# Topic: Re: How can I calculate the power number and mixing time?

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Since you measure both the power and mixing time, you already have the answer.&nbsp; The power number is just the power at the speed and impeller diameter, the same as for one impeller.&nbsp; If the impellers are different sizes, pick the diameter of one, but remember which one you used for the diameter.&nbsp; You can use the new power number the same way you would use the power number for a single impeller.&nbsp; The only difference is that the power number for two impellers will be larger than the power number for either impeller separately.&nbsp; If the impellers are of equal size and type the total power number for both impellers on the shaft, will be approximately two times the power number for one impeller.&nbsp;The same type of analysis can be done with mixing time.&nbsp; If the impellers are of the same size and type, the mixing time will be two thirds to three fourths of the mixing time for a single impeller of the same type and size.&nbsp; Using the results will be the same as if you had just one impeller.&nbsp;The only real difference, which can also be measured, is a condition where only the lower impeller is submerged in the liquid.&nbsp; In that case, you can measure both power and mixing time for one impeller.&nbsp; By difference, you can estimate the power number of the other impeller and distinguish the difference between one impeller and two impellers.
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