## 4 Conclusions  
The linear model between the load and the instrumental signal was significant for all the sensors in the two-point bending tests, even for displacements above the SDL value. Moreover, the results were under the classical theory only for small displacements. The sensors embedded within 30 to 45 percent of the axis of the sensing area showed a linear trend between the load and the instrumental signal, an approximation error of the load inferior to 5 percent, and a coefficient of determination greater than 0.99. The sensors embedded on the edge and in the center of the sensing area of the strain gauge presented a significant linear trend, an approximation error of the load greater than 15 percent, and a coefficient of determination between 0.93 and 0.98. This was due to the error of specification of the model for these cases. For loads inferior to 20 percent of the maximum load, a loss of linearity was observed in the bending tests, which is attributed to an initial shift because of a lack of perfect embedment; whereas for loads superior to 20 percent of the maximum load, a loss of linearity was observed that can be attributed to the loss of friction between the loading nose and the surface of the sensor. The parameters of the linear model, intercept, and slope were observed to be unique for each sensor. The value of the slope was inversely proportional to the location of the embedment on the sensor. In contrast, the intercept was directly affected by the value of the initial instrumental signal, \(\mathrm{V_{so}}\), and constituted a source of error. Therefore, it is advisable to consider the \(\mathrm{V_{so}}\) value as a parameter of adjustment of the regression equation to reduce the uncertainties in the linear model. Finally, the strain gauge itself, subjected to pure bending, can be employed to measure forces of small magnitude, in the order of mN, through a linear relationship between the load and the instrumental signal, provided that it is embedded within 30 to 45 percent of the sensing area. However, to the best of the authors' knowledge, the present study is an isolated attempt to use strain gauges for the direct measurement of shear stress in open channels. Thus, it was not possible to compare the sensitivity results shown in the present research against other studies. Future studies may consider a FEM study that may determine pressure sensors' sensitivity in terms of device dimensions and linearity or improve the understanding of the dependence of various geometrical parameters on the overall sensor performance. In addition, for future research, a set of simulations to prove the capability of the proposed sensors may be conducted. Conceptualization, L.S. and D.R. ; methodology, L.S. and D.R. ; validation, L.S., D.R., and E.F.; formal analysis, L.S. and D.R. ; investigation, L.S., D.R., and E.F.; resources, L.S. ; data curation, L.S. and D.R. ; writing--original draft preparation, L.S. and D.R. ; writing--review and editing, E.F. and L.S. ; visualization, L.S. ; supervision, L.S. and D.R. ; project administration, L.S. ; funding acquisition, L.S. and E.F. All authors have read and agreed to the published version of the manuscript. Figure 9: Experimental test in an open channel. **Funding:** The APC was funded by the Universidad del Bio-Bio, Chile (Funding Number VRAPROYE-UBB2055). **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** Not applicable. **Acknowledgments:** The authors want to acknowledge the support provided by the Department of Agroindustry of the College of Agricultural Engineering at the Universidad de Concepcion, Chile and by the Department of Civil and Environmental Engineering at the Universidad del Bio-Bio, Chile. In addition, the first author would like to acknowledge the scholarship provided by the National Commission for Scientific and Technological Research (CONICYT) to carry out his doctoral studies.