Liquid hydrogen and oxygen are widely used as fuels in space vehicles. Because both are highly dangerous
materials prone to explosion, detection of the liquid level in fuel tank becomes a critical element for the
safety and efficiency in space operations. Two liquid level sensing techniques are presented in this paper.
The first technique is based on optical fiber long period gratings. In this technique, the full length of a
specially fabricated fiber is the body of the probe becomes the length of the sensing fiber that is submerged
in the liquid can be detected by the interrogation system. The second system uses optical fibers to guide light
to and from an array of point probes. These probes are specially fabricated, miniature optical components
which reflects a substantial amount of light back into the lead fiber when the probe is gas but almost no light
when it is in liquid. A detailed theoretical study by computer simulation was carried out on these two
techniques in order to determine which technique was more suitable for experimental investigation. The
study revealed that although the first technique may provide more potential benefits in terms of weight and
easy installation; a number of technical challenges make it not suitable for a short term solution. The second,
probe array based technique, on the other hand, is more mature technically. The rest of the research program
was therefore focused on the experimental investigation of the probe array detection technique and the test
results are presented in this paper.
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