Microthermal sensors for determining fluid composition and flow rate in fluidic systems

Novel microthermal sensor principle for determining the mixture ratio of binary fluid mixtures using Föppl vortices

“…At a very low flow rate, the sensitivity is nearly independent of the position and corresponds to the sensitivity for undisturbed flow of approx. 0.2 K % −1 , which is comparable to earlier results using similar microheaters in a stopped fluid (Schmitt et al, 2013a). With increasing flow rate, the sensitivity initially decreases for all heaters.…”

“…Regarding the influence of the flow rate on the length of the vortices for water and assuming a linear correlation, an increase from 1.19 mm at 0.06 mL s −1 to 2.47 mm at 0.1 mL s −1 is observed, resulting in a sensitivity of 32 mm (mL s −1 ) −1 . In a previous work, Schmitt et al (2013a) showed that using the proposed time-of-flight measurement principle for determination of the flow rate achieved a measurement uncertainty of less than 1.34 µL s −1 in the flow rate range between 0.027 and 0.055 mL s −1 which is relevant for DMFCs. Using error propagation, this results in a measurement uncertainty of the volume fraction of approx.…”

“…If the flow velocity is known, knowledge of the vortex length allows the determination of the mixture ratio. To address applications with varying flow velocity this can be determined simultaneously by a time-of-flight method investigated by Schmitt et al (2013a) in an earlier work. Here, the temperature sensor (depicted in green on the right side at the end of the heater array in Fig.…”

“…2) is designated to be used to detect the temperature pulse induced by one of the heaters of the array. The time required for the pulse to reach the temperature sensor is inversely proportional to the flow velocity and unaffected by the mixture ratio which influences the thermal properties of the fluid, as shown by Schmitt et al (2013a). The second temperature sensor (depicted in green on the left side in front of the bluff body; cf.…”

“…To ensure this, a simple physical sensor principle for determining the mixture ratio is investigated because of the inherently greater stability and higher robustness compared to chemical sensor principles. First, sensor designs developed and experiments performed by Schmitt et al (2013a) which were based on direct measurement of thermal fluid properties, have shown promising results, but improved sensitivity and accuracy are required to meet the application requirements. The presented measurement principle is subject to patent application (Kiefer et al, 2015).…”