Humidity standards are used for verification and calibration of water activity meters. Our SAL-T humidity standards are about over-saturated salt solution according (Greenspan report, 1977). Although a defined air humidity is created over the surface of those standards which can be used for verification or calibration of a water activity sensor. The unique design of the salt standard allows re-use over a period of 3-5years (field experience) or even longer, depending on handling. This fact brings maintenance costs of a Novasina water activity meter to low levels.