You should clearly understand the definite relationships of the three temperatures-dry-bulb, wet-bulb, and dew point.

1. When air contains some moisture but is not saturated, the dew-point temperature is lower than

Figure 17-2.-Exposed view of a motorized psychrometer.

the dry-bulb temperature; the wet-bulb temperature lies between them.

2. As the amount of moisture in the air increases, the difference between the dry-bulb temperature and the wet-bulb temperature becomes less.

3. When the air is saturated, all three temperatures are the same.

4. By using both the wet-bulb and the dry-bulb temperature readings, you can determine the percent of relative humidity and the dew-point temperature by using a psychrometric chart. (See fig. 17-3.) The dry-bulb temperatures are equally spaced vertical lines. The wet-bulb temperature lines are angled across the chart. The dew-point temperature lines are horizontal straight lines across the chart, as are the lines labeled GRAINS OF MOISTURE PER POUND OF DRY AIR. Find where the wet-bulb and dry-bulb lines cross and interpolate the relative humidity from the nearest curved relative humidity lines to the temperature-line crossing point. Then find the dew point by following the straight dew-point line closest to the intersection across to the left of the chart. Now read the dew-point temperature.

We will use the following example to help you learn how to use a psychrometric chart to determine the percent of relative humidity and the dew-point temperature. Use the following conditions: the dry-bulb temperature is 95F and the wet-bulb temperature is 70F. Locate the line for 95F along the bottom of the chart (1). Next, locate the wet-bulb temperature of 70F on the curved line marked with values of temperature (2). Use the intersection of these two lines (3) to determine the percent of relative humidity. As shown in the figure, the point of intersection occurs between two curved lines labeled 20% and 30%. Since the point is closer to the 30% line, we can interpolate and estimate a value of about 28% relative humidity. To determine the dew-point temperature, follow over to the left in a straight line from the point of intersection. This line intersects the curved line labeled WET-BULB AND DEW-POINT TEMPERATURES at a point between 55F and 60F (4). Since the intersection is closer to 55F, we can interpolate again and estimate that the air must be cooled to a temperature of about 57F to cause the water vapor in the air to condense into a liquid (which will cause the air to dry out).

Figure 17-3.-Simplified psychrometric chart.