If we examine the froth column of this bank of cells you will notice that the column becomes SHALLOWER from cell to cell.
This is to capture the particles that are lightly mineralized, and is floating very low in the froth. The last cell will have the shallowest froth column, in fact in many cases, the pulp level will be so close to the surface, and the froth so thin that there is always the danger of the pulp slopping over into the overflow. The mineral that is floating on the surface of this cell is a fair representation of the amount of mineral that is being discharged from the bank of cells.
The method that the froth level is controlled in a bank of cells take different forms, depending on the design of the cell. There is one method where each cell is separated from each other by a gate. This gate is called a WEIR gate and it travels up and down to increase or decrease the pulp level, which will increase or decrease the froth column. Of course this would be considered a variable and if it was changed the other variables in the circuit would have to be changed to compensate for it.