Saturated Salts as a Humidity Calibration Reference
Perhaps you have a requirement to calibrate humidity measurement instruments in the field. By “field” I mean, outside a controlled laboratory area. For example, you may have hygrometers installed in several locations around a critical warehouse storage area or in a continuous process where removal of the instruments is not practical or it is expensive and time consuming. When you make decisions about your field calibration process, one thing that must be decided is whether you perform a single point or a multi-point calibration. Typically, if your process could see a wide range of relative humidity or temperature, a multi-point calibration is prudent. As with any decision you make around critical or regulated environment, you must be able to articulate your decision and defend your decision should you ever be challenged (by a customer or regulatory agency).
Assuming that you have chosen a multi-point calibration, your next decision is about how you will generate the multiple points of humidity reference. You could choose to purchase a relatively expensive humidity generator that relies on two pressures or two temperatures or both. These types of generators are an excellent choice with low uncertainties and high reliability. If you don’t have the budget and are comfortable with higher uncertainties and lower reliability, saturated salts is a fine alternative.
Certain types of salt will generate specific relative humidity. Lewis Greenspan published a list of salts and the relative humidity that is generated in 1977, 'Humidity fixed points of binary saturated aqueous solutions', National Bureau of Standards. See Figure 1 below for a summary table.
I’ve been teaching humidity measurement seminars for six years and typically, there is a lot of interest in using saturated salts as a reference. Someone invariably will ask about traceability. Is it possible to claim traceability based on the physical principle of the salt? Yes, you could defend this position as long as you document the process for maintaining and using the saturated salts in accordance with an accepted standard such as ASTM E104-02 (2007). However, I typically recommend using a reference hygrometer to achieve traceability to a national standard whereby the saturated salt is simply the medium to generate the calibration environment. Saturated salts require a lot of maintenance and patience to use properly, but they are an inexpensive and efficient means of creating multiple points of relative humidity.
Bruce McDuffee is the global marketing manager for the Life Science Division of Vaisala, which helps life science companies reduce risk of lost or poor quality products and reduce risk of regulatory problems by providing highly reliable and proven measurement, monitoring, and validation solutions. McDuffee holds a MBA specializing in international management and marketing, bachelor of science in civil engineering and served as a Surface Warfare Officer in the U.S. Navy for several years before entering the test and measurement instrumentation industry. McDuffee teaches humidity seminars throughout the United States and webinars offered around the world.