The accuracy of the app mostly depends on the accuracy of the test strips. Test strips typically just give a rough indication of the actual concentration. The graph below gives the results from tests at two temperatures (20 and 4 degrees Celsius or 68 and 41 Fahrenheit). From these result, we estimated the accuracy at 30%. The accuracy of conventional lab measurements is ca. 10%.
A known issue with testing strips in general is that the color reaction has a slight temperature dependency. At low temperatures, this leads to an underestimation of the higher nitrate concentrations for which a correction is possible. Note that the app is designed for outdoor use in good light conditions. See also the Tips and Tricks for getting the most accurate readings!