Standard Liquid Helium Level Sensors The liquid helium level sensor operates by measuring the resistance of a superconductive filament contained within a protective tube. The current through the sensor maintains the portion of the filament in helium gas in the normal (resistive) state, while the portion in liquid remains in the superconducting state (zero resistance). The resulting voltage along the sensor is proportional to the length of filament above the liquid helium and provides a continuous measure of the helium depth. Voltage measurements are made using a four wire technique to eliminate errors resulting from variations in the length of the leads. The small amount of heat generated in the probe is dissipated primarily in the helium gas rather than in the liquid helium. These sensors are rugged, reliable, and can function in magnetic fields up to 10 Tesla. Standard AMI liquid helium sensors are constructed utilizing 3/16", 1/4", and 3/8" phenolic (normally G-10) tubing. Flexible tubing is available in 3/32" and 5/16" outer diameters. Sensor active lengths from 1" to 36" inches in 1 inch increments utilizing 1/4" phenolic tubing are stocked by AMI for immediate shipment. Other active lengths can be fabricated to customer specifications up to 80" standard or 98" on a custom basis. The overall sensor length typically exceeds the active length by one inch for phenolic tubing sensors, and two inches for flexible sensors. View the sensor operations manual for additional information - LHe Sensor Operations Manual Contact AMI with your specifications for a quotation.

*other operating currents available
**other resistances available

Custom Liquid Helium Level Sensors

Custom sensors are available from AMI to meet your individual application requirements. Below is a partial list of the type custom helium sensors that AMI provides.

• 2K Sensors - These sensors are designed to provide uninterrupted level readings over a wide temperate range - a minimum of 1.8 to 4.2K. This is ideal for operation in systems where it is important to continuously monitor the liquid helium level over transitions through the Lambda Point. The combination of a Model 135-2K instrument + 2K sensor will accurately measure the level without experiencing the traditional "dead zone" where erroneous and erratic readings can occur due to the heat transfer dynamics in this region. These sensors are painted red to easily differentiate them form normal sensors. The specially designed Model 135-2K instrument must be used with the 2K sensor for best results. Automatic helium level sensor vacuum burnout protection is a standard feature of the instrument. A 2K sensor used with a traditional instrument will not function through the lambda point.

• Radiation Proof Sensors - In high radiation environments such as particle accelerators it is important to use materials that will perform well over many years of service life. These sensors contain no Teflon or other materials that are prone to degrade.

• Vacuum Environment Sensors - In certain applications it is preferred to run the level sensor signal wires through the vacuum space of the cryostat. This means that the helium sensor must be leak tight between the helium and vacuum environment. Sensors are provided with a glass-to-metal hermetic feedthru connector that is seal welded to the top of the sensor and the signal wires are routed through the vacuum space to a secondary feedthru connector. The sensor itself is sealed to the helium chamber via welded mini-Conflat vacuum flange and crushed metal seal.

• Spray-Proof Sensors - The nature of NbTi superconductors when used to measure liquid helium levels is that they are very sensitive to the thermal and thermodynamic conditions of the helium environment. One such application that can be difficult is the liquid collection chamber of a helium recovery or recondensing system. The helium can enter the chamber as a sprayed mist and even droplets of liquid that enter through the sensor tube holes can impact the filament and cause erroneous and erratic readings. Spray-Proof sensors were designed with this specific application in mind and solve this problem by adequately shielding the filament against such spray.

• Rugged Service Sensors - Normal 1/4" fiberglass tubing works fine for sensors up to 4-5 feet in length. Beyond this point a heavier walled tubing is recommended in a 3/8" or 1/2" inch O.D. size to resist the rigors of shipping, handling and installation.

• Special Service Sensors - AMI can design and build almost any configuration of helium level sensor to suite the special needs of a particular application. Some examples include sensors designed to survive high G-force loading and vibrations associated with space flight systems or rocket sleds. Other examples include sensors with integral heaters built into the bottom of the sensor for the purposes of pressure building in helium storage dewars during filling operations (left).

• Removable Sensors - In many systems it is preferred to have a removable helium level sensor. For continuous level readouts AMI offers both fiberglass tube sensors and stainless steel tube sensors. On fiberglass sensors of this type AMI has chosen to offer only a high quality metal sleeve on the upper section with integral locking Lemo connector (lower left). The locking plug ensures a positive electrical connection to the sensor. The metal sleeve provides a suitable sealing surface for compression fittings. Standard sizes available include sleeve of 1/2˝, 3/8˝, 8, 10 or 12 mm O.D.

  Stainless steel sensors are offered in only 1/4" O.D. and also have the locking Lemo connector (bottom center). Situations requiring only a periodic point measurement can utilize either a rigid wand with point sensor on the tip or a flexible wand with point sensing. A photo of this flexible wand is shown (lower right) which eliminates any problems with ceiling height interferences.

• Non-magnetic Sensors - In certain sensitive experimental systems the presence of even a small amount of weakly magnetic material can cause unwanted signal distortion (SQUID measurements, etc.). In such cases sensors can be provided that further minimize the small amounts of magnetic material in the sensor.

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