maintaining liquid nitrogen levels

A controller for filling and maintaining the level of liquid nitrogen Dewars is described, which has been used in experiments where the boil-off rate is large due to the proximity of an atomic beam oven to a beam dump cooled by the liquid nitrogen. The control system uses either a pair of copper–constantan thermocouples, or a pair of green LEDs to monitor the upper and lower levels of the filled Dewar. The level of the Dewar is then maintained between these two detectors. The controller is simple to build and is inexpensive, thereby making it accessible to many undergraduate and research laboratories