The Ostwald viscometer is used to determine the viscosity of Newtonian liquids by measuring the time required for a fixed volume of liquid to flow through a capillary tube under the influence of gravity.

The principle is based on Poiseuille’s equation, which relates viscosity to the flow time of a liquid through a capillary. The viscosity of a liquid is calculated using the following relationship:

η₁ = η₂ × (ρ₁ t₁) / (ρ₂ t₂)

Where:
η₁ = Viscosity of the test liquid
η₂ = Viscosity of water
ρ₁ = Density of the test liquid
ρ₂ = Density of water
t₁ = Flow time of the test liquid
t₂ = Flow time of water

Assumptions of Ostwald Viscometer :
The working of the Ostwald viscometer is based on certain assumptions:
• The liquid flows through the capillary under laminar (streamline) flow conditions, not turbulent flow.
• The flow is driven only by gravity, and no external pressure is applied.
• The temperature is constant during the experiment, as viscosity is highly temperature-dependent.
• The viscometer constant remains the same for both the reference liquid (water) and the test liquid.

Importance of Temperature Control :
• Viscosity of liquids decreases with an increase in temperature. Therefore, even a small change in temperature can significantly affect the flow time and calculated viscosity.
• Hence, the viscometer is kept in a constant temperature water bath (usually at 25°C) before taking measurements to ensure accurate and reproducible results.

The Ostwald viscometer measures viscosity based on the time taken by a liquid to flow between two marks under laminar flow and constant temperature conditions.


Figure 1. Viscometer

Label Explanation :
• Upper Bulb (Reservoir): Holds the liquid initially
• Lower Bulb: Collects the liquid after flow
• Capillary Tube: Thin tube where flow occurs
• Mark A (Upper Mark): Starting point for timing
• Mark B (Lower Mark): Ending point for timing

The viscosity of water at 25°C is approximately 0.89 cP. Temperature plays a critical role — hence the viscometer must be immersed in a constant-temperature water bath to maintain accuracy.