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 . <EOS>