Abstract: Calibration experiments are done on multiphase flow loop using water and air separately. Experimental data have been used to calculate the wall friction factors in the test section. It is found that the Prandtle, Miller and Blasius equations all are not valid in the smooth pipe when the Reynolds numbers are more than 0. 3 Re₁(the Re₁ is the upper limit of smooth flow) because there are errors from 5% to 16. 7% between friction factors computed by experimental data and above equations. It is also found that the friction factors obtained by experiment data could be modeled more precisely by the Colebrook equation with a wall roughness of 0. 018 mm. Analysis and evaluation are made for above equations and their valid ranges for accurate hydraulic calculations are advised.