We are using hydrofoil impeller with power number of 0.31. Regarding measurement of diameter we take the distance between the two edge of the impeller diagonally. Speed we check by some calibrated instrument but don't know the name.
From the description of the impeller diameter measurement, I am not sure I understand what “the distance between the two edge of the impeller diagonally” means. The “diagonally” term does not sound right regardless of which edge is used.
I believe that the correct measurement for impeller diameter should start with blade radius measurements from the shaft to the center of each blade tip (see attached figure). Alternatively measure from the shaft to the most distant corner of each blade tip. Then add half (1/2) the diameter of the shaft to each blade measurement. Then average the three blade radii and finally double the average radius to get the diameter. Measuring from the shaft to the blade tip is more consistent than trying to measure from the estimated center of the shaft. Averaging the three blades eliminates any minor differences in the diameter of each blade.
Your power error can be significant if the impeller diameter is not correct. A slight larger impeller diameter would explain your under estimate of the impeller power.
The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.