Dry-Bulb? Wet-Bulb? What's the Difference?
Although the acts of melting ice and boiling water are mundane occurrences, the underlying physics are not so simple. The illustration shows what happens to the temperature of water as heat is added (Btu per pound on the horizontal axis). The graph shows plateaus at 32ºF and 212ºF. Why does the temperature hold steady, even as heat is being added, right at water’s melting and boiling points?
The temperature holds steady because it takes so much energy to accomplish the phase changes. Only when the last molecule of water has changed phase does the temperature of the material rise. All the energy it takes to accomplish phase changes is hidden.
We call the energy expressed in the sloped sections of the graph sensible heat because a thermometer can “sense” this heat. We call the energy expressed in the plateaus latent heat because it is hidden from the thermometer. Look at how much energy is hidden as the latent heat of vaporization of water at 212°F: 970 Btu in just one pound of water—the difference between the heat energy in water at its boiling point (575 Btu/lb) and the amount needed to produce steam (1,545 Btu/lb).
This is how perspiration and other types of evaporative cooling work: because the phase change from liquid to vapor requires so much energy, the molecules must absorb that energy from the nearest heat source (when you’re exercising, that heat comes from you; in an evaporative swamp cooler, it’s from warm ambient air).
Measuring sensible and latent heat requires two different types of thermometer—dry-bulb and wet-bulb. Unlike a typical (dry-bulb) thermometer, which measures sensible air temperature, the wet-bulb thermometer is wrapped in a cotton wick; when the wick is completely wet, you can swing the thermometer around, and the water evaporating off the wick pulls the wet-bulb thermometer’s temperature down in direct proportion to the water content of the air around it. The drier the air, the more water evaporates off the wick and the lower the wet-bulb temperature goes; the more humid the air, the less water evaporates and the more similar the dry-bulb and wet-bulb temperatures are.
The wet-bulb thermometer measures the latent heat content of the moisture-laden air. But even more important is the difference between the two temperatures: that tells us the relative humidity—the moisture content of the air compared with how much moisture it can hold. Identical dry- and wet-bulb temperatures mean that the air is holding as much moisture as it possibly can—100% relative humidity.
The device that holds both thermometers is called a sling psychrometer; its measurements (and other elements of the science known as psychrometrics) can be expressed in the psychrometric chart—the topic of an upcoming BackPage Primer.