So you want to measure the humidity and dew point so you can trump the Meteorologists? This is good, but before you get started there are a few basic concepts you should understand. Don’t worry, you don’t need to take notes and you can even skip this section if you like! Jump to step two and get to work. Just be sure to follow the pictures I've included and to read the captions! This step is for the nerds (like myself) who are curious about the theory behind the madness.
To measure the humidity you are going to take two readings from a thermometer and record them on a piece of paper. This is referred to as the data collection, and I have split it into two parts.
Part 1: The first reading is called the “dry bulb” temperature. Let's just say that Scientists like to call it “dry bulb” because it unnecessarily complicates the reading of an ordinary thermometer.
Part 2: The second reading is referred to as the “wet bulb” temperature. You’re going to have to get a little frisky to obtain this reading... you're going to use an improvised sling psychrometer. A sling psychrometer is a fancy name for some string attached to a small base which can hold and secure a wetted thermometer. The string is used to spin the base and thermometer around at high speeds, above someone’s head. Yes, you read that correctly. You’re going to spin a thermometer around your head.
How It Works:
The difference between the dry and wet bulb temperatures is the key to finding the humidity. The dry bulb temperature is the ordinary air temperature and will be used as a standard to compare the wet bulb temperature reading to. The wet bulb temperature is lower than the dry bulb temperature because it is taken using a thermometer that is wet with water. The wetted thermometer is spun around to cause the water to evaporate. This evaporation occurs because energy is pulled out of the bulb and into the water by the convective air current caused by spinning the thermometer around. Transferred energy is used by the water to change phases from the liquid phase to the gas phase. The loss of energy in the thermometer is relative to the thermometer's temperature reading in addition to the water’s heat of vaporization; the amount of water that evaporates depends on how much water is in the air (aka the air's humidity).
What does this mean? If the air is very humid less water will evaporate from the thermometer because there is already a lot of water in the air. This means the wet bulb temperature will be closer to the dry bulb temperature.
If the air isn’t very humid then more water will evaporate off of the thermometer into the air. This will cause greater loss of energy from the thermometer and a much lower temperature reading with respect to the dry bulb temperature. Thus, the difference between the dry and wet bulb temperature readings is directly proportional to how humid the air is!
The Psychrometric Chart is a product of experimental results at varying temperatures and a constant pressure that summarizes the correlations of changing wet bulb temperatures to dry bulb temperatures as a function of humidity. In other words, a bunch of scientists performed this experiment hundreds of times at different temperatures and levels of humidity; this graph is the result of their work. This means that all we have to do is look at the chart and trace a line or two to find our humidity and dew point. Thank you, Carrier Corporation!
It should be noted that this method will not work when the temperature is below freezing. In addition, there is also a case when this method can give unreliable results. If you happen to live in the mountains or a high elevation city the values you find using the psychrometric chart will be less accurate. This is because the psychrometric chart was made from data taken at atmospheric pressure, so the chart is the most effective at sea-level elevations. However, there are charts made from data taken at higher elevations, so those can be used as a substitute for the one I provide in step eight.
Now that the basics of finding the humidity and dew point have been covered it's time to collect some materials scattered throughout the house. On to step two!