Our team at NASA’s Armstrong Flight Research Center developed a highly accurate method for measuring liquid levels using optical fibers. Unlike liquid level gauges currently on the market that rely on discrete measurements to give broad approximations of liquid levels, Armstrong’s innovative fiber optic method provides precise and accurate measurements. Specifically, this novel method is capable of providing measurements at ¼-inch intervals within a tank. This significant leap forward in precision and accuracy in liquid level sensing offers significant benefits to many industries. Dubbed CryoFOSS because it was originally designed to monitor cryogenic fuel levels in rockets using fiber optic sensing system (FOSS) technology, this innovation can be used in many industrial, medical, and pharmaceutical applications.

The sensing system uses fiber optic Bragg sensors located along a single fiber optic cable. These sensors actively discern between the liquid and gas states along a continuous fiber and can accurately pinpoint the liquid level. The technology uses a resistive heater wire bundled with the optical fiber. The heater is pulsed to induce a local temperature change along the fiber, and the fiber Bragg grating data is used to monitor the subsequent cooling of the fiber. The length of fiber in the liquid cools more rapidly than the portion of the fiber in the gas above the liquid. The measurement system accurately establishes the location of this transition to within ¼-inch.

Armstrong’s liquid level sensing technology represents a significant advancement in the state of the art for measuring cryogenic liquid levels. Conventional methods for measuring cryogenic liquid levels rely on cryogenic diodes strategically placed along a rod or rack. The diodes are mounted in pre-selected, relatively widely spaced positions along the length of a rod; this configuration provides limited, imprecise data. Furthermore, each diode on the rod has two wires associated with it, which means a single system may require a large number of wires, making installation, connectivity, and instrumentation cumbersome.

The innovation provides liquid measurements with much greater precision, achieving measurements at ¼-inch intervals. The streamlined system uses just two wires and a fiber optic strand, which greatly simplifies installation and instrumentation. Furthermore, the innovation is not susceptible to electromagnetic interference (EMI) and can be used in corrosive or toxic liquids without damaging the fiber or contaminating the liquid.

Due to its extraordinary accuracy and ease of use, this measurement system offers important advantages for a wide range of applications beyond cryogenic liquids. It can be used across various industries that require level measurement within large containers or for materials that are difficult to measure (e.g., cryogenic liquids):
• Aerospace, especially liquid fuel launch vehicles and satellites
• Chemical or refinery plants to monitor the facility’s fluid flow
• Industrial tanks to measure level of cryogenic or other liquids (e.g., for liquid natural gas storage and transport)
• Oil and gas industry for storage applications
• Food and beverage manufacturing
• Pharmaceutical manufacturing
• Medical and hospital operations

http://www.nasa.gov/offices/ipp/centers/dfrc/technology/DRC-012-006-Liquid-Level-Sensing.html