independent values plus barometric pressure, one dot on the chart is a complete description of the air in a room.

For restoration that matters because your meter gives you temperature and relative humidity, but your decisions depend on grains, dew point and how far the air is from saturation. The chart converts one into the other at a glance, and it shows you the direction a process is pushing the air rather than just a pair of before-and-after numbers.

The chart on this page is drawn from the ASHRAE Hyland-Wexler equations at sea-level pressure. It renders immediately with three default points already plotted, so nothing has to load before you can read it.

Locate the dry-bulb temperature on the bottom axis, go straight up until you meet the relative humidity curve you measured, and that intersection is your air. Everything else is read from that one point.

1. Take a temperature and relative humidity reading with a thermo-hygrometer, in the middle of the space and away from equipment discharge.
2. Type both into one of the point boxes under the chart above. The point appears with a horizontal and a vertical crosshair.
3. Moisture content: follow the horizontal crosshair to the right-hand axis. That number is grains per pound. The readout under the point box prints it too.
4. Dew point: follow that same horizontal line left until it meets the heavy saturation curve, then drop straight down to the temperature axis. The temperature you land on is the dew point.
5. Wet-bulb: follow the fine dotted lines that slope up and to the left from your point until they reach the saturation curve; the temperature there is the wet-bulb.
6. Enthalpy: the dashed amber lines running roughly parallel to the wet-bulb lines are constant total heat, in Btu per pound of dry air.
7. Hover anywhere on the chart to read the temperature, grains and relative humidity at the cursor without plotting a point at all.

• Vertical lines — dry-bulb temperature. Straight up and down, one per labelled temperature. Moving along a vertical line changes temperature without changing moisture content, which is exactly what a heater does.
• Horizontal lines — humidity ratio. Grains per pound, or grams per kilogram with the toggle on. Moving along a horizontal line changes temperature at constant moisture; moving vertically means water was added or removed.
• Curved lines — relative humidity. Drawn every 10%. They fan upward to the right because warmer air can hold more vapor, so the same grains represent a lower percentage as temperature rises.
• The heavy curve — saturation, 100% relative humidity. Air cannot sit above this line. The temperature directly below the point where your horizontal line meets this curve is the dew point, which is why dew point has no separate family of lines.
• Fine dotted lines every 5°F — wet-bulb temperature. They start on the saturation curve and slope down to the right. Along one of these lines, evaporation and cooling trade off exactly: water added, heat removed, total heat nearly unchanged.
• Dashed amber lines — enthalpy. Total heat per pound of dry air, sensible plus latent. Nearly parallel to the wet-bulb lines, which is why the two are often confused.
• Specific-volume lines are deliberately omitted from this chart. They matter for airflow and duct engineering, and leaving them out keeps the lines a restorer actually reads legible on a phone screen.

On a water loss the chart earns its keep by holding four readings side by side: outside air, unaffected interior air, the affected chamber, and the dehumidifier exhaust. The four presets above are named for exactly those readings, each in its own colour.

The drying gap is the vertical distance between the unaffected point and the affected point. Both sit somewhere on the chart; the one that is higher is holding more water. A chamber plotted level with or above the unaffected reading is not a chamber yet. Select those two points in the process-line control and the chart prints the grain difference on the line itself.

Grain depression is the same measurement applied to a single machine: plot the intake and the exhaust, and the vertical drop between them is what the dehumidifier removed from the air it processed. The exhaust point of a working refrigerant unit lands to the right and below the intake point — warmer, drier, because condensing water releases heat into the airstream.

Outside air is worth plotting even when you never intend to open a window, because it tells you what is leaking in. If outdoor air sits well above your chamber on the chart, every gap in the containment is working against the equipment, and a rising chamber point overnight usually means infiltration rather than material releasing water.

No target numbers are printed here for any of these gaps. What the chart gives you is direction and magnitude — and a record you can put in a file. For how those readings feed into an actual drying plan, see structural drying and the S500 overview.

• Sensible heating. A horizontal move to the right. Temperature climbs, grains stay put, relative humidity falls. Heat alone never dries a building; it only raises the air's appetite for water.
• Cooling to condensation. A horizontal move to the left until you hit the saturation curve, then the point slides down along the curve as water condenses out. This is what happens on a cold windowpane and inside an air conditioner's coil.
• Refrigerant dehumidification. Left and down to the coil condition, then right along a lower horizontal line as the dried air is reheated by the condenser. The net move is right and down: warmer, drier.
• Desiccant dehumidification. Up and to the right along a line of roughly constant enthalpy. The desiccant adsorbs vapor and releases the heat that vapor was carrying, so process air leaves noticeably hotter and much drier without ever reaching dew point. That is why desiccants keep working in cold air, where a refrigerant coil would frost.
• Evaporation from wet materials. Up and to the left along a wet-bulb line. The air gains grains and loses temperature, because the heat driving the evaporation comes out of the air itself. This is the process you are trying to cause — and the reason chamber air cools when air movers first go in.

Use "Download chart (PNG)" for a two-times-resolution image, or "Print chart" for a one-page landscape copy. Both include whatever points you have plotted, so you can print the chart with the day's readings already marked and drop it straight into the job file.

The printed version drops the input boxes and buttons and prints the graph alone with the site address in the footer. A blank printable chart is easy to make too: untick every point before printing.

One caveat worth writing on the copy you carry: this chart is drawn at sea-level barometric pressure. At altitude the same temperature and relative humidity correspond to more grains than a sea-level chart shows. The GPP calculator takes an elevation input if you need the corrected figure.

If you are studying rather than working, print one and mark it up by hand. Reading dew point, wet-bulb and grains straight off the chart is the fastest route through the psychrometry questions on the IICRC WRT certification exam.