Coal is transported by conveyor and tripper cars to coal hoppers, day vessels, or silos. These hoppers feed ball mills, which in turn feed the pulverized coal to the boilers. There will be at least one hopper for each boiler, and there are typically two boilers per unit. These hoppers and silos storing the pre-pulverized coal are large and very dusty. The silos hold several hours’ supply of coal and can continue to supply coal to the boiler in the event of a problem in the coal handling system. As coal is critical for the continuous process, it is necessary to monitor and control the actual amount of coal in order to prevent process stoppages.