High resolution and large sensing range liquid level measurement using phase-sensitive optic distributed sensor.

Liquid level sensor with large sensing range and high-resolution is essential for the application of industry monitoring. In this work, a distributed optical fiber liquid level sensor is proposed and demonstrated based on phase-sensitive optical time domain reflectometry (φ-OTDR). In the basic of the thermal optic effect, the temperature change will induce the fluctuation of the effective refractive indexes of the fiber core, as well as the fluctuation of the optical path of the light transmitting in the fiber. Therefore, the φ-OTDR can detect the liquid level with a large measurement range by interrogating the phase information along the fiber due to the temperature difference between the liquid and air. Further, the scattering enhanced optical fiber (SEOF) is used as the sensing fiber to improve the signal to noise ratio (SNR) of the phase signal. Moreover, a high sensitivity liquid level sensing head by wrapping the SEOF on a heat conductive cylinder is designed and optimized to improve the sensing resolution. In the experiment, the proposed distributed liquid level sensor presents a high sensitivity of 73.4 rad/mm, corresponding to a competitive liquid level resolution of 142μm based on the noise floor of 10.4 rad within 160 s. The field test validates a large sensing range of 20 cm which is limited by the cylinder length, while a potential sensing range could reach 320 m with the sensing fiber of 40 km, proving a dynamic range of 127.1 dB. The proposed liquid level sensor with large dynamic range and high sensing resolution can benefit potential application in smart industry platforms and biomedicine monitoring.