# Design of a Mass Air Flow Sensor Employing Additive Manufacturing and Standard Airfoil Geometry
## 4 Discussion
The specific flow rate for this particular housing inlet area corresponds to an incoming velocity of 15.2 m/s; thus, the reduced velocity inside the bypass is estimated at 7.6 m/s (considering an approximate 0.5 decrease factor), which is significantly lower than 14 m/s and in accordance with the previously performed analysis, concerning the saturation of the sensor's signal. Similarly, in the case of the Low Velocity Bypass pathway for the maximum applied inflow of 6944 SLPM (485 kg/h), the estimated incoming velocity is 26.3 m/s, while the reduced velocity inside the bypass is estimated at 5 m/s, considering an approximate 0.19 decrease factor according to the CFD simulations performed. Therefore, no saturation at the sensor's signal is expected for the specific bypass topology, even for the maximum air intake (615 kg/h), since the estimated value is substantially below 14 m/s; note that this is supported by the obtained experimental results. As a reference, in the case of the sensing element directly mounted on the main inflow pathway of the housing, saturation occurred at an incoming velocity of 11.4 m/s, which was closer to the value of 14 m/s in comparison to the other topologies, as expected. We should note at this point that the exact values of air velocity above the sensing element cannot be directly compared for the three different topologies without incorporating a non-negligible error, since even a modest difference regarding the location of the element inside each bypass pathway, with respect to the one assumed for the simulated flow profile, can result in a reasonable difference between the velocity estimated and experimentally measured; apparently, presuming that two or three elements are situated concurrently at the exact same locations, with regard to the assumed positions at the corresponding simulations, can enhance this error. Furthermore, the obtained results from the simulation study, regarding the optimization of each topology, can potentially incorporate, to some extent, different modelling errors (different models/meshing topology, etc.). Regarding the foremost measurement errors present in the specific study, we should take into consideration the errors introduced by the sensing element (RTD) employed and the reference MAF sensor integrated in the diesel engine testbed. In more detail, considering 126 \({}^{\circ}\)C as the maximum RTD temperature, the introduced error by the sensing element (IEC 60751:2008, Class F0.6) [26; 27] was equal to \(\pm\)1.86 \({}^{\circ}\)C, less than 4.6% of the measured temperature decrease for all three topologies; for constant current mode of operation with 40 mA supplied to the element, the above figure corresponds to a maximum variation of the sensor's output of \(\pm\)0  
in order to quantify the above statement, further electrical characterization of the sensor should be performed at the engine testbed at different air temperatures, a procedure that is practically not feasible for adequate temperature fluctuation and with no additive value for a preliminary investigation of the developed device's functionality, employing the CI mode of operation. That being said, the above analysis, based on the experimental findings, strongly supports the CFD study results, concerning the optimum design geometry of the housing, while the adaptation of standard airfoil geometry to the housing design leads undoubtedly to a substantial increase in the measurement range of the final sensing device.