It is most convenient from the view point of data reduction that the functions and be close to each other, i.e. wires 1 and 2 be identical and their overheat ratios should also be equal. The measurement of and directly gives the values of and through the calibration curves. These can be related to and as follows.

For wire 1, the normal velocity component is and the longitudinal velocity component is . The heat transfer coefficient associated with each velocity component is different. Hence the effective velocity that accounts for cooling represented by the voltage is written as

where is the ratio of heat transfer coefficients in parallel flow to cross-flow past a cylinder under attached flow conditions (i.e., Re based on wire diameter . The commonly used value of is Similarly, for the second wire