In the apparent viscosity calculation, at what rotor speed (rpm) is the dial reading used?

• A. 300
• B. 600
• C. 1200
• D. 1800

Answer: (C)

Apparent viscosity is tied to the shear rate you apply when you take the dial reading. For this calculation, you use the torque read at a specific rotor speed that represents a high-shear condition, then convert that dial reading into an apparent viscosity with the instrument’s constant. Using the highest standard rotor speed ensures the result reflects the fluid’s resistance to flow under strong shear, which is how apparent viscosity is usually defined in these tests. The other speeds would give readings corresponding to different shear rates and thus different apparent viscosities, so they aren’t used for this particular calculation.