Model 269 Controller allows unattended filling of dewars and cylinders, by the use of a Single Thermistor sensor. When liquid nitrogen is detected, the filling system will SHUT OFF.

When properly installed both systems (269DF and 269LCF) rely on liquid nitrogen (LN2) to activate the sensor probe (thermistor). At room temperature the thermistor is around 10ohms and in liquid nitrogen the thermistor is around 30000. Due to the heating effect of the thermistor via the 269 circuit, liquid nitrogen will reduce the thermistor operation to 5-10000 ohms.

During the filling process the LN2 gas has no effect on the sensor probe. Not until LN2 sufficiently covers the thermistor, causing the system to shut down.