Simultaneous measurement of flow and volumetric heat capacity using a microfluidic sensor

This paper reports on the simultaneous measurement of flow and volumetric heat capacity of a fluid using a microfluidic sensor. This work uses a thermal flow sensor with highly doped silicon heaters integrated into the sidewalls of the flow channel. The heaters are actuated using a 2 Vrms AC voltage at frequencies from 0.1 Hz to 10 Hz. The mass flow rate is measured through the calorimetric readout principle. The readout of the up- and downstream resistive temperature sensors is synchronised with the heater power to avoid fluctuations in the output signal. Measurements are conducted using water, isopropanol, and nitrogen up to a flow rate of 1.4 g/h. A linear flow range of 0.4 g/h is recorded, and a resolution of at least 19 mg/h and 8 mg/h is obtained for water and isopropanol, respectively. Simultaneously with the flow measurements, a temperature sensor on top of the heater loop records the temperature changes at the 2ω frequency using a lock-in amplifier. The amplitude of the voltage fluctuations at the 2ω frequency scales with the inverse of the volumetric heat capacity of the fluid. The volumetric heat capacity measurements show a flow-independent response in the linear regime of the flow response. The simultaneous measurement of mass flow rate and volumetric heat capacity is reported for a given set of fluids up to 0.4 g/h. For the identification of fluids and fluid mixtures, the thermal conductivity is also needed. Furthermore, the accuracy of the applied 2ω[jls-end-space/]-method is determined at zero flow using binary liquid mixtures of isopropanol in water with a step size of 10% wt.