### IAM Search

## Fluid Mechanics - Fluid Viscosity

There are two different ways to quantify the viscosity of a fluid: absolute viscosity and kinematic viscosity. Absolute viscosity (symbolized by the Greek symbol “eta” η, or sometimes by the Greek symbol “mu” μ), also known as dynamic viscosity, is a direct relation between stress placed on a fluid and its rate of deformation (or shear). The textbook definition of absolute viscosity is based on a model of two flat plates moving past each other with a film of fluid separating them. The relationship between the shear stress applied to this fluid film (force divided by area) and the velocity/film thickness ratio is viscosity:

* *

Where,

η = Absolute viscosity (pascal-seconds)

F = Force (newtons)

L = Film thickness (meters) – typically much less than 1 meter for any realistic demonstration!

A = Plate area (square meters)

v = Relative velocity (meters per second)

Another common unit of measurement for absolute viscosity is the poise, with 1 poise being equal to 0.1 pascal-seconds. Both units are too large for common use, and so absolute viscosity is often expressed in centipoise. Water has an absolute viscosity of very nearly 1.000 centipoise.

Kinematic viscosity (symbolized by the Greek letter “nu” ν) includes an assessment of the fluid’s density in addition to all the above factors. It is calculated as the quotient of absolute viscosity and mass density:

Where,

ν = Kinematic viscosity (stokes)

η = Absolute viscosity (poises)

ρ = Mass density (grams per cubic centimeter)

As with the unit of poise, the unit of stokes is too large for convenient use, so kinematic viscosities are often expressed in units of centistokes. Water has an absolute viscosity of very nearly 1.000 centistokes.

The mechanism of viscosity in liquids is inter-molecular cohesion. Since this cohesive force is overcome with increasing temperature, most liquids tend to become “thinner” (less viscous) as they heat up. The mechanism of viscosity in gases, however, is inter-molecular collisions. Since these collisions increase in frequency and intensity with increasing temperature, gases tend to become “thicker” (more viscous) as they heat up.

As a ratio of stress to strain (applied force to yielding velocity), viscosity is often constant for a given fluid at a given temperature. Interesting exceptions exist, though. Fluids whose viscosities change with applied stress, and/or over time with all other factors constant, are referred to as non-Newtonian fluids. A simple example of a non-Newtonian fluid is cornstarch mixed with water, which “solidifies” under increasing stress then returns to a liquid state when the stress is removed.

**Go Back to Lessons in Instrumentation Table of Contents**

### Related Articles

- Disassembly of a sliding-stem control valve
- Instrumentation Documents - Process and Instrument Diagrams
- Instrumentation Documents - SAMA Diagrams
- Conservation Laws
- Analog Electronic Instrumentation
- Machine Vibration Measurement - Vibration Sensors
- Machine Vibration Measurement - Monitoring Hardware
- Machine Vibration Measurement - Mechanical Vibration Switches
- Signal Characterization
- Doctor Strangeflow, or how I learned to relax and love Reynolds numbers
- Practical Calibration Standards - Temperature Standards
- Practical Calibration Standards - Pressure Standards
- The International System of Units
- Practical Calibration Standards - Flow Standards
- Fluid Mechanics - Torricelli’s Equation
- Fluid Mechanics - Flow Through a Venturi Tube
- Elementary Thermodynamics - Temperature
- Elementary Thermodynamics - Heat
- Industrial Physics Terms and Definitions
- Elementary Thermodynamics - Heat Transfer
- Elementary Thermodynamics - Specific Heat and Enthalpy
- Positive Displacement Flowmeters
- Mathematics for Industrial Instrumentation
- True Mass Flowmeters
- Process/Instrument Suitability of Flowmeters
- Machine Vibration Measurement
- Continuous Analytical Measurement - Safety Gas Analyzers
- Industrial Physics for Industrial Instrumentation
- Metric Prefixes
- Unit Conversions and Physical Constants
- Dimensional Analysis for Industrial Physics
- Classical Mechanics
- Elementary Thermodynamics
- Fluid Mechanics
- Chemistry for Instrumentation
- Continuous Analytical Measurement - Conductivity Measurement
- Fluid Mechanics - Pressure
- Fluid Mechanics - Pascal's Principle and Hydrostatic Pressure
- Fluid Mechanics - Fluid Density Expressions
- Fluid Mechanics - Manometers
- Fluid Mechanics - Systems of Pressure Measurement
- Fluid Mechanics - Buoyancy
- Fluid Mechanics - Gas Laws
- Fluid Mechanics - Reynolds Number
- Fluid Mechanics - Viscous Flow
- Fluid Mechanics - Bernoulli’s Equation
- Elementary Thermodynamics - Phase Changes
- Elementary Thermodynamics - Phase Diagrams and Critical Points
- Elementary Thermodynamics - Thermodynamic Degrees of Freedom
- Elementary Thermodynamics - Applications of Phase Changes
- Continuous Analytical Measurement - pH Measurement
- Continuous Analytical Measurement - Chromatography
- Continuous Analytical Measurement - Optical Analyses
- Chemistry - Terms and Definitions
- Chemistry - Atomic Theory and Chemical Symbols
- Chemistry - Periodic Table of Elements
- Chemistry - Electronic Structure
- Chemistry - Spectroscopy
- Practical Calibration Standards - Analytical Standards
- Chemistry - Formulae for Common Chemical Compounds
- Chemistry - Molecular Quantities
- Chemistry - Energy in Chemical Reactions
- Chemistry - Periodic Table of the Ions
- Chemistry - Ions in Liquid Solutions
- Chemistry - pH
- Final Control Elements - Control Valves
- Final Control Elements - Variable-Speed Motor Controls
- Principles of Feedback Control
- Basic Feedback Control Principles
- On/Off Control
- Proportional -Only Control
- Proportional-Only Offset
- Integral (Reset) Control
- Derivative (Rate) Control
- Summary of PID Control Terms
- P, I, and D Responses Graphed
- Different PID Equations
- Pneumatic PID Controllers
- Analog Electronic PID Controllers
- Digital PID Controllers
- Practical PID Controller Features
- Classified Areas and Electrical Safety Measures
- Concepts of Probability and Reliability
- Process Characterization
- Before You Tune...
- Quantitative PID Tuning Procedures
- Tuning Techniques Compared
- Process Safety and Instrumentation
- Notes to Students with Regards to Process Dynamics and PID Controller Tuning
- Basic Process Control Strategies
- Lessons in Instrumentation TOC
- Supervisory Control
- Cascade Control
- Ratio Control
- Relation Control
- Feedforward Control
- Feedforward with Dynamic Compensation
- Limit, Selector, and Override Controls
- Safety Instrumented Functions and Systems
- Instrument System Problem-Solving