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

Answer 1

The Correct Option is D

The correct option is (D): Ostwald viscometer.

Answer 2

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

1. Cup and bob viscometer: This is a rotational viscometer that measures viscosity across a range of shear rates. It is built for fluids whose viscosity changes with shear rate, that is, non-Newtonian fluids, since a single reading is not enough to characterise them.
2. Brookfield's viscometer: This is also a rotational instrument, using a spindle turning in the fluid at different speeds. Like the cup and bob type, it is chosen when viscosity needs to be checked at multiple shear rates, which points to non-Newtonian behaviour.
3. Pycnometer: A pycnometer measures density, not viscosity at all, so it does not answer this question regardless of fluid type.
4. Ostwald viscometer: This is a simple capillary (U-tube) viscometer that times how long a fixed volume of liquid takes to flow through a narrow tube under gravity. Because a Newtonian fluid has one fixed viscosity value no matter the shear rate, a single efflux-time measurement is enough to characterise it fully, which is exactly what this instrument gives.

Since Newtonian fluids need only one viscosity value, a single-point capillary instrument is sufficient, and that is the Ostwald viscometer.

So the correct answer is Ostwald viscometer.