Sulphur dioxide SO2 No.5EC ppm 5EC 2 5 40 30 20 10 SO2 + I2 + H2O → 2HI + H2SO4 Reaction principle Possible coexisting substances and their interferences Nitrogen dioxide − (unclear demarcation) Hydrogen sulphide ≧ 1/10 + White (unclear demarcation) Substance Concentration Interference Changes colour by itself to Permeation tube method Calibration gas generation 0.4 ppm(1 pump stroke) Purple → White Temperature 0 to 40 ℃ (32 to 104 ℉) correction used Relative humidity 0 to 90 % correction not used 10 %(for 2 to 10 ppm) , 5 %(for 10 to 40 ppm) 10 tubes for 10 tests 36 months Detecting limit : Colour change : Operating conditions : Relative standard deviation : Tube quantity and number of tests per box : Shelf life : Performance 2 to 40 ppm 1(50mL) 1 45 sec Measuring range Number of pump strokes Correction factor Sampling time Scale