Nepheles and Hydronepheles are steam generators expressly designed for the dental laboratory, and provided with a capacious boiler entirely in stainless steel. The high thermic insulation and the immersion resistance grant high performances with reduced power consumption. A specially provided valve cap prevents the boiler opening under pressure and protectsthe equipment from the danger of over pressure. Hydronepheles is equipped with a water level control device allowing the automatic refilling when connected directly to a water tap, through frequent micro-refilling operations in order to keep temperature and pressure constant.