Psychrometric Chart

A full chart at sea level in °F or °C, drawn from the ASHRAE Handbook equations. Plot a reading on it, print it on one page, or read on for how to use it.

How to read it ↓

Psychrometric chart at sea level

12.513.013.514.014.515.010%20%30%40%50%60%70%80%90%3035404550556065707580859015202530354045505560103035404550556065707580859095100105110115120102030405060708090100110120130140150160170180190200Dry bulb temperature, °FHumidity ratio, gr/lb of dry airEnthalpy, Btu/lb of dry airSea level, 14.696 psiaSaturation, 100% RHRelative humidity, %Wet bulb, °FEnthalpy, Btu/lb of dry airSpecific volume, ft³/lb of dry air
  • Saturation, 100% RH
  • Relative humidity, %
  • Wet bulb, °F
  • Enthalpy, Btu/lb of dry air
  • Specific volume, ft³/lb of dry air
0.780.800.820.840.860.880.900.920.9410%20%30%40%50%60%70%80%90%02468101214161820222426283032342030405060708090100110120100246810121416182022242628303234363840424446485024681012141618202224262830Dry bulb temperature, °CHumidity ratio, g/kg of dry airEnthalpy, kJ/kg of dry airSea level, 101.325 kPaSaturation, 100% RHRelative humidity, %Wet bulb, °CEnthalpy, kJ/kg of dry airSpecific volume, m³/kg of dry air
  • Saturation, 100% RH
  • Relative humidity, %
  • Wet bulb, °C
  • Enthalpy, kJ/kg of dry air
  • Specific volume, m³/kg of dry air

The chart is wider than your screen. Swipe it sideways to see all of it.

Standard sea level pressure, 14.696 psia (101.325 kPa). Dry bulb along the bottom, humidity ratio up the right side, and enthalpy on the scale past the saturation curve.

From pockethvac.com/psychrometric-chart/. Drawn from the ASHRAE Handbook equations (Fundamentals, 2017, chapter 1) at standard sea level pressure.

How to read a psychrometric chart

Every point on the chart is one state of the air. Two readings put you on the point, and the lines through it give you the rest. The curved edge at the top left is the saturation curve, 100% relative humidity. On it, dry bulb, wet bulb and dew point are all the same temperature, and that's where you read wet bulb and dew point.

1. Find the point

Find the dry bulb along the bottom and go straight up until you reach the relative humidity curve for your reading. With a wet bulb reading instead, stop where you cross that wet bulb line. With a dew point, stop where you cross the level line that starts at the dew point on the saturation curve.

2. Read the rest

  • Wet bulb: follow the blue wet bulb line up and to the left to the saturation curve, and read the temperature there.
  • Dew point: go straight left to the saturation curve and read the temperature there.
  • Humidity ratio: go straight right to the scale on the right side, in grains of moisture per pound of dry air (grams per kilogram in metric).
  • Enthalpy: follow the dashed enthalpy lines up and to the left, past the saturation curve, to the enthalpy scale. They run close to the wet bulb lines, but they're not the same lines.
  • Specific volume: read between the steep green lines, in cubic feet per pound of dry air (cubic metres per kilogram).
  • Relative humidity: if you started from a wet bulb or dew point, read the curve the point sits on, or between.

Worked example: 75°F and 50% RH

Find 75°F along the bottom and go straight up to the 50% curve. That's the point. From there you read:

Wet bulb
62.6°F, a little past halfway from the 60 line to the 65 line
Dew point
55.1°F
Humidity ratio
64.6 gr/lb
Enthalpy
28.1 Btu/lb of dry air
Specific volume
13.68 ft³/lb of dry air

On the °C chart, the nearest round reading, 24°C and 50% RH, gives 17.1°C wet bulb, 12.9°C dew point, 9.30 g/kg, 47.8 kJ/kg and 0.854 m³/kg. Type either one into the boxes under the chart to see the guide lines. Start from the wet bulb or dew point instead of the RH and the other numbers can be off in the last digit, since the readings above are rounded. The psychrometric calculator works out the same numbers without reading lines, at any elevation.

How the chart is drawn

The chart is drawn when the site is built, from the same ASHRAE Handbook equations as the calculators (Fundamentals, 2017, chapter 1), at standard sea level pressure. Nothing is traced from another chart. Its saturation curve is within 0.6% of ASHRAE's Table 2 for saturated air, which adds real-gas corrections the equations leave out.

Above sea level the air is thinner, so the same dry bulb and RH hold more moisture per pound of dry air and take up more room. At 5,000 ft, 75°F and 50% RH is 77.9 gr/lb, 30.2 Btu/lb and 16.49 ft³/lb with a 61.8°F wet bulb, against 64.6 gr/lb, 28.1 Btu/lb, 13.68 ft³/lb and 62.6°F at sea level. Only the dew point stays the same. Above sea level, use the psychrometric calculator, which takes your elevation.

Saturation pressure: ASHRAE eqns 5 (over ice) and 6 (over water)
Humidity ratio W = 0.621945 × pw ÷ (p − pw)
Wet bulb: ASHRAE eqns 33 and 35, solved by bisection
Dew point: eqns 5 and 6 solved for the temperature where pws = pw
Enthalpy h = 0.240 t + W (1061 + 0.444 t) Btu/lb, or 1.006 t + W (2501 + 1.86 t) kJ/kg
Specific volume v = 0.370486 (t + 459.67)(1 + 1.607858 W) ÷ p ft³/lb, or 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p m³/kg with p in kPa
Sea level: p = 14.696 psia (101.325 kPa)

Sources

Updated