Automatic nitrogen fill is controlled by the Cryostat Control Unit (CCU). It uses a two point sensor with an overheated platinum sensor.
The values shown in SEA Client (N2_upper / N2_lower) are the temperatures of the sensors, beeing about 80 K when immersed and above 100 K else.
The mechanism is based on a vapor pressure switch (containing argon gas) and a timer box. Every 8 hours the filling is started, and stopped when the LN2 reservoir is full. The filling time is limited to 10 minutes in oder to avoid damage in case of sensor failure. A sensor with a leak would indicate "vessel full", which is also a safety feature.