NASA Goddard Space Flight Center has developed a sensor for microanalytical systems that measures, in real time, the flow rate and temperature of the liquid being sampled. Current sensors divert the liquid to separate temperature and flow sensors, which can result in fluid leakage and the need for a larger initial sample. This design eliminates that diversion. The system sensors measure flow rates in the nanoliter-per-minute range and temperatures from greater than 150 °C down to below -80 °C.
The sensors in this system are encased in Silicon Nitride (SiN) to electrically isolate them from the fluid flow and are suspended in the middle of the channel to maximize their sensitivity and response time. The temperature sensors are utilized as part of the flow sensor design. The temperature sensor in front (before) of the heater gives the initial temperature reading for the fluid. The sensor behind (following) the heater measures how much heat the heater was able to inject into the fluid. By knowing the thermal conductance and the heat capacity of the fluid, in addition to the power into the resistor and the change in temperature, the flow rate of the fluid can be calculated.
The sensors are fabricated on a silicon (Si) wafer that has a microchannel etched into it. A sacrificial layer is deposited on to the Si wafer to backfill the channels; this provides a planar surface to fabricate the sensors.