Saturated salt solutions have a long tradition in relative humidity calibration because they make it relatively simple to create a set of fixed relative humidity points. The use of saturated salt solutions is covered by several standards including ASTM E104-02 and JIS Z8806:2001.
A saturated salt calibrator always contains three different phases: gas (saturated air), liquid (water), and solid salt. As long as both the liquid and the solid salt phase are present the relative humidity in the air space will stabilize at well-known fixed points, which depend on the type of salt used. The ratio of liquid water to solid salt is not important. Typical salts used and their respective relative humidity levels are shown in the table below:
The Vaisala HMK15 salt calibrator is a simple relative humidity calibrator that allows you to easily perform two or three-point calibrations – useful if your measurement conditions vary over the full range. Even small temperature differences between the probe being calibrated and the salt solution will result in big differences in relative humidity, so the HMK15 is designed to rapidly reach equilibrium with the ambient temperature.
Making the salt solutions is easy as pre-weighed bags of analytical grade salts and even clean ion-exchanged water can be included in the order. Simply measure out the correct amount of water and mix it with the contents of the salt bag. The salts are even batch calibrated to ensure that you will get the correct humidity value, and each bag comes with a certificate showing the results of this calibration.
If the salt solution is not contaminated or spilled, you should be able to use the same salt solution for at least one year.
Instead of the relying on the values in the table above, some users prefer to rely on a separate traceable calibration standard, for instance a Vaisala HM70 handheld device with accredited calibration. The HMK15 makes this possible as the calibration chamber has space for up to four probes at a time.
Wherever possible, try to avoid handling the probe you are calibrating as the heat from your hands can raise the temperature of the probe above ambient temperature, meaning it will take much longer to stabilize.
If the calibrator and the probe start out at the same temperature, it will usually take between 10 and 30 minutes before equilibrium is reached in each calibration chamber. It is good practice to monitor the probe readings for stability. On instruments with graphical displays, it is also recommended to check the graph view of the relative humidity reading.
By following these basic precautions during calibration you can achieve accuracies of +/- 1.3 … 2%RH with this relatively simple setup.
Watch the video below to see how to calibrate Vaisala HMT330 with HMK15 Humidity Calibrator.
Please note that for critical measurements, we recommend sending the HMT330 to one of Vaisala's Service Centers for the best possible calibration. See all the HMT330 calibration options here.
Lars Stormbom works at Vaisala in the Industrial Instruments product area. He holds a MSc in Technical Physics and has an extensive experience in humidity measurements. Lars is the specialist in a wide range of humidity sensors and multiple technology applications.