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.
- Mix common table salt with tap water until you have a completely saturated solution with no standing water.
- 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.
- 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.
- Leave the sensor in the bag for four days or 96 hours to completely stabilize.
- 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.
- If the sensor is adjustable or calibrateable, set the humidity on the display to 75%.
- 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.