Humidity standards are used for verification and calibration of probes and transmitters. The SAL-SC humidity standards are based on over-saturated salt solution according to Greenspan (Greenspan report, 1977). Thereby a defined air humidity is created inside the standards which can be used for verification or calibration of air humidity sensors. The optimised design allows to re-use the standards for a period of 3-5 years. The SAL-SC container is put on the probe and after a short time a humidity equilibrium is established. Field experiences have shown that thanks to these re-usable standards the maintenance costs could be reduced considerably.