Features:
- Adjustable Alarms
- English, Metric or Percent Units
- Continuous, Timed or One-Time Level Readings
- RS-232C Serial I/O Port
- 3.5-Digit Custom LED Front Panel Display with Annunciators
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.
The 241 Lake Shore Cryotronics Liquid Helium Level Monitor may still be available for purchase and support from Artisan Technology Group beyond End-Of-Life (EOL) by the manufacturer (OEM).
Artisan Scientific Corporation dba Artisan Technology Group is not affiliated with or a distributor for Lake Shore Cryotronics. The depiction, description, or sale of products featuring names, trademarks, brands, and logos is for identification and/or reference purposes only and does not indicate any affiliation with or authorization by any rights holder.