On the Development of a Robust Optical Fiber-Based Level Sensor

A novel optical fiber-based probe was used to determine the fluid level in a nitrate salt up to 400 °C. The probe consisted of a single mode optical fiber and heating element running coaxially, both encapsulated in a protective stainless steel sheath. The temperature was determined continuously along the length of the optical fiber with the use of Rayleigh backscatter based optical frequency domain reflectometry. With the heater activated, the convection coefficient of the surrounding medium defined the axial temperature profile of the probe. The gas-fluid interface was determined by fitting a numerically calculated temperature profile with actual optical fiber data by minimizing the sum of square residuals. The probe was successfully tested over a range of 18 cm, revealing an accuracy of ±1.7 mm and a response time as low as 5.3 s.