Flowmeters - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 67

The fact that spinner mountings are not frictionless causes the real response to
differ from the ideal. Additionally, the effects of the viscosity of the fluid cause
further deviations from the ideal. Figure 5.9 shows the same plot of rps against fluid
velocity but adds the effects of fluid viscosity. The viscous-fluid response lines are
shown as being continuous; however, the friction of the spinner in its mounting
results in a threshold below which the spinner will not turn. The true response there-
fore is as shown in Fig. 5.10 . Note that where fluid velocities are small, regardless
of direction, the spinner will not turn at all.
Fig. 5.9 Spinner response in viscous fluids. Courtesy Schlumberger
Fig. 5.10 True response of spinner. Courtesy Schlumberger