Conductance values will depend not only on the pressure and the nature of
the gas which is flowing, but also on the sectional shape of the conducting
element (e.g. circular or elliptical cross section). Other factors are the
length and whether the element is straight or curved. The result is that various
equations are required to take into account practical situations. Each of
these equations is valid only for a particular pressure range. This is always
to be considered in calculations.