How to Measure Viscosity

Several Types of Instruments Can be Used to Determine the Viscosity of Fluids from Milk to Honey

In this In The Mix blog, we look at how viscosity is measured.Photo of honey dripping into a glass bowl The viscosity of fluids in the mixing process is key to determining the correct tank agitator specifications.

What is Viscosity?

Before we investigate how to measure viscosity, let’s start with a definition for it. Per Princeton University, viscosity is a measure of a fluid’s resistance to flow. A fluid with large/high viscosity (e.g., molasses) resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity (e.g., water) flows easily and fast because its molecular makeup results in very little friction when it is in motion.

The viscosity of various common fluids is shown in the table below. The values shown are in centipoise (cps). Centipoise is unit of measure for viscosity equivalent to one-hundredth of a poise. The poise is the unit of dynamic viscosity in the centimeter–gram–second system of units. It is named after Jean Léonard Marie Poiseuille, a French physicist and physiologist who lived from 1797 to 1869. Poiseuille was a pioneer in the study of fluid flow, especially human blood (Wikipedia).

Approximate Viscosity of Common Fluids (at room temperature)
Fluid Viscosity (centipoise – cps)
Water 1
Milk 3
Kerosene 10
Antifreeze 15
Vegetable Oil 40
Tomato Juice 180
SAE 30 Motor Oil 350
Honey 1500
Glue 3000
Mayonnaise 5000
Molasses 10,000
Sour Cream 15,000
Peanut Butter 250,000
How is Viscosity Measured?

Knowing fluid viscosity is crucial for ensuring the quality and performance of products used and manufactured by many different industries lubricants to sealants to food. But how exactly is viscosity measured?

A viscometer is the most common instrument used to measure viscosity. Viscometers can measure only constant viscosity (i.e., does not change with flow conditions). A rheometer is used when viscosity changes with flow conditions.

There are various types of viscometers. The following are some of the most common (Wikipedia):

  • Capillary (or U-Tube or Oswaldt) – Capillary viscometers feature a U-shaped glass tube with a capillary in one arm. A bulb sits above the capillary. Another bulb sits on the lower part of the other arm. Suction draws fluid into the upper bulb. It then flows down through the capillary into the lower bulb. Kinematic viscosity is propositional to the time it takes for the liquid to pass between marks above and below the upper bulb.
  • Vibrational – Vibrational viscometers use an oscillating electromechanical resonator to measure fluid viscosity. The damping on the resonator increases with the fluid viscosity.
  • Rotational – Rotational viscometers measure the torque required to rotate a disk or spindle in a fluid to determine viscosity.
  • Falling Sphere – With a falling sphere viscometer, a sphere of known size and density drops through stationary fluid in a glass tube. The time it takes to pass two marks on the tube is the terminal velocity. Stokes’ law and the following formula can then be used to calculate the viscosity (\(\eta\)) of the fluid:

  \(\eta = \frac{2 (ps-pl)ga^2}{9v}\)

 \(ps\) is the density of the sphere, \(pl\) is the density of the liquid, \(g\) is the acceleration due to gravity, \(a\) is the radius of the sphere, and \(v\) is the velocity of the sphere as it falls through the liquid.

A rheometer differs from a viscometer in that it can control the environment around the fluid being measured. This includes the application of a wide range of stress, strain, and strain rate.

ProQuip provides viscosity measurement in conjunction with an agitator quotation for both Newtonian and non-Newtontian fluids.

For More Information

If you need assistance with Newtonian or non-Newtonian fluids as it relates to your industrial mixing application, email ProQuip at applications@proquipinc.com or call us at 330-468-1850.

Related: 3 Ways  Viscosity Affects Your Industrial Mixer Specifications and Mixing Processs