Johnson A421 Digital Temperature ControllerThe Digital Temp Controller operates by overriding the internal thermostat in a refrigerator or freezer and gives the brewer greater control over the fermentation. Precise control of fermentation temperature is one of the most beneficial things you can do to improve your home brew beer. It is also ideal for controlling the serving temperature of your kegerator or keezer. The digital controller has a digital display with adjustable differential. It can