AMI helium sensors are rugged, reliable, and can function in magnetic fields up to 10 Tesla.
Standard AMI liquid helium sensors are constructed using 3/16", 1/4", and 3/8" phenolic (normally G-10) tubing. Flexible sensors are available in 3/32" and 5/16" outer diameters. Immediate shipment is available for 1/4" diameter sensors having active lengths of 1" to 36" inches (1 inch increments) and 6" Teflon-insulated leads. Other active lengths can be made to customer specifications up to 80", and can incorporate longer leads or, for 1/4" and 3/16" sensors, stainless steel sleeved Lemo connectors. Overall sensor length typically exceeds the active length by at least one inch for phenolic tubing sensors, and two inches for flexible 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. A standard sensor will not function properly through the lambda point (see Model 135-2K ).
Rigid: 3/16", 1/4", and 3/8"
Flexible: 3/32" and 5/16"
Metal Sleeve Diameter
(for Lemo Connector):
|1/2", 3/8", 12mm, 10mm or 8mm|
|Active length:||Customer specified|
Rigid: Active length plus 1 in.
Flexible: Active length plus 2 in.
|Std. sensor current:||75 mA|
|Std. sensor resistance:||
11.6 ohms/in. at 20 K
13.7 ohms/in. at 300 K
|Magnetic field operation:||Unaffected to at least 10 Tesla|