Those scientific and industrial equipment that require a very low temperature cooling system to operate often require continuous maintenance and constant control of the cryogenic liquid level. For this reason, some plants are equipped with remote surveillance systems based on “ad hoc” developed technologies for very low temperatures. We present a patented technical solution to be able to use, in cryogenic conditions, a common ultrasound sensor that costs 4 euros. This cap for dewar tanks, due to its particular shape, allows to obtain this result. Consequently, it also offers the possibility of being able to use other lowcost technologies, such as the microcontrollers of the Arduino series. This allows an unlimited range of developments ranging from remote monitoring to the total automation of the ordinary maintenance of the cooling system. With this idea an “active cap” for Dewar was created at very low cost. It monitors the level of liquid nitrogen and provides online measurements updated in real time, the forecast of intervention for topping up and any alarm messages. A simple upgrade can allow automatic refilling of the liquid in the dewar. This solution, for inexpensiveness, can also represent an alternative to closedcircuit cooling and recycling systems.