Wet Bulb Dew Point Chart

The dry bulb wet bulb and dew point temperatures are important to determine the state of humid air.

Wet bulb dew point chart.

Higher the mositure content of the air higher is the dew point temperature. Lines of constant wet bulb temperatures run diagonally from the upper left to the lower right in the psychrometric chart. By definition wet bulb temperature is the lowest temperature a portion of air can acquire by evaporative cooling only. The dew point temperature of air depends upon its mositure content.

Combining the dry bulb and wet bulb temperature in a psychrometric diagram or mollier chart gives the state of the humid air. At lower humidity the wet bulb temperature is lower than dry bulb temperature because of evaporative cooling. The knowledge of only two of these values is enough to determine the state of the moist air including the content of water vapor and the sensible and latent energy enthalpy in the air. Enter the two different temperatures.

This script will determine the relative humidity from the entered dry and wet bulb temperatures data. Dew point is expressed as a temperature. Dew point temperature tdp. On the right side of the chart locate the vertical line labeled dew point.

For the saturated air the dry bulb temperature the wet bulb temperature and dew point temperature are same. At 100 relative humidity the wet bulb temperature is equal to the air temperature dry bulb temperature. The key difference between dewpoint and wet bulb temperature is that dewpoint temperature is the temperature to which we should cool the air to saturate the air with water vapor whereas wet bulb temperature is the temperature that we get from a moistened thermometer bulb that is exposed to air flow. Therefore the change on the chart is down and to the right.

The wet bulb temperature wbt is the temperature read by a thermometer covered in water soaked cloth wet bulb thermometer over which air is passed. Looking at the psychrometric chart we see the dew point is 55 f. The dew point lines traverse the chart as horizontal lines. The coil of the air conditioner is at 45 f so the system will condense water from the air.

What this tells you is that dew point does not change as the dry bulb temperature varies. Dew point and wet bulb temperatures are very important in indicating the state of the humid air. If the supply air leaving the system is at 55 f dry bulb and 53 f wet bulb we see from the chart that the dew point is 51 5 f.