Calibration

• 1). Read your hygrometer's documentation for instructions on how to adjust its settings and how to read it. If your hygrometer can't be adjusted, you will still be able to calibrate it.
  
  
• 2). Put the tap water and salt into the coffee cup.
  
  
• 3). Stir the mixture with a spoon for several seconds.
  
  
• 4). Put both the coffee cup and your hygrometer into the plastic bag. Avoid spilling any saltwater on the hygrometer.
  
  
• 5). Seal the bag. Place the bag and its contents in a draft-free area that is out of direct sunlight.
  
  
• 6). Check your hygrometer's relative humidity reading after 8 to 12 hours. It should read about 75 percent.
  
  
• 7). Adjust your hygrometer so that it reads 75 percent, if it can be adjusted. If it can't be adjusted, record the difference in its reading. Whenever you take a reading, you will have to adjust for this difference.
  
  For example, if your hygrometer reads 70 percent instead of 75 percent, you know that your hygrometer gives a reading 5 percent lower than the actual relative humidity, so add 5 percent to any future readings.
  
  

Testing Relative Humidity

• 1). Place your hygrometer in the area you plan to paint in. Don't set your hygrometer in front of heat registers, chimneys, or any other location where it could be directly exposed to heat.
  
  
• 2). Wait two hours to get an accurate reading. Most hygrometers take some time to adjust to new locations.
  
  
• 3). Check your hygrometer's reading, and compare it to the threshold for the type of paint you're using.