The Optical Hotspot Flow Sensor (OHFS) enables optical measurement of electrically conductive liquid metal flow rate by exploiting the electrically conductive nature of liquid metals or any other conductive fluid to enable thermal tagging of liquid flow in tubes (US Patent 7,409,875). It is well suited for measuring very low flow rates, on the order of mg/s.
A pulse of thermal energy (derived from a current pulse and associated joule heating) is applied near the inlet of the sensor. This thermal feature is convected downstream by the flowing fluid. A downstream fiber optic, pointed at the fluid and connected to a high-gain photodiode, optically records ripple in the light emission associated with the increased local temperature as the hotspot in the propellant passed the fiber optic location. By measuring the time between the upstream generation and downstream observance of the thermal feature, the flow speed can be calculated using a time-of-flight analysis.
The hotspot in the upstream flow is generated by pulsing current directly across the conductive liquid channel flow; doing so exploits the high resistivity of the fluid and obviates the need for a separate resistive heating element. In order for the sensor to provide useful results, the spatial integrity of the hotspot must be maintained until it reaches the thermocouple location. The hotspot will tend to flatten out as it propagates, due to thermal diffusion. Therefore, the thermal diffusion time scale is much larger than the convective time scale, which varies from fluid to fluid. Thermal light emission only starts to be visible to photodiodes at higher temperatures (about 300 °C), so this measurement technique is limited to temperatures above this lower limit.