Liquid level sensing for harsh environment applications using distributed fiber optic temperature measurements

This work demonstrates the measurement of liquid level from spatially distributed temperature measurements using optical frequency domain reflectometry. The goal of this work is to provide initial evidence for a liquid level sensor concept that could be applied in harsh environments such as those with: conductive, flammable, or other chemically aggressive media; electromagnetic interference; high temperatures (up to ∼1000 °C); and neutron/gamma radiation. The sensor concept includes fiber optic sensors, along with an insulated heater wire assembled inside a protective Inconel 600 sheath. When the heater is not powered, the sensor provides spatially distributed temperature measurements along the length of the fiber. When the heater is powered, the difference in heat transfer in the liquid vs. vapor sections leads to a higher fiber temperature at locations above the liquid/vapor interface. The initial demonstration described in this work tested the sensor in a tank of water that was drained in increments of 2.5 cm. A model for determining liquid level from the distributed temperature measurements was developed that resulted in an accuracy of ±0.5 cm. The paper addresses potential effects of axial conduction through the sensor sheath, as well as ways to limit these effects to avoid “smearing” of what would otherwise be a step change in temperature at the liquid/vapor interface.