Ryuten ondokei (maximum-registering mercury thermometer)

A ryuten ondokei is a type of mercury thermometer, distinguished in particular by its ability to show the maximum temperature reached. It is widely used to measure the temperature of koji making (seikiku) and of moromi (fermentation mash). Because proper temperature control is important in yeast fermentation and in the koji-making process, this thermometer is an indispensable tool for ensuring reliable temperature measurement in sake brewing.