# Design of a Mass Air Flow Sensor Employing Additive Manufacturing and Standard Airfoil Geometry
## 3 Results  
For the purposes of a preliminary electrical characterization of the developed sensor, hot-wire principle of operation on constant current (CC) mode was selected. In more detail, a constant current of 40 mA was supplied to one of the RTDs on the sensing element, while the corresponding voltage decrease was monitored for different operating points of the engine. The power supplied, taking into consideration the initial resistance of the RTD element, was 238 mW. The air intake varied from 0 to 485 kg/h (0 to 6944 SLPM) by running the engine at appropriate operating points of speed and mechanical load applied; the maximum tested inflow corresponded to a mechanical load of 78.8%, which was not further increased due to safety reasons related to the testbed.  
We should note that the estimated pressure drop (\(\Delta\)P) between the inlet and outlet of the housing, employing the optimum topology with two bypasses pathways, was approximately 1000 Pa, according to the performed CFD simulations for a maximum air intake of 615 kg/h, while the maximum allowed air intake restriction in the testbed's IC  
Figure 10: (**a**) Engine testbed experimental setup; (**b**) the developed sensor mounted in series with the integrated MAF of the testbed.  
engine is 6500 Pa [14], significantly higher than the estimated pressure drop. Furthermore, during all the performed sequential runs of the engine, no alteration was noticed at any of the engine performance parameters monitored, which could be induced by the presence of the developed device under characterization. Thus, the presence of the bypasses on the airflow did not influence in any way the performance of the testbed's IC engine.  
For each operating point, three sets of measurements were performed, with respect to the three different configurations available; that is, for the three locations on which the RTD element was mounted (main inflow pathway, High Velocity Bypass, and Low Velocity Bypass). The recorded sensor's output is presented in Figures 11, 12 and 13 respectively; note that approximation curves were fitted in the specific experimental data, employing simple polynomial functions with a high accurate fit (R\({}^{2}\) = 0.997 or above). A comparative graph for the three sensor configurations is presented at Figure 14. According to the IEC 60751:2008 standard (international standard for industrial PT resistance sensors), the sensing RTD's resistance variation, with respect to temperature at a range of 0 to 155 \({}^{\circ}\)C, is given using the following equation [26; 27]:  
\[R(T)=R_{0}\times(1+A\cdot T+B\cdot T^{2}) \tag{1}\]  
where:  
R\({}_{0}\) is the nominal resistance value at 0 \({}^{\circ}\)C  
\(T\) is the operating temperature in \({}^{\circ}\)C  
\(A\) is an appropriate coefficient according to IEC 60751 (A = 3.9083\(\cdot\)10\({}^{-3}\)\({}^{\circ}\)C\({}^{-1}\))  
\(B\) is an appropriate coefficient according to IEC 60751 (B = \(-\)5.775\(\cdot\)10\({}^{-7}\)\({}^{\circ}\)C\({}^{-2}\))  
Figure 11: RTD on main inflow pathway. (**a**) Sensor’s output vs. incoming flow; (**b**) RTD temperature and resistance change vs. incoming flow.  
Figure 12: RTD on High Velocity Bypass. (**a**) Sensor’s output vs. incoming flow; (**b**) RTD temperature and resistance change vs. incoming flow.