Cryogenic Level Instrumentation

Helium Level Measurement and Control

Liquid helium level measurement is nearly indispensable in that it gives an instantaneous and reliable readout of the liquid helium level in the dewar, a must for avoiding operation of the magnet with too little helium.

Unlike single point sensor methods of determining the liquid helium level in dewars, AMI's liquid helium level instruments provide a continuous readout of the liquid helium level in a reliable and convenient manner.

The liquid helium level sensor is comprised of a fine superconducting wire in a ¼-inch outside diameter, non-conductive tube. During operation, a small current is conducted through the wire, causing it to be resistive in the helium gas and superconducting in the liquid. This technique results in a voltage being established along the sensor that is proportional to the length of the wire above the liquid helium, and provides a continuous measure of the helium depth. Measurements of this voltage are made using a four wire method to avoid errors caused by variations in the length of the leads to the sensor. The small amount of heat generated in the probe is dissipated primarily in the helium gas rather than in the liquid helium. The sensor is rugged, reliable, and functions in magnetic fields up to 10 Tesla.

A host of helium level monitors are available from AMI, from a simple 0% to 100% analog meter display to the more sophisticated, microprocessor-based, sample-and-hold design. Helium level monitors which incorporate AMI’s patented sample-and-hold design are ideal for operation in systems where it is important to monitor the helium level and minimize the liquid helium losses.

In order to minimize liquid helium loss, sample-and-hold monitors automatically energize the liquid helium level sensor at predetermined time intervals and track the normal (resistive) zone as it progresses from the top of the sensor toward the liquid surface. As soon as the normal zone reaches the liquid surface the level reading is saved, and the current in the sensor is turned off until the next sample interval occurs. The sample-and-hold design can reduce helium boil-off substantially compared to operating the helium level sensors continuously.

Liquid Nitrogen Level Measurement and Control

For magnet systems that use nitrogen shielded dewars, an AMI liquid level controller can automate the process of transferring nitrogen to the dewar. AMI's capacitance-based sensor utilizes a very reliable liquid nitrogen level measurement method. The sensing element is a linear capacitor consisting of two concentric stainless steel tubes in which the cryogenic liquid becomes the dielectric. The sensor capacitance is directly related to the liquid level in the sensor. Level is typically displayed from 0% to 100% by a liquid level instrument.