A method of dispensing an additive, comprising: dispensing an additive from a secondary feeder to a primary feeder; dispensing the additive from the primary feeder to a mixing device; obtaining a primary measurement from the primary feeder feeding the mixing device, wherein the primary measurement is a total weight of additive fed from the primary feeder over a time interval; characterised by obtaining a secondary measurement from the secondary feeder that feeds the primary feeder, wherein the secondary measurement is a total weight of additive fed from the secondary feeder over the time interval; comparing the primary measurement and the secondary measurement; and (i) when the difference between the primary measurement and the secondary measurement is less than a selected threshold, continuing either or both of (i-a) dispensing the additive from the primary feeder to the mixing device or (i-b) dispensing the additive from the secondary feeder to the primary feeder; or (ii) when the difference between the primary measurement and the secondary measurement is greater than the selected threshold, halting either or both of (ii-a) dispensing the additive from the primary feeder to the mixing device or (ii-b) dispensing the additive from the secondary feeder to the primary feeder, and further recalibrating at least one of the primary feeder or the secondary feeder. dispensing an additive from a secondary feeder to a primary feeder; dispensing the additive from the primary feeder to a mixing device; obtaining a primary measurement from the primary feeder feeding the mixing device, wherein the primary measurement is a total weight of additive fed from the primary feeder over a time interval; characterised by obtaining a secondary measurement from the secondary feeder that feeds the primary feeder, wherein the secondary measurement is a total weight of additive fed from the secondary feeder over the time interval; comparing the primary measurement and the secondary measurement; and (i) when the difference between the primary measurement and the secondary measurement is less than a selected threshold, continuing either or both of (i-a) dispensing the additive from the primary feeder to the mixing device or (i-b) dispensing the additive from the secondary feeder to the primary feeder; or (ii) when the difference between the primary measurement and the secondary measurement is greater than the selected threshold, halting either or both of (ii-a) dispensing the additive from the primary feeder to the mixing device or (ii-b) dispensing the additive from the secondary feeder to the primary feeder, and further recalibrating at least one of the primary feeder or the secondary feeder. (i) when the difference between the primary measurement and the secondary measurement is less than a selected threshold, continuing either or both of (i-a) dispensing the additive from the primary feeder to the mixing device or (i-b) dispensing the additive from the secondary feeder to the primary feeder; or (i-a) dispensing the additive from the primary feeder to the mixing device or (i-b) dispensing the additive from the secondary feeder to the primary feeder; or (ii) when the difference between the primary measurement and the secondary measurement is greater than the selected threshold, halting either or both of (ii-a) dispensing the additive from the primary feeder to the mixing device or (ii-b) dispensing the additive from the secondary feeder to the primary feeder, and further recalibrating at least one of the primary feeder or the secondary feeder. (ii-a) dispensing the additive from the primary feeder to the mixing device or (ii-b) dispensing the additive from the secondary feeder to the primary feeder, and further recalibrating at least one of the primary feeder or the secondary feeder.