LN2 Level Monitors

ILL Liquid Nitrogen Level Monitors

The liquid nitrogen levels of our cryostats are monitored with electronics designed from industrial components. The levels are measured with capacitive sensors from Cryomagnetics or other companies. The calibrations, measurements, alarms and automated refills are easily managed with a process controller (Eurotherm model 3504).

Intuitive interface

The unit measures and displays permanently the liquid nitrogen level with a bar graph and a number that are visible quite far away from the cabinet. Only one button is required to start / stop manually a refill. The message "Refilling..." is displayed when filling.

When the low level is reached, an alarm is displayed in red and a fill is automatically launched. If the fill fails after a certain amount of time, it is automatically stopped. The alarm is maintained and the level continues to be measured and displayed until the user acknowledges the alarm.

Easy settings

The level monitor is calibrated in a very few steps:

  • fill the cryostat in liquid nitrogen
  • enter the Settings mode
  • raise the capacitive sensor and confirm the low level value
  • lower the capacitive sensor and confirm the high level value
  • quit the Settings mode

The low and high levels, the alarm level and the fill timeout can easily be modified from the Settings menus according to the user wishes.

Full remote control

Computer control via RS-485 and Ethernet is available. All parameters can be read and set, even the configuration of the sensor. The communications are based on the Modbus communications protocol which has become a de facto standard in industry. We provide a Modbus XOP and a Procedure for controlling the level monitor with Igor Pro.

The ILL liquid nitrogen level monitor has been carefully designed and manufactured to provide hours of trouble free service. Its user interface has also been designed so as to simplify as much as possible its operation. The monitor also has several features that are not available on other commercial systems: a large display for easy reading on large instruments and a full remote access allowing users to monitor different cryostats without the need for recalibrating.

The level monitor uses a capacitive sensor connected to an oscillator. The oscillator is powered by the monitor with a 10V DC signal and read with the +/-10V DC voltmeter integrated into the level monitor. The monitor also provides a 24V DC signal to control the cryogenic solenoid valve mounted on the liquid nitrogen dewar. It is delivered fully tested and ready to be calibrated. We describe below how to install, operate and remotely control the level monitor.


The monitor manages the readings and the fills. The user can easily set the low and high levels, the alarm level and the fill timeout. The refills are started when the low level is reached or when the user presses the command "REFILL". The fill cycle will end when the level reaches the high iimit, when the fill timeout expires, or when the user presses the "STOP" command. When the fill timeout expires, the refill is aborted and the alarm remains active until the user acknowledges it.


The monitor can be remotely controlled through a RS485 or Ethernet link using the Modbus communication protocol (public function code 3).

The ILL liquid nitrogen level monitor is presently delivered with the version 1.07 of the soft-wired program installed and preset with Orange cryostats parameters. Remote control is performed using the registers listed below.

Firmware Update

The latest release 1.07 adds the ability to detect a sensor break, fixes a bug allowing the modification of the release number, and another bug allowing the display of negative values.

Size and Weight

The monitor fits within a 19"/3U chassis.

  • Height: 129 mm (5.1 inches)
  • Width: 143 mm (5.6 inches)
  • Depth: 355 mm (14 inches)
  • Weight: 3.02 kg (6.66 pounds)

Modbus registers

LN2 level value-2120619
Alarm set point in %010010257
Low level set point in %010010241
High level set point in %01004963
Fill command014976
Fill timeout value in min (default: 30min)010011272
Fill timeout error status0111273
Firmware version1104977

Electrical and operating requirements

Contact us if you need a 100V AC / 60 Hz unit.- Line voltage: 240V AC

  • Frequency: 50Hz
  • Operating temperature: 10 to 35°C (50 to 95 °F)
  • Storage temperature: -24 to 45°C (-13 to 113 °F)
  • Relative humidity: 5 to 85% noncondensing

Please find below the release history and some instructions for updating the firmware. To get the most recent release of the firmware, please contact us

Release History

  • 1.00 - Initial release
  • 1.01 - Modified the pages for simplifying the use of the controller.
  • 1.02 - Fixed a bug where the alarm cannot be acknowledged until the high limit is reached.
  • 1.03 - Added the ability to modify the refill timeout.
  • 1.04 - Fixed a bug in the routine managing the timeout. 
  • 1.05 - Added a page for displaying the firmware release number.
  • 1.06 - Added a variable for remotely requesting the fill of the nitrogen bath.
  • 1.07 - Added the ability to detect a sensor break, fixed a bug allowing the modification of the release number, and another bug allowing the display of negative values.

Instructions for updating the controller

  • Install iTools (available here)
  • Connect the controller to the computer with an Ethernet cable,
  • Configure the TCP/IP parameters of the computer:
    • Select the Connection panel and click on TCP/IP Properties,
    • Enter the fixed IP address ( for NOMAD),
    • Enter the subnet mask,
    • Enter the gateway address,
  • Launch iTools and open the OPC Server window,
  • Open the "Scan for TCP hosts..." panel from the Add/New TCP Network Port menu command,
  • Enter the TCP/IP address (standard address of the LN2 level monitor),
  • Launch the scan and stop it once the controller detected,
  • Reduce the OPC Server window (do not close it!),
  • Click on the Scan icon of the main iTools toolbar and launch a search for connected instruments,
  • After the scan, click on the Load icon of the main toolbar, select the new firmware and launch the update.