The criterion in terms of acceptable error or desired accuracy was different for the upward and downward orientation and was decided based on the preliminary results obtained from the prediction of the void fraction in each orientation. It was discussed earlier that the void fraction in the range typically 0–0.25 is sensitive to the increase in the gas flow rate and also the percentage error in the prediction is large due to low values of the void fraction. Hence a less restrictive criterion was used for the lowest of the four specified void fraction ranges. Due to the better accuracy involved in the measurement and the high values of the void fraction typically in a range of 0.75 < a < 1, the associated percentage error was expected to be small and hence a stringent criterion was set in this range for the satisfactory performance of the void fraction correlations