Question

Which of the following type of viscometer is used for the measurement of viscosity of a Newtonian fluid:

• A. Cup and bob viscometer
• B. Brookfield's viscometer
• C. Pycnometer
• D. Ostwald viscometer

Hint:

For Newtonian fluids (fluids with constant viscosity), the Ostwald viscometer is commonly used in laboratory settings for accurate viscosity measurements.

Answer 1

The Correct Option is D

The Ostwald viscometer is used for the measurement of the viscosity of a Newtonian fluid. It works on the principle of measuring the time it takes for a fluid to flow through a narrow tube under the influence of gravity.

Answer 2

The question asks which type of viscometer is suited to measuring the viscosity of a Newtonian fluid, one where viscosity stays constant regardless of the shear rate applied. Let's look at each instrument's working principle.

1. Cup and bob viscometer: This is a rotational instrument that measures viscosity across a range of shear rates by rotating one cylinder inside another. Its real strength is in capturing how viscosity changes with shear rate, which is exactly the behaviour of a non-Newtonian fluid, making it more suited to those systems than to simple Newtonian ones.
2. Brookfield's viscometer: A Brookfield viscometer is also a rotational, spindle-based instrument, widely used for creams, gels, and suspensions whose viscosity depends on the speed of rotation. Because it is built to reveal shear-dependent behaviour, it is again primarily suited to non-Newtonian fluids.
3. Pycnometer: A pycnometer is a precision flask used to measure the density (specific gravity) of a liquid, not its viscosity at all, so it does not answer this question's requirement in the first place.
4. Ostwald viscometer: This is a capillary, one-point viscometer. Liquid is drawn up past two etched marks on a U-shaped glass tube and the time it takes to flow back down through a narrow capillary under gravity is timed. Since the flow here occurs at a single, fixed shear rate set by the capillary geometry, this method only gives a meaningful, single viscosity value for fluids whose viscosity does not change with shear rate, which is the defining property of a Newtonian fluid.

Because Newtonian fluids have one true viscosity value regardless of shear rate, a simple single-point capillary measurement like the Ostwald method is sufficient and appropriate, unlike the rotational instruments built to probe shear-dependent, non-Newtonian behaviour.

Therefore, the correct answer is Ostwald viscometer.