AMI helium sensors are rugged and reliable and can operate in magnetic fields up to 10 Tesla. Standard AMI liquid helium sensors are made using 3/16", 1/4" and 3/8" phenolic (usually G-10) tubing. Flexible sensors are available in 3/32" and 5/16" O.D. 1/4" diameter sensors are available in effective lengths of 1" to 36" inches (1" increments) and 6" PTFE Insulated wire is available for immediate shipment. Other effective lengths can be manufactured to customer specifications up to 80" and can include longer leads or, for 1/4" 3/16" sensors, stainless steel sleeved Lemo connectors.
The total sensor length typically exceeds the effective length by at least one inch for phenolic tube sensors and two inches for flexible sensors.
Liquid helium level sensors operate by measuring the resistance of a superconducting filament contained within a protective tube. The current through the sensor maintains the portion of the filament in helium in a normal (resistive) state and the portion in liquid in a superconducting state (zero resistance). The voltage generated along the sensor is proportional to the length of the filament above the liquid helium and provides a continuous measurement of helium depth.
Voltage measurements are made using a four-wire technique to eliminate errors caused by variations in the length of the filament. The small amount of heat generated in the detector is dissipated primarily in the helium, not in the liquid helium. Standard transducers will not function properly through the lambda point.