Question

What is the working principle of Hoeppler viscometer?

• A. Flow rate of fluid is inversely proportional to its viscosity
• B. Rise of liquid in Capillary correlates with its viscosity
• C. Time of Fall of a ball in a fluid filled tube correlates with viscosity
• D. Torque required to shear a cone against the fluid lined plate correlates with viscosity

Hint:

Hoeppler = Falling/Rolling Ball model. Viscosity is evaluated by logging how much time a calibrator sphere takes to sink through the testing matrix.

Answer 1

The Correct Option is C

Concept: Viscosity measurement principles differ based on the geometry and design of the viscometer. The Hoeppler viscometer is a classic rolling/falling ball viscometer used to determine the dynamic viscosity of Newtonian liquids. Detailed Mechanical Evaluation:
• Hoeppler Mechanism: A ball of known density and diameter is allowed to roll/fall under gravity through an inclined glass tube filled with the sample liquid. The tube is angled at a precise deviation (usually around $10^{\circ}$) from the vertical axis.
• Mathematical Correlation: The absolute dynamic viscosity ($\eta$) is directly related to the rolling/falling time ($t$) of the ball across a designated distance by the equation: \[ \eta = t \cdot (S_b - S_f) \cdot K \] where $S_b$ is the density of the ball, $S_f$ is the density of the fluid, and $K$ is a characteristic viscometer constant.
• Deconstruction of incorrect variants:
• Option (A) links to Ostwald or capillary flow models under external driving forces.
• Option (B) reflects surface tension measurements or simple capillary rise phenomena, not dynamic viscometry.
• Option (D) reflects rotational instruments like the Cone-and-Plate viscometer used for non-Newtonian profiles. Thus, the falling time mechanism directly characterises the working rule of a Hoeppler apparatus.