Two optical level point sensors were designed and tested for use in cryogenic fluids, such as hydrogen, whose liquid and vapors phases are difficult to distinguish visually. Two separate designs were manufactured, both using the same operating principle of differing refractive indices between liquid/glass and vapor/glass. One sensor design was a fused silica rod with a 45° angle cut at one end. The second sensor design used a commercial-off-the-shelf fused silica prism collimated with a laser and diode and connected to an oscilloscope to display signal output. Both sensors were tested in a liquid nitrogen environment as an initial risk reduction step. For the glass rod design, a binary visual response (dark or light) could be directly observed that indicated whether liquid or vapor was present at the end of the rod with the 45° angle. The prism sensor produced a signal on the oscilloscope that changed sharply in amplitude depending on whether liquid or vapor was present at the sensor location.
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