LM31/34 Liquid Nitrogen Level Monitors

Teragon's LM31 and LM34 Liquid Nitrogen Level Monitors provide reliable and accurate LN₂ level measurements in dewars, cold traps and other cryogenic reservoirs. The LM31 continuously displays the current liquid level and evaporation rate of a single LN₂ reservoir. The LM34 monitors up to four LN₂ reservoirs simultaneously. Applications include foreline traps, radiation detectors, cold fingers and cryopreservation.

The LM31/LM34 monitors employ high-precision capacitive probes (model LP30) which are sensitive to the different dielectric constants of liquid and gaseous nitrogen. The monitors are connected to the LP30 probes using coaxial cable with BNC connectors. The monitors can support cable lengths up to 100 feet. The LP30 probes are assembled to customer specification and are available with active lengths from 4 inches to 10 feet.

Low and high level alarm functions are provided for each probe. The user-defined alarm setpoints are entered from the front panel of the monitors and no manual adjustment of the probes is necessary. If an alarm condition occurs, the monitor provides an audible tone and the display indicates the reason for the alarm.

Both LM31 and LM34 monitors have RS-232 serial interfaces which permit remote monitoring of the LN₂ reservoirs. The interface provides level and rate information, alarm status, as well as setpoint and calibration functions for each probe. Virtually all keypad commands can be issued using the serial interface. Connectors for the RS-232 interface and the probe cables are provide on the monitor's rear panel.

The monitors' high-reliability features include an internal watch-dog timer, non-volatile memory and a keypad lockout capability. Each monitor comes with a power cord and user manual and probes come with a 10 foot probe cable. All parts are covered by a 1 year warranty. The LM31/LM34 are durable units designed for years of precise and reliable service. Inquiries regarding custom applications are welcome.

Standard features of the LM31/34 include:

	Continuous Monitoring: The LN2 level is continuously monitored and displayed on the front of the monitor. The LM31 also displays the rate of evaporation for a single probe while the LM34 displays up to four levels simultaneously. The LM34 display can be toggled to show the corresponding rate information.
	User Defined Alarm Points: High and low level alarm points for each probe can be entered using the monitor's keypad. If any probe's liquid level exceeds either alarm point (high or low), an audible alarm sounds and the display indicates a fault condition.
	Digital Precision: With an embedded microprocessor, digital filtering is used to achieve a level precision of 0.1%. All setpoints are stored with this precision in non-volatile E2PROM memory.
	Serial Interface: The RS-232 serial interface allows remote level monitoring. Virtually all keypad commands can be accessed via the serial interface and a keypad lockout feature is provided for added security.
	Effortless Calibration: Probe calibration is quick and easy: with the probe at room temperature, press the EMPTY key. Place the probe in a filled reservoir and press FULL. Calibration results are stored in non-volatile E2PROM memory.
	High-Visibility Display: The backlit, high-contrast display is visible with or without ambient light. The contrast is adjustable from the front panel to provide maximum visibility in any environment.
	Alarm Versatility: Each probe alarm can be configured as high, low, neither or both. Pressing the alarm acknowledge key temporarily silences the alarm. The RS-232 interface allows detailed interrogation of the alarm settings and status. An optional fail-safe alarm relay can be used to activate an external alarm or auto-dial phone system.

Monitor and Probe Specifications

LM31 / LM34 Monitors
Power: 120VAC @ 0.1 Amps, 50-60 Hz
Serial Interface: 1200, 2400 or 9600 baud
Dimensions excluding tilt handle: 3.4 in. H x 7.5 in. W x 7.0 in. D (86 mm x 190 mm x 178 mm)
Weight: 2 lbs. (0.9 kg)
Probe Inputs: Female BNC, 1 for LM31 and 4 for LM34
Each Monitor comes with a 6 foot power cord, a user manual and the appropriate number of 10 foot probe cables.

LP30 Level Probes
Probe Type: Capacitive transducer
Sensing Lengths: From 4 in. to 10 feet (100 mm to 3050 mm)
Wetted Parts: 316 Stainless Steel, PTFE & Epoxy
Probe Connector: Female BNC

Monitors, probes and cables are covered by a one year warranty.
Specifications are subject to change without notice.

Options & Accessories

Fail-safe alarm relay

Monitor rack mounting

100, 200, or 230VAC line power

Probe mounting adapters & flanges

Long probe cables (up to 100 ft.)

Placing Orders

Please specify the required Sensing and Insertion Lengths as shown in the LP30 Probe diagram. The Sensing Length is the distance from the bottom of the probe to the vent. For proper operation the vent must be located within the LN₂ reservoir. The Sensing Length is the distance over which the liquid level ranges from 0% to 100%. The minimum Sensing Length is 4 inches (100 mm).

The Insertion Length must include the distance over which the temperature changes from ambient to cryogenic. Normally this includes the height of the dewar cap and any mounting fittings. The minimum Insertion Length is the Sensing Length plus 2 inches (51 mm).

Note that the probe should not be mounted flush against the floor of the reservoir. A minimum clearance of 0.040 inch (1 mm) is required at the end of the probe to allow the liquid to enter.

U.S. Pricing

LM31 Level Monitor: $595

LM34 Level Monitor: $695

Monitor rack mounting (single or side by side): $100

LP30 Probe with Insertion Length up to 12 inches (305 mm): $295

For each additional inch (25 mm) of LP30 Insertion Length: $3

Optional Adapter: 1/4" MPT to compression fitting: $15

Prices are FOB San Francisco and are subject to change.