Viscosity is a measure of a fluid’s resistance to flow, often described as its “thickness”. It is measured using devices called viscometers and rheometers, with common methods including timing how long it takes a fluid to flow through a narrow tube or how fast a ball falls through it. The results are typically reported in units like Pascal-seconds (Pa·s) (SI unit) or Poise (P) and centipoise (cP) (CGS units).
How viscosity is measured？
Capillary viscometers:
Measure the time it takes for a specific volume of fluid to flow through a calibrated capillary tube under gravity or pressure. Ubelode can be taken as reference.
Falling ball viscometers:
Measure the time it takes for a ball of known size and density to fall a set distance through the fluid. The speed of the ball is inversely related to the fluid’s viscosity. A formula can be used to calculate viscosity from this data. Hoppler is the most frequently used one. Also Tylose use this viscometer
Rotational viscometers:
Measure the torque required to rotate a spindle or bob submerged in the fluid. A higher viscosity results in greater resistance, which requires more torque to overcome at a constant speed. The most common one is Brookfield.
Viscosity cups:
Use gravity to measure the time it takes for a set volume of fluid to flow through an orifice at the bottom of the cup. Ford, Zahn, and Ford Dip cups are common examples.
Units of viscosity：
- Pascal-second (Pa·s): The standard international (SI) unit of dynamic viscosity. It is equal to one newton-second per square meter.
- Poise (P): The unit of dynamic viscosity in the centimetre-gram-second (CGS) system.
- Centipoise (cP): A more commonly used subunit of the poise, where 1cP=0.01P .
- Stoke (St): The unit of kinematic viscosity in the CGS system.
- MilliPascal-second (mPa·s): Also a common unit, equivalent to one centipoise.
The following table provides a clear comparison of three different instruments represent different fundamental principles for measuring viscosity:
| Measurement Method | Brookfield (Rotational) | Hoppler (Falling Ball) | Ubbelohde (Capillary) |
| Core Principle | Measures the viscous drag torque on a rotor rotating in the fluid. | Measures the time for a ball to fall at a constant speed through a liquid in an inclined tube. | Measures the time for a fixed fluid volume to flow through a capillary tube under gravity. |
| Viscosity Type | Dynamic Viscosity (relative measurement) | Dynamic Viscosity | Kinematic Viscosity |
| Primary Fluid Suitability | Non-Newtonian Fluids (viscosity changes with shear rate), wide range. | Transparent Newtonian Fluids. | Newtonian Fluids. |
| Key Characteristics | – Versatile, can study fluid rheology. – Requires careful selection of spindles and speeds. |
– Operation is simple and direct. -Requires transparent samples. -Requires precise temperature control and timing. |
– High accuracy, ideal for thin liquids and polymer solution research. – Can avoid issues like polymer adsorption. |
| Typical Application Scenarios | Petroleum, chemicals, power, metallurgy, food, pharmaceuticals, etc. | Beverages, coatings, cosmetics, cleaners, petroleum products, pharmaceuticals, polymers, etc. | cientific research, quality monitoring, intrinsic viscosity determination for calculating molecular weight. |
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