Patent ID: 7668694

Claim:
A method of determining a fluid level Xc in a wellbore, where the wellbore extends downward from a surface and has a centrifugal pump disposed therein for transferring fluid within the wellbore, the method comprising: determining a zero-flow input speed Uz to the centrifugal pump at which output flow from the centrifugal pump is substantially zero, without using output flow from the centrifugal pump, by monitoring a change in torque dT and a change in input speed dU and calculating a quotient dT/dU of the change in torque dT divided by the change in input speed dU, as the input speed Up is reduced, and determining Uz to be substantially a speed at which dT/dU achieves a maximum; and using the zero-flow input speed Uz to calculate the fluid level Xc in the wellbore.