Ambient Weather Support

Testing The Accuracy Of A Hygrometer - Saturated Salt Test

The saturated salt solution test is an easy method for testing the accuracy of your humidity sensor. Most electronic sensors are accurate from +/- 3 to 5% humidity, and some weather stations or weather centers can be calibrated from the display console by applying a bias.

  1. Mix common table salt with tap water until you have a completely saturated solution with no standing water.
  2. Place about 1 inch of the saturated salt solution in an open container or bowl that is approximately 4 inches high. The 3 inches above the solution will contain air with a predictable humidity value. You can use a cut away container as shown in the figure below. One edge of the container is cut slightly lower to rest the sensor on the lip.
  3. Place this container and the sensors in a waterproof zip lock plastic bag, as shown in the figure below. Leave one of the corners open and remove all of the remaining air with a straw. Close the air tight bag.
  4. Leave the sensor in the bag for four days or 96 hours to completely stabilize.
  5. The sensor should read 75% plus or minus the sensors rated margin of error. For example, if the sensor is rated at +/- 5%, the display should read between 70 and 80% relative humidity.
  6. If the sensor is adjustable or calibrateable, set the humidity on the display to 75%.
  7. Repeat this procedure once per year to account for electronic drift (sensor contamination).

    The images above show a table salt slurry of 1″ height mixed in a container about 3″ tall. This solution is then placed in a plastic bag with the sensor.