Linear Velocity and Flow Rate

The flow rate through a capillary column whose inner diameter is less than 0.53 mm is difficult to measure accurately and reproducibly by a conventional soap-bubble meter. Instead, the flow of carrier gas through a capillary column is usually expressed as a linear velocity rather than as a volumetric flow rate. Linear velocity may be calculated by injecting a volatile nonretained solute and noting its retention time, tM (seconds). For a capillary column of length L in centimeters. 

For example, the linear velocity of carrier gas through a 30-m column where methane has a retention time of 2 minutes is 3000 cm per 120 s or 25 cm/s. If desired, the volumetric flow rate F (mL/min) may be computed from the relationship 

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At the same flow rate, columns of varying IDs operate at different linear velocities therefore, to compare the performance of several different columns, you need to operate each column at the same linear velocity. Figure 6-6 gives the relationship between linear velocity and flow rate for selected column diameters. This figure assumes the same packing density in each column. The linear velocity is marked on the top line. The corresponding flow rate for various diameter columns would be found on the lower line. For instance, a linear velocity of 0.5 cm/sec would occur at 4.0 mL/min on a 4.6-mm ID column, at 3.0 mL/min on a 4.0-mm ID column, and at 0.75 mL/ min on a 2.0-mm ID column. Linear velocity can be calculated for each column by the equation... 

Recommended Linear Velocities and Flow Rates with Helium and... 

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