Psychrometric charts are complex graphs that can be used to assess the physical and thermodynamic properties of gas-vapour mixtures at a constant pressure. Where these rows meet, you will see the dew-point. How do you find the dew point of a wet and dry bulb?įind the dry-bulb temperature on the left side of the “Dew-point Temperatures” ESRT, then find the difference between the wet and dry-bulbs, also known as the wet-bulb depression, on the top of the chart. … The dew point will be the lowest number, and the wet bulb will fall between those two.

Mike Moss: In a saturated air parcel (with relative humidity 100%) the wet bulb, dry bulb and dew point temperatures are all the same. How do you read dew point charts? Is dew point the same as wet bulb? Given the same amount of water vapor in the air, the absolute humidity does not change with the temperature at a fixed volume. It is calculated by dividing the total mass of water vapor by the volume of the air.

Use the horizontal lines extending from this axis to trace the humidity ratio throughout the chart. The vertical, or “Y” axis of the chart marks the amount of moisture per pound or kilogram. How do you find the humidity ratio on a psychrometric chart?įind the humidity ratio labeled on the right vertical axis. The intersection of the vertical 78☏ dry-bulb line and the diagonal 65☏ wet-bulb line has now established a state point for the measured air. Wet-bulb temperature is located along diagonal dotted lines leading to scale readings at the upper, curved boundary marked saturation temperature. How do you find the wet bulb temperature from a psychrometric chart? The dew point is measured vertically and usually measured in degrees Fahrenheit or degrees Celsius, on this chart it is measured in Fahrenheit. The dew point is represented on the chart by Point 3. For this specific graph, it is measured in inches of mercury. Adapted from Moore, 1993.Vapor pressure is measured in units of mercury. HVAC systems usually change both the sensible and latent heat of an outdoor air stream by running it through hot and cold (and sometimes additional hot) coils to alter the sensible heat - and subsequently the humidity-of the air.įigure 6: Mechanical cooling typically involves cooling air below the dew point until the desired humidity ratio is achieved, and then reheating the air to achieved the desired dry-bulb temperature. However, this is representative of the typical mechanical air-conditioning process. This may seem like a roundabout and inefficient process.

As the the air-vapor mix is reheated, the condition moves horizontally to the right on the chart ( Figure 6). In order to lower the relative humidity, or to bring the conditions into the human thermal comfort zone, sensible heat will need to be added while the moisture content (latent heat) is maintained. This is the dew point-the critical temperature at which condensation occurs as the dry-bulb temperature of an air-vapor mix is lowered.Īfter being cooled beyond the dew point temperature, the humidity ratio of the air-vapor mix will be reduced and the air will remain at 100 percent relative humidity. water) starts to form on the cool window surface. As the temperature of the air adjacent to this surface is pulled beyond the saturation line, its limit for holding moisture is reached and 'dew' (i.e. An example of this condensation occurrence would be when hot-humid indoor air comes into contact with a cool window surface during the winter.

Dew point psychrometric chart