Measuring "viscosity number" most commonly refers to determining kinematic viscosity, a fundamental fluid property that quantifies a fluid's resistance to flow under the influence of gravity. This measurement is crucial across various industries, from manufacturing to quality control.
The precise measurement of viscosity involves determining the time in seconds required for a fixed volume of fluid to flow a known distance under gravity through an orifice of a calibrated viscometer at a controlled temperature. This efflux time is then converted into kinematic viscosity.
The Kinematic Viscosity Measurement Process
Kinematic viscosity is typically measured using a capillary viscometer, such as the Ubbelohde or Ostwald types. These instruments rely on the principle of fluid flow through a narrow tube under gravitational force.
1. Essential Equipment
- Calibrated Viscometer: Specific types of glass capillary viscometers (e.g., Ubbelohde, Ostwald, Cannon-Fenske) are chosen based on the fluid's expected viscosity range. These viscometers are precisely manufactured and calibrated to ensure accurate results.
- Constant Temperature Bath: Maintaining a precisely controlled temperature is critical, as viscosity is highly sensitive to temperature changes. A water bath or oil bath capable of maintaining temperature within ±0.01 °C (or even tighter tolerances for high precision) is used.
- Stopwatch: A digital stopwatch accurate to at least 0.1 seconds is required to measure the efflux time.
- Pipette/Syringe: For carefully introducing the fluid into the viscometer.
2. Measurement Procedure
The standard procedure for measuring kinematic viscosity (e.g., as per ASTM D445 or ISO 3104) involves these key steps:
- Temperature Control: The viscometer, containing the test fluid, is immersed vertically in the constant temperature bath. It must reach thermal equilibrium with the bath before any measurements are taken, typically for at least 15-20 minutes.
- Fluid Introduction: A precisely measured volume of the fluid is introduced into the viscometer's reservoir. Care is taken to avoid air bubbles.
- Drawing Fluid: The fluid is drawn up through the capillary tube to a specified mark above the upper timing mark, often using suction or gentle pressure.
- Timing the Efflux: The suction is released, allowing the fluid to flow back down through the capillary under gravity. The time (in seconds) it takes for the fluid meniscus to pass between two calibrated timing marks is recorded.
- Replication: Multiple readings (usually at least three) are taken. These readings must be within a specified percentage of each other (e.g., 0.3% to 0.5% agreement) to ensure accuracy. If they are not consistent, the measurement is repeated.
3. Calculation
Once the efflux time (t) in seconds is determined, the kinematic viscosity (ν) is calculated using the following formula:
ν = C × t
Where:
- ν is the kinematic viscosity, typically expressed in square millimeters per second (mm²/s), also known as centistokes (cSt).
- C is the calibration constant of the viscometer, provided by the manufacturer or determined through calibration with certified reference fluids. This constant is specific to each viscometer and often varies with temperature.
- t is the efflux time in seconds.
Understanding "Viscosity Number" in Polymer Science
While the direct measurement method primarily yields kinematic viscosity, the term "viscosity number" can also refer to specific concepts in polymer science, particularly related to the characterization of polymer solutions. In this context, "viscosity number" is often synonymous with reduced viscosity or is a key component in determining the intrinsic viscosity (also known as limiting viscosity number).
These "viscosity numbers" are derived from measurements of the kinematic viscosity of the polymer solution and the pure solvent.
- Relative Viscosity (η_rel): The ratio of the solution viscosity to the solvent viscosity.
- η_rel = (Solution Viscosity) / (Solvent Viscosity)
- Specific Viscosity (η_sp): The increase in viscosity due to the polymer solute.
- η_sp = η_rel - 1
- Reduced Viscosity (η_red or Viscosity Number): Specific viscosity divided by the polymer concentration (c). This value gives an indication of the hydrodynamic volume of the polymer molecule.
- η_red = η_sp / c
- Intrinsic Viscosity (η or Limiting Viscosity Number): This is the limit of the reduced viscosity as the concentration approaches zero. It reflects the individual polymer molecule's contribution to viscosity in a solvent and is an important measure of a polymer's molecular size and shape.
- η = lim (c→0) (η_sp / c)
| Term | Formula | Unit | Significance |
|---|---|---|---|
| Kinematic Viscosity | C × t | mm²/s (cSt) | Fluid's resistance to flow under gravity |
| Reduced Viscosity | η_sp / c | dL/g (or cm³/g) | Measure of polymer's effect on solution viscosity |
| Intrinsic Viscosity | lim (c→0) (η_sp / c) | dL/g (or cm³/g) | Individual polymer molecule's hydrodynamic volume |
Factors Affecting Measurement Accuracy
To ensure precise and reliable viscosity measurements, several factors must be carefully controlled:
- Temperature Control: As mentioned, even small temperature fluctuations can significantly alter viscosity readings.
- Cleanliness of Viscometer: Any residue or particles in the capillary can obstruct flow and lead to inaccurate times. Viscometers must be meticulously cleaned after each use.
- Fluid Homogeneity: Ensure the fluid is well-mixed and free from air bubbles or suspended particles.
- Vertical Alignment: The viscometer must be perfectly vertical in the bath to ensure accurate gravitational flow.
- Operator Technique: Consistent timing and proper fluid handling are essential for reproducibility.
By adhering to standardized procedures and maintaining strict control over environmental conditions, accurate and reliable viscosity measurements can be achieved, whether for direct kinematic viscosity or as a precursor to calculating polymer-specific "viscosity numbers."
