The Cryogenic Fiber Optic Sensing System (CryoFOSS) offers an approach to liquid-level sensing that uses fiber optic Bragg sensor technology. The innovation is able to sense liquid levels to within 1/4-inch spatial resolution and can actively discern between liquid and gas states.

The technology improves upon liquid-level detection techniques that employ temperature measurements. Such methods are difficult to use in cryogenic tanks that contain partial gas and partial liquid, which are precooled and at a steady state. In such closed systems, temperature differences are very small, especially in the transition region between liquid and gas, thus making accurate and precise liquid-level measurements using temperature alone very challenging.

More accurate methods employ cryogenic diodes placed serially along a rod or rack at strategic points to indicate whether liquid is present. Although this currently preferred method works, it has significant limitations. Installation and instrumentation are cumbersome due to each diode requiring two associated wires. Furthermore, the technique requires that the diodes be mounted in preselected, squarely placed positions, which may not be suitable for all liquid-level detection applications.

The CryoFOSS technology enables measurement of liquid levels in applications in which discriminating between liquids and gas states is difficult because of a small thermal gradient. It is composed of a sensing fiber and a heater element (the length of both of which can be customized to fit the application) inserted together into a thin-wall tube. The CryoFOSS sensor is attached to an interrogator, capable of electrically driving the heating element and interrupting the optical temperature gradients along the sensor. Uniquely driving the heating element causes the local area around the fiber to heat and cool rapidly, thus causing the portion of the CryoFOSS sensor in liquid to cool at a faster rate than the portion in gas. Plotting the data along the length of the sensor provides a distinguishable point that indicates the location of the liquid-to-gas transition, enabling measurement of the precise amount of liquid in the structure.

The technology is ideal for spacebased applications in which cryogenic conditions are present, and it can be used in many other applications in which liquid-level detection is required.

This work was done by Allen Parker of Armstrong Flight Research Center. For more information, contact the NASA Armstrong Technology Transfer Office at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to DRC-012-006.

Photonics Tech Briefs Magazine

This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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