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How Can I Measure Viscosity?

Posted by Art Gatenby on Nov 12, 2013 2:29:00 PM

 

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How Can I Measure Viscosity? The answer, like most answers in the measurement world, is it depends. To find out what it depends on we'll start with defining viscosity.

What is Viscosity?  

Viscosity is the mechanical friction between molecules in motion, and the resistance to deformation because of mutual attraction of the molecules (in other words, resistance to flow). 
There are two types of viscosity.  
1. Dynamic viscosity, also known as absolute viscosity, is the tangential force per unit area required to move one horizontal plane with respect to another horizontal plane at a unit velocity when maintained a unit distance apart by the fluid. 
viscosity picture
Kinematic viscosity is the ratio is the ratio of absolute/dynamic viscosity to density.

Learn More About Viscosity

Ways to Measure Viscosity

Before the begining of the 20th Century, in the 1800's, the first measurements of vicsosity were made using capillary tube methods. Start with that the following is a brief summary of the different techniques/instruments that have been developed and are in use today. 

Capillary Viscometer 

The earliest methods for measuring viscosity were based on using capillary tubes and measuring the time it took for a volume of liquid to pass through the length of the tube. These developments were in place before the turn of the 20th century and are known as Ostwald or Ubbelohde viscometers.

Zahn Cup

Similar to this method is the Zahn Cup, which is a small container with a handle and a small hole in the bottom. The time it takes to empty the cup through the hole is correlated to viscosity. The Zahn cup is often used in the paint industry.

Falling Sphere Viscometer

Another technique is the Falling Sphere Viscometer, in which a sphere of known density is dropped into the fluid sample and the time it takes for the sphere to fall to a specified point is recorded. This method has been used on ships to monitor the quality of the fuel going into the ship’s engine. A similar product is the Falling Piston Viscometer.

 Vibrational Viscometer

Vibrational Viscometers measure the damping of an oscillating electromechanical resonator immersed in a fluid. This technique is often used in-process to give continuous readings in a product stream, batch vessel, or in other process applications. 

Rotational Viscometer

The rotational viscometer measures the torque required to turn an object in a fluid as a function of that fluid’s viscosity. This method is frequently used in quality control and production laboratories.

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The simple liquids react in a simple way to speed of flow and time and are called pure or Newtonian fluids. These pure fluids include things like water and milk.

However, there is a class of non Newtonian fluids that react in very differnt ways.  The techniques for measuring these viscosities require complex testing methods and which may use other types of instruments.

I hope this has helped to explain some of the ways viscosity is measured. Let us know what questions you have by leaving a comment, or sending us an email at viscosity@cscscientific.com.

Until next time, I remain bewildered as ever.

 Art

P.S. If you are not subscribed to these Test Equipment rants and muses, sign up at the top of the page. 

By Art Gatenby

Topics: "Ask Art", Newtonian Fluid, Measure Viscosity, Viscometers, Viscosity

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