Nobody wants to be uncomfortable in their own home. If you are a homeowner having comfort issues then understanding different temperature types will help you diagnose the issue. Dry bulb temperature and wet bulb temperature are two essential measurements of thermal comfort.
What is the difference between dry bulb and wet bulb temperature? Dry Bulb temperature is simply air temperature. Wet bulb temperature is air temperature that takes into consideration evaporation.
You are probably asking “what do bulbs have to do with temperature?” To answer that question let’s first discuss how thermometers work.
We are all familiar with the glass thermometers from science class. On one end of the device is a glass chamber filled with a liquid (usually an alcohol solution or mercury). This is the bulb of the thermometer.
Dry Bulb Temperature (DBT)
Even if you have never heard the term dry bulb temperature you likely use them every day. Whenever you discuss the temperature outside you use a DBT.
Dry bulb temperature is simply the atmospheric temperature measured using a thermometer that is dry.
More accurately, it is the air temperature where the thermometer is shielded from direct radiation (such as sunlight) and moisture. This is important because irradiation can alter the reading. Thus, no longer representing the temperature of the air.
The dry bulb temperature is a measurement of the heat content in the air. It is also an important part of psychrometrics. More on that in a bit.
Wet Bulb Temperature (WBT)
Wet bulb temperature is measured using a thermometer with a wet cloth wrapped around the bulb. The temperature reading will drop due to the evaporative cooling.
Wet bulb temperature is the lowest temperature that air can reach by evaporating water into the air.
Don’t get me wrong, dry bulb temperature is important. However, if you like to exercise at home then it doesn’t tell the whole story.
Your body relies on evaporative cooling via sweat to cool itself. At 90°F WBT your body can no longer cool itself. Even worse, it is deadly to remain in an environment with a 95°F WBT for too long. Your body will heat up with no way to cool itself down. In other words, you will not be able to acclimate to the heat.
It is important to note that this isn’t just dangerous for humans, but animals as well.
How to use Wet Bulb Temperature
I live in Colorado. The relative humidity here is between 30-40% year-round. In other words, it is dry. Therefore, when you get out of the shower the drop in temperature is extremely noticeable.
How cold? Let’s find out!
You can use the relative humidity and dry bulb temperature to find the wet bulb temperature on a psychrometric chart. First, find the line where the dry bulb temperature intersects with the relative humidity line. Second, follow the diagonal line up to the wet bulb temperature. You will notice that at 100% relative humidity the dry bulb and wet bulb temperatures have the same value.
Remember, at 100% relative humidity the air can not absorb more water. Since no evaporative cooling can take place, wet bulb temperature will be equal to dry bulb temperature. In other words, the drier the air the more evaporative cooling can occur and the lower the wet bulb temperature.
Now that we know how to use the chart, let’s get back to the shower problem. Let’s assume that I set my thermostat to 70°F DBT. At 30% RH the air will feel like it is 53°F. That’s down right chilly!
So what do we do? Let’s install a whole home humidifier and set it to 50% RH. Now when I step out of the shower the air feels like it is 58°F. That’s a 5 degree improvement!
Final Thoughts
You can think of DBT as the air temperature and WBT as the apparent temperature when sweating. It’s important to consider both when setting your thermostat setting for maximum comfort. Especially in dry environments.
Your thermostat will only ever give you a dry bulb temperature measurement. If you live in a dry area then understanding how to adjust for wet bulb temperature can help keep you comfortable.
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Anthony Marini, PE is a Mechanical Engineer licensed in the state of Colorado. He graduated from Colorado School of Mines in 2012 and has been designing residential HVAC systems ever since. He has designed the mechanical systems for tens of thousands of homes all over the continental United States.