The volume of mobile phase passing through the column in unit time. The flow rate is usually measured at column outlet, at ambient pressure (p a) and temperature (T a, in K); this value is indicated with the symbol F. If a water-containing flowmeter was used for the measurement (e.g. the so-called soap bubble flowmeter) then F must be corrected to dry gas conditions in order to obtain the mobile phase flow rate at ambient temperature (Fa): \[F_{\text{a}}=F\ (1- \frac{p_{\text{w}}}{p_{\text{a}}})\] where p w is the partial pressure of water vapour at ambient temperature. In order to specify chromatographic conditions in column chromatography, the flow-rate (mobile phase flow rate at column temperature, Fc) must be expressed at T c (kelvin), the column temperature: \[F_{\text{c}}=F_{\text{a}}\ \frac{T_{\text{c}}}{T_{\text{a}}}\]