Typical cryogenic tank metering systems use a series of thermocouple, RTD, or other temperature or resistive devices in a rake or array configurations. Since these operate using the thermal change between the liquid and gas fluid phases, they are limited by thermal latency (the time it takes the sensing element to respond to the temperature). In addition, cryogenic fluids often create a volatile boundary or sloshing layer. This layer causes uncertainties of the true fluid boundary in a tank. Finally, accuracy and resolution are determined by the number of sensing segments used. These are typically tied to individual data channels, which puts a strain on data acquisition systems to achieve continuous and high-accuracy values.
Using the properties of optical transmission of light through a waveguide, a method was developed, prototyped, and tested. The instrument is capable of determining the liquid level of a fluid, such as a cryogen, in ground tank metering applications or in a zero-G system that employs stratification or settling techniques.
The sensor and sensing method use a segmented optical waveguide. The loss of an optical signal through the gap between waveguides is well documented and observed. If the gap contains the fluid state, then a certain step of signal attenuation is seen. If the gap contains the gas state (or air state), different attenuation is seen. By introducing light into the waveguide and detecting the resulting signal, a single data channel can be used to resolve the liquid level. Accuracy and resolution are determined by the number of segments employed. The system is inherently safe for sterile applications and explosive or toxic fluids.
This work was done by John Wiley of Marshall Space Flight Center, Amanda Duffell of the University of Alabama Huntsville, and Valentin Korman.