How to Use Phycrometric Charts

Investigate the graph components of the psychrometric chart piece by piece. Look at the axes on the graph. Find the dry-bulb temperature scale on the horizontal axis, or x-axis. Find the humidity ratio scale (or moisture content) on the vertical axis, or y-axis. Look for the curved line that represents the 100% moisture saturation or the moisture holding capacity. The chart also shows other important moist air properties such as wet-bulb temperature, enthalpy (sensible and latent energy), dewpoint or saturation temperature, relative humidity and specific volume.

Use a sling psychrometer to obtain a dry-bulb and wet-bulb temperature of your sample. Locate the intersection of the dry-bulb and the wet-bulb temperatures on the psychrometric chart. First look for the dry-bulb temperature along the x-axis and find the line the runs for the dry-bulb temperature that runs vertically through the chart. The wet-bulb temperature is the dotted line leading to the curved boundary saturation line.

Indicate the intersection reading for the vertical dry-bulb temperature line and the diagonal wet-bulb temperature line. This is known as the state point for the air temperature measured. The state point for the air temperature can tell you the content of water vapor and the enthalpy in the air.

Determine the relative humidity through the labeled relative humidity line. The line should curve from left to right along the chart. In addition, find the dewpoint temperature on the indicated line on the chart. This line should also run from left to right up through the chart towards the saturation boundary line.

Evaluate the air properties obtained from the psychrometric chart and take any necessary actions to find the environmental solutions needed.