There are esssentially two types of sensors used for measuring water activity. This is the primary method approved by the AOAC International. It's major advantage is speed. Some can take readings in as little as 5 minutes. This compared to 30-90 minutes for the capacitive sensors.
Some manufacturers produce instruments that come precalibrated.
Others require calibration using a variety of saturated salt solutions. Be sure to note the design of the instrumentation with particular attention to how easy the mirror is to get to for cleaning. Another disadvantage of this style is that foods that contain propylene glycol cannot be analysed because the proplyene glycol will condense on the mirror.