Miller Magazine Issue: 131 November 2020

56 COVER STORY MILLER / November 2020 the laws of gases and thermodynamics. See Figure 2 and Figure 3. This fluctuation can be as wide as of 500 ppm in 24 hours, dropping concentration from 550 ppm to 50 ppm and turning the fumigation into a certain failure. Gas equilibrium Fumigators tend to believe that gas equilibrium is achieved automatically in most fumigations. This can be true only in very small volumes like in containers. Most other storage volumes like silos, warehouses, grain bags, bunkers, sheds, ship holds and large stack fumigations require specific actions to achieve and maintain gas equi- librium. Forced gas recirculation (also called ‘j-system’) or silo-thermo-siphoning are ways to help gas distribu- tion through grain mass. In most fumigations the moni- toring points are few and the frequency of measuring is once a day. Under these terms it is impossible to capture fumigant fluctuation and thus impossible to test if the Figure 3: Phosphine concentration fluctuating between 850 and 450 ppm during a 12-day treat- ment, tracking the day-night temperature. Figure 4: The black vertical line is the moment when gas recirculation is stopped. Equilibrium is immediately lost and product parts remain untreated as phosphine concentration drops below 200 ppm on the third day of the treatment.