AL1 Liquid Nitrogen Level Alarm

The Teragon AL1 Alarm continuously monitors the liquid level in cryogenic reservoirs. The alarm is used to warn of the impending loss of liquid nitrogen. Applications include the protection of cryobiological samples or high-voltage radiation detectors. The alarm can also be used to prevent the hazardous overfilling of portable dewars.

The alarm uses a liquid sensor that is placed at the desired alarm level. As the liquid level passes the sensor, the alarm will sound. The alarm can be user configured as either a low-level alarm (sensor dry) or a high-level alarm (sensor wet). In either configuration, the alarm is also activated if the sensor is damaged or unplugged.

During normal operation, a green LED indicates that the sensor is operating properly. When an alarm condition occurs, a red LED is lit, a loud audible signal is emitted, and a relay is actuated. The fail-safe alarm relay allows the AL1 to activate a remote alarm or auto-dial phone system. The AL1's features include:

	Audible and Visual Alarm Signals
	High or Low Level Alarms
	Sensor Signal Indicator
	Fail-Safe Relay for External Use
	Small Size
	Low Cost
	Easy Installation
	1 Year Warranty
	Made in the USA

The AL1 system comes with the alarm module, level sensor, AC power adapter, velcro mounting strips, and a user guide. Sensors are available in lengths of 2, 3, and 4 feet. All parts are covered by a 1 year warranty. The Teragon AL1 alarm will provide the level of protection that your cryogenic application deserves.

AL1 Specifications

Power Supply: 12-18 VDC @ 0.1 A
Power Jack: P-5 (2.1mm male)
Alarm Relay: 200 VDC @ 0.5 A
Relay Jack: P-6 (2.5mm male)
Audible Tone: 2800 Hz continuous
Visual Indicators: Green LED for sensor signal, Red LED for alarm condition
Alarm Dimensions: 1.65 in. x 2.27 in. x 3.72 in.
Sensor Type: Thermally biased Teristor
Sensor Bias: 44 mW
Sensor Lengths: 2, 3, or 4 feet
1 year warranty
Specifications are subject to change without notice.

Application Notes

The AL1 sensor, shown below, detects the different thermal conductivities of liquid and gaseous nitrogen. This is done by biasing the sensor with a small amount of heat input (44 mW). If the sensor is submerged in LN₂, the heat is conducted away from the sensor with negligible temperature rise. However, if the sensor is uncovered, a small but measurable temperature rise occurs at the sensor tip. The AL1 alarm analyzes the sensor signal to determine if (1) the sensor is installed & operating properly and (2) if an alarm condition exists. If the sensor is operating correctly, the green "sensor signal" LED is lit whether or not there is an alarm condition.

The alarm can be configured as either a high- or low-level alarm by a jumper on the AL1 circuit board. As a high-level alarm, the AL1 will signal an alarm whenever the sensor is submerged in LN₂. The alarm signals include (1) lighting the red "alarm condition" LED, (2) sounding the audible alarm and (3) actuating the alarm relay. Conversely, as a low-level alarm, the AL1 will signal an alarm whenever the sensor is dry. In either configuration, if the sensor is unplugged or malfunctioning, an alarm will sound with the green LED unlit.

The alarm relay, accessible via an electrical jack on the bottom of the alarm module, allows the AL1 to interface with external equipment. A jumper on the AL1 circuit board allows the relay to operate in one of two modes: either conventional or fail-safe. In the fail-safe mode, the alarm condition is signaled by open relay contacts. The relay is closed only when there is no alarm. If power is removed from the AL1, the relay will open to indicate a hazard. In the conventional relay mode, the alarm condition is signaled by closed relay contacts. The relay is open when there is no alarm condition or if power is removed from the AL1.

The small size of the AL1 alarm permits it to be mounted on the side of monitored dewars. This is easily done using the adhesive-backed velco strips provided with the AL1. The position of the mounting can be chosen so that the sensor will freely hang at the desired alarm height (see diagram above). An integral splash guard on the end of the sensor inhibits premature triggering in applications involving turbulence.

AL1 Options

A mute version of the alarm is available for customers that desire only the visual and relay alarm signals. Specify part number AL1-M. An optional 6-foot mounting strap allows the alarm to be mounted on the dewar side without applying velcro adhesive to the dewar itself. Rather, the velcro is applied to the strap which is tightened around the dewar. Straps can be daisy-chained to accommodate dewars of larger circumference.

The AL1 liquid nitrogen sensor is designed to work with unpressurized (15 ± 5 psia) LN₂. Sensors can be fabricated for other pressure ranges. Sensors can also be assembled with feed-throughs for vacuum and high-pressure applications. Noncryogenic sensor extensions permit the alarm module to be located as far as 12 feet from the sensor. Extensions are available in 3 and 6 foot lengths. Custom sensors with Cryoflex leads longer than 4 feet can be made to customer specifications. Please contact Teragon with your requirements.

U.S. Pricing

The AL1 system comes with a sensor, alarm, 120 VAC adapter, adhesive-backed velcro strips, relay plug and a user guide. Please specify the desired sensor lead length: 2, 3, or 4 feet.

AL1 alarm system: $275

AL1 replacement sensor (2, 3 or 4 foot lengths): $55

3 or 6 foot sensor extensions: $45

6 foot mounting strap option: $25

Prices are FOB San Francisco and are subject to change.