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 testing results from the TReNDS project by SEGES (agricultural cooperation in Denmark) and Arhus University. The graph shows Nitrate App NO3-N readings after 30 sec and 60 sec at temperatures of 2oC, 10oC and 20oC using lab standard samples. The 60 sec Nitrate App readings gave good results, while after 30 sec the Nitrate test strips underestimate the nitrate concentations at low temperatures.
From these result, we advise to scan the test strips after 60 sec (especially in cold weather) and estimated the accuracy of the Nitrate App measurements at ca.+/-30%. The accuracy of conventional lab measurements is ca. +/- 10%.
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!
A field test was performed by Water Authority Limburg, who collected both in-field Nitrate App measurements and laboratory samples. The comparision plot for this dataset is shown below.