Can 10ppm Really Be Zero?
Test strips make fairly accurate measurements of pH levels, concentrations of sanitizers such as chlorine & hydrogen peroxide or the amount of Vitamin C in juice. The common factor in all these is that the measurement is made on a bulk solution. In other words, the liquid in question occupies some volume of a test tube, bottle or bucket.
Several of these strips have been repurposed for detecting residual levels of quat sanitizers & sulfite preservatives on the surfaces of things. Unfortunately, this has lead a misinterpretation of government regulations by so called experts who are either training or inspecting commercial food operations.