As for the second dimension the total length of columns is the addition of both columns, i.e. 80 to 90 meters, the flow will be lower than for the first dimension, which only has a column of 50 to 60 meters. To balance this, we need to add to the column of the first dimension an empty column with the same internal diameter and with a length of 25 to 30 meters. The column switching systems are generally controlled by the GC itself (on–off relay) or by external modules like the one proposed by SGE International, the MDS series for one GC .