The risks associated with introducing new hardware and methods into an operational environment have in part prohibited cryogen level measurement technology from advancing. In prior art, measurements have been made with invasive probes immersed in the cryogenic fluid. Implementing this approach would require physical retrofitting, as well as requiring the measuring instrument to make contact with the harsh cryogen fluid environment. However, an externally mounted optical measurement system would mitigate these concerns. Therefore, an optical approach was developed that uses and augments existing validated technology in a manner that does not interfere with the current infrastructure.

A proof-of-concept study was conducted to assess the feasibility of using a commercially available optical range finder coupled with the existing reed switch float system to determine if this combined instrumentation can measure the liquid cryogen tank level during run time. This approach would enhance the existing reed switch level sensing method by integrating a laser rangefinder into the cryogen tank using fiberoptic feedthroughs.

To accomplish this feasibility study and simulate runtime operation, an Acuity AR1000 laser rangefinder coupled with capacitive and temperature sensing was used to provide reference measurements of the liquid cryogen tank level while being drained. The test apparatus enabled the implementation of an optical-based level measurement utilizing an operational environment similar to current practice. This noncontact optical approach was demonstrated combining the laser rangefinder and a spherical float within a cryogenic laboratory test apparatus. The laser rangefinder was located outside the tank and emitted a signal that was reflected from a target floating on the surface of the cryogen. The rangefinder was mounted on the upper surface of the tank using an existing port, and didn’t present the potential of becoming foreign object debris (FOD). Additionally, an athermal telescope was also constructed to collect and focus light on a fiber optic bundle mounted inside the tank. By using the same material for the optics and structure, the system compensated for temperature changes, and by using an aluminum structure, the FOD concerns could be virtually eliminated.

Results demonstrated that the laser rangefinder/float repeatedly tracked simultaneous measurements made with a calibrated capacitance liquid level sensor during filling and draining operations. Additionally, an algorithm based on Kalman filter signal processing was successfully applied to both remove expected measurement fluctuations due to bobbing floats on a turbulent surface, and combined the nearly continuous optical measurement with the discrete sensor measurement to improve fluid height estimation.

This work was done by Robert Ryan of I2R, and Randy Buchanan and Anton Netchaev of the University of Southern Mississippi for Stennis Space Center. For more information, please contact I2R at 228-688-2776. Refer to SSC-00395.

NASA Tech Briefs Magazine

This article first appeared in the September, 2014 issue of NASA Tech Briefs Magazine.

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