It is not usually necessary to perform a new calibration when using the Relative Humidity Sensor (RH-BTA). You can simply connect the sensor to a Vernier data-collection interface, start your Vernier data-collection program, and the stored calibration is used.

If you do want to calibrate the Relative Humidity Sensor, you can do so by comparing it to another calibrated instrument that measures relative humidity (hygrometer or psychrometer). You can then perform a one-point calibration following the calibration instructions for your data-collection program from the links below.

A two-point calibration can be done by using two moist salts. In a sealed container of moist salts, the air above the salts will reach a known relative humidity. The relative humidity above the salt does depend slightly on the temperature. See the table below for a list of salts and their known relative humidity at different temperatures.

Here is the procedure for doing a two-point calibration with moist salts:

  1. Place a handful of salt in the bottom of a jar (quart or liter size is fine).
  2. Add a little water to the jar so that the salt is wet. Note: The goal is to end up with wet salt, not to totally dissolve the salt.
  3. Place the Relative Humidity Sensor in the jar. Note: Do not get the salt or salt solution on the sensor.
  4. Seal the jar. Plastic wrap and rubber bands will do. Allow some time (2 to 6 hours) for the air inside the sensor to reach the proper relative humidity level.
  5. Connect the Relative Humidity Sensor to your interface and start the data-collection program.
  6. Following the calibration instructions for your data-collection program from the links below, go through the procedure for the first calibration point in a two-point calibration. Enter the relative humidity for the salt you used, as determined from the table below.
  7. Repeat the procedure for the second calibration point using a different salt. Be sure to allow enough time for the Relative Humidity Sensor to adjust for the change in humidity.

Note: Even though none of the salts listed here is especially dangerous, use normal precautions with these chemicals.

Salt 15°C 20°C 25°C 30°C 35°C
Lithium Bromide 6.86 6.61 6.37 6.16 5.97
Lithium Chloride 11.3 11.31 11.3 11.28 11.25
Potassium Acetate 23.4 23.11 22.51 21.61 –
Magnesium Chloride 33.3 33.07 32.78 32.44 32.05
Potassium Carbonate 43.15 43.16 43.16 43.17 –
Magnesium Nitrate 55.87 54.38 52.89 51.4 49.91
Potassium Iodide 70.98 69.9 68.86 67.89 66.96
Sodium Chloride 75.61 75.47 75.29 75.09 74.87
Ammonium Sulfate 81.7 81.34 80.99 80.63 80.27
Potassium Chloride 85.92 85.11 84.34 83.62 82.95
Potassium Nitrate 95.41 94.62 93.58 92.31 90.79
Published: January 18, 2019. Updated: March 18, 2020.