I realized that the value of my sense resistor is too high. To get a reasonable RPMs value in the range of desireable viscosities, the voltage across it must be about 4.5V. That caused problems with my 3-comparator circuitry because Vref is only 5V. The lowest-acceptable viscosity will produce an output voltage that is higher than Vref, that's the core of the problem.
The solution is easy, just (1) change the value of the sense resistor to 25 ohms and (2) reduce the voltage to the motor-control circuit to about 2.25V. Then the comparator circuitry will have enough range to deal with a low/good/high viscosity range.
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