NASA engineer Allen Parker and a team at Armstrong Flight Research Center have developed a fiber-optic-based sensing technology that accurately pinpoints and measures liquid levels. The CryoFOSS, or Cryogenic Fiber Optic Sensing System, uses fiber optic Bragg sensors, located along a single cable, to actively discern between liquid and gas states. The technology can be employed in a variety of applications, from NASA’s rockets to a winery’s storage tanks.

Sensor Technology: What is the CryoFOSS sensor?
Allen Parker

Allen Parker: The CryoFOSS sensor was developed in an attempt to do liquid-level measurement or mass gauging in a cryogenic tank environment, for a rocket application like a fuel tank. The actual sensor itself is made of a fiber optic strand. The strand of fiber has gratings, or little sensors, that measure stress at every quarter inch along the length of the fiber. This fiber is placed within a small polyimide or PTFE tube. Inside the tube, with the fiber, we have a small heating element, roughly about the same diameter as the actual fiber itself. That heating element, along with the fiber, allows you to make a liquid-level measurement along the length of that structure.

ST: How is the sensor used to monitor a rocket’s cryogenic fuel levels?

Allen Parker: We have a couple modes of operation. One is more of a continuous mode, while another is more of a “one-shot” mode. Basically, you energize the heating element. You apply a current, and that will heat up the local area around that heating element, which also contains the fiber. The fiber will tell you how the environment is absorbing that heat. You use the gratings inside the fiber as temperature devices. You simply monitor how the environment will suck away the heat that you’re injecting through the heating element.

For the portion of the sensing structure that is located in the liquid, the thermal conductivity of the liquid would be higher than in the gas region. You would see in your data that the temperature would be transferred into the liquid a lot faster than it would be in the gas region. You would see that by way of a significantly lower temperature reading in that particular portion of the fiber data set.

ST: How is the CryoFOSS sensor better than current liquid-level gauges?

Allen Parker: The CryoFOSS sensor is better (in my view) because we are making a continuous liquid-level measurement. The sensors that I have seen demonstrated, whether they use a silicon diode or a thermocouple rake, are very discrete in nature. So you maybe have a sensor at the 75-percent fuel point, the 50-percent fuel point, or at the 25-percent fuel point. Once the liquid reaches that level, then you would have in your data set either an “on” or “off” to indicate if you were in liquid or in the gas region.

The CryoFOSS system includes a fiber optic connector and sensor. (Image Credit: NASA)

This particular CryoFOSS sensor gives you a continuous indication of liquid level throughout the length of the fiber optic sensor itself. So if you take this CryoFOSS sensor and install it in your tank from the top to the bottom, you can get a quarter-inch spatial resolution of liquid-level indication from the top to the bottom.

ST: Why are these measurements so important?

Allen Parker: Being able to get a real indication of liquid level, especially for rockets, is important — both on the fueling part of the process, and knowing when your tank is full or coming to a point where it’s full. Also, what’s probably more important: When you’re expelling the fluid through a burn process, you want to be able to monitor that in real time and feed that back into the computer that controls the burning of that process. Being able to have an accurate indication of where the liquid level is is very important for that whole burn process.

ST: How is accuracy ensured, and how accurate are these measurements?

Allen Parker: It really is on a per-application basis. We’ve seen that it can vary a little based upon whether you’re purging with a hot gas, not purging with a hot gas, or using liquid hydrogen or liquid nitrogen. In terms of an ideal situation, we are able to resolve it to within a half of an inch, or a quarter of an inch, based upon how we have the system set up — within at least one percent of accuracy.

ST: In what other applications could this be used?

Allen Parker: Another potential area is in the oil and gas industry. For instance, you may have a storage tank that has crude oil with water mixed in, or maybe even some sediment, based upon the type of processing being done in extracting the oil from the ground. We believe that this technology could be used to tell you how much oil, water, or gas that you have in a storage tank.

Of course, there are industries like wine and beer, where you have these large storage tanks, and they want to know exactly what the liquid levels are. This sensor is not limited to just cryogenic environments. It can be used in normal environments — wherever you have an application where you need to know liquid level.

ST: What was your role in the development of the CryoFOSS sensor?

Allen Parker: NASA Kennedy, along with Marshall, came to us because we were developing systems using this FOSS technology. They asked a question: Could we use this measurement technology, based on fiber optics, to give us a continuous indication of cryogenic liquid level? Our first response was: “Well, we don’t know, but we can definitely go out and do some crude measurements at Marshall [Space Flight Center, located in Huntsville, AL],” where they were conducting these experiments.

We took our system out and put it in an environment where we had some liquid nitrogen and did our first liquid-level measurement attempt. We found that the fiber in and of itself was not able to make that measurement. That’s when we came up with the idea — this was my direct part — to couple that fiber with a heating element, to excite the local area around the fiber to see how the environment would absorb the heat, and use the fiber as an indication of that. It was really a team effort. My part was coming up with using a heating wire to help us make that liquid-level measurement.

ST: What’s next with the development of CryoFOSS?

Allen Parker: We’re continuing to do testing at NASA Marshall. We’ve been testing liquid hydrogen in a large tank that’s symbolic of a fuel tank. Then, the thought is that we would move to liquid-oxygen-type applications and additional tank applications with different configurations. We’re continuing to test, and continuing to partner with the rocket manufacturers out here to try and have this technology adopted.

ST: What do you think is most exciting about the sensor and its capabilities?

Allen Parker: What’s really exciting is that the sensor itself is fairly simple. You have a tube, you have a fiber, and you have a heating element. It’s relatively inexpensive. The egress inside the tank is, I believe, one of the simplest solutions: You have two metallic conductors and an optical fiber that passes through the barrier of the tank. That’s pretty simple in comparison to some of the other tanks being used.

It’s also a part of the fiber optic sensing family; you have a sensor that can give you liquid-level measurement, but that sensor plugs into a system as just another fiber that could make additional strain or temperature measurements. That’s the versatility of the technology. One system could do liquid-level measurement, strain measurement, temperature measurement, shape measurement — all simultaneously on one system. So for a rocket, which is a very complex structure made up of multiple different types of systems that require different types of parameter measurements, the FOSS technology really has a lot of potential.

The CryoFOSS sensor is available for licensing. Through partnerships and licensing agreements with industry, NASA’s Technology Transfer Program ensures that NASA’s investments in pioneering research find secondary uses that benefit the economy, create jobs, and improve quality of life. For more information, visit www.technology.nasa.gov .


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This article first appeared in the October, 2016 issue of Sensor Technology Magazine.

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