241 Lake Shore Cryotronics Liquid Helium Level Monitor

The Lake Shore Cryotronics 241 Liquid Helium Level Monitor is a microcontroller-based instrument which measures the level of liquid helium using a hollow sensor with a superconductive Niobium-Titanium (NbTi) wire within it. The sensor, equipped with small liquid access holes, is placed in the cryogenic dewar.

In normal operation, part of the sensor is in the liquid and part in the gas. A current passes through a 3 Ω heater at the top of the sensor that heats the wire and causes the wire in the gas to be non-superconducting. The current permits measurement of the wire resistance. The instrument reads the current with one set of leads, and the voltage with the other, for a four-wire resistance measurement.

With the sensor vertically fixed in the dewar, and a known active sensor length, the instrument calculates sensor depth in the gas and converts it to the sensor depth in the liquid in inches, centimeters, or percent.

To avoid liquid boil-off, current is applied to the superconductive wire for only 5 seconds when a reading is taken. A sensor not partially submersed in liquid helium within a vacuum overheats to destruction. To avoid this, the Lake Shore Cryotronics 241 reads the voltage across the sensor within 200 milliseconds after applying the current. If resistance is equal to or greater than resistance which implies a zero level (Maximum Resistance), the current turns off immediately and a zero level displays.

The instrument produces a current of 70 mA in the superconductive wire and has a compliance of 30 volts. The maximum readable resistance is 428.6 Ω. The actual maximum is slightly less than 428.6 Ω because of lead and heater resistance. This corresponds to a maximum active sensor length of 104.5 cm (41.1 in.) for a superconductive wire of 4.1 Ω/cm (10.41 Ω/in.), or 79.4 cm (31.2 in.) for a superconductive wire of 5.4 Ω/cm (13.7 Ω/in.).

If a sensor connection breaks, or the sensor wire opens, an alarm sounds, and the display shows error E27. Initiate sensor calibration from a recessed Cal key on the rear panel and control calibration inputs from the front panel. The enclosure need not be opened. The calibration constants store in EEPROM nonvolatile memory.

To take a reading, press the front panel Read key. To take readings automatically at selected intervals, set the Sample Period with the front panel keys or send a command over the Computer Interface. Press the Continuous Read key to take readings at 1 to 2 second intervals. This is useful during a fill operation.

User-defined alarms indicate maximum level during a fill operation and minimum level during normal operation. A Serial I/O port provides a digital computer interface in RS-232C electrical format.

An audible alarm alert users to a level below the Minimum Alarm Setpoint or, during the Continuous Read mode (as in a fill operation), to a level above the Maximum Alarm Setpoint. Alarm status can also be read over the Computer Interface. Enter and read Alarm Setpoints from the front panel or over the Computer Interface.