How to Read a Psychrometer

A psychrometer's chart.

A psychrometer, sometimes called a "wet bulb/dry bulb thermometer," is just that. It has two thermometers, one red and one blue. They may read in the Celsius scale or in degrees Fahrenheit. One thermometer, usually containing red liquid, is called the dry bulb. It measure the ambient air temperature. The other thermometer, which contains blue liquid, is called the wet bulb, and is separated from the dry bulb by a small chart. The wet bulb and the chart, when used with the dry bulb, make it possible to determine the relative humidity.

Instructions

1. Look at the dry bulb thermometer and record the temperature. This is the actual temperature of the air, not how hot or cold it feels.

2. Look at the wet bulb thermometer. Its temperature will be lower than the dry bulb temperature unless it's raining. Record the temperature displayed.

3. 

   Look at the chart. Find the temperature you recorded from the dry bulb on the chart. The dry bulb temperatures are on the the horizontal axis, the bottom edge of the chart.

4. 

   Find the temperature you recorded from the wet bulb on the chart. The wet bulb temperatures are along the outermost curved edge of the chart.

5. 

   Draw a line straight up from the dry bulb temperature and a line straight across from the wet bulb temperature. The point where the two lines intersect is the relative humidity.

Tips & Warnings

• If your thermometer reads in Celsius and you want to convert to Fahrenheit, take the Celsius temperature, add 40 to it, multiply by 1.8, then subtract 40. If it reads in Fahrenheit and you want to convert to Celsius, add 40, divide by 1.8 (instead of multiplying) and subtract 40.

References

• National Snow and Ice Data Center: Psychrometer

Resources

• All About Relative Humidity