HVAC Reference Library · Wall Charts

How to Read a Psychrometric Chart

Where to find dry bulb, wet bulb, dew point, relative humidity, humidity ratio and enthalpy on a psychrometric chart, and which way common HVAC processes move across it.

Ref HV-0052ASHRAE Handbook—Fundamentals

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A psychrometric chart plots the properties of moist air on one page. If you know any two properties of an air sample, you can find the point on the chart and read all the others. The chart is drawn for one barometric pressure, usually sea level. Use an altitude chart or software at high elevation. The data behind the chart is published in the ASHRAE Handbook—Fundamentals.

How to read it

Measure two properties, most often dry bulb and wet bulb with a psychrometer, or dry bulb and relative humidity.
Find the dry bulb line on the bottom scale and follow it straight up.
Find the wet bulb value on the curved saturation line and follow its diagonal line down and to the right.
Where the two lines cross is the state point. Read the other properties from that point.

Chart lines

Property	How the lines run	Scale location
Dry bulb temperature	Vertical lines	Bottom axis
Wet bulb temperature	Diagonal lines sloping down to the right	Read on the saturation curve
Dew point temperature	Horizontal lines	Read where the line meets the saturation curve
Relative humidity	Curved lines; 100% RH is the saturation curve	Labeled on the curves
Humidity ratio (moisture content)	Horizontal lines	Right axis, in grains or lb of water per lb of dry air
Enthalpy	Diagonal lines close to the wet bulb lines	Separate scale along the upper left edge
Specific volume	Steep diagonal lines	Labeled along the lines, in cubic feet per lb of dry air

There are 7,000 grains in one pound.

Common processes

Sensible heating: moves straight right. Dew point and moisture stay the same; RH falls.
Sensible cooling: moves straight left until it reaches the dew point.
Cooling and dehumidifying: moves down and to the left, as at a wet coil.
Humidifying with steam: moves mostly straight up.
Evaporative cooling: moves up and to the left along a wet bulb line.

Key points

At saturation (100% RH), dry bulb, wet bulb and dew point are equal.
The change in enthalpy between two points, times the air mass flow, gives the total heat added or removed.
Wet bulb readings need good airflow over a clean, wet wick.

Common mistakes

Using a sea-level chart at high altitude without correction.
Reading the enthalpy scale off the wet bulb line without using a straightedge.
Confusing dew point (moisture content) with wet bulb (evaporative limit).

Governing standards: ASHRAE Handbook—Fundamentals (psychrometrics); chart publisher's notes on pressure and units.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

More in Wall Charts

← All HVAC sheets