What is Viscosity?

The viscosity of a liquid is a measure of how easily a liquid flows typically at a set temperature. Viscosity can be measured in numerous units such as centistokes, centipoise, staybolts, mpas to name a few. In science, water is often used as the basis of comparison and with viscosity, the situation is no different. The viscosity of water is 1cp and an example of a viscous or thick fluid is tomato puree or peanut butter which is 500,000 cps meaning it is 500,000 times thicker than water. Different pumping technologies are required depending on the viscosity of the liquid as some liquids will flow more easily than others such as thick oils and greases. 

Viscosity can be altered greatly by temperature which is why when viscosity is quoted its temperature is detailed after the viscosity. Some liquids such as oils viscosity can alter wildly between a few cst and 1500cst. Positive displacement pumps prefer thicker fluids as they can achieve higher flow rates and pressures than they can with low viscosity liquids as in positive displacement pumps the liquids will sometimes recirculate within the pump head known as slip. 

Fluids can also alter when being pumped if not handled in the correct way. Some fluids remain at a constant viscosity when agitated (providing environmental conditions remain the same e.g temp)  such as water as no matter how much water is whisked, stirred or shaken its viscosity remains constant. This is known as Newtonian.

Non-Newtonian liquids alter depending on the force applied – referred to as shear in one of four ways:

• Dilatant – Liquid thickens when agitated e.g. single cream, egg whites, milk

• Psuedoplastic – Thins when agitated e.g. tomato ketchup
  

• Rheopectic – Thickens or solidifies when a force is applied e.g ink, body armour
  

• Thixotropic – Becomes thinner when force is applied e.g. Manuka Honey
  

It is vitally important when pumping shear sensitive liquids that the correct pumping technology is used to ensure liquids do not change state unintentionally and that the relevant accessories such as pulsation dampeners, frequency drives and heating jackets are selected to reduce or eliminate this risk

Low shear pumping action ensures solids remain intact such as olives, fruit nuts coleslaw eggs and base mixtures such as sauces, mayonnaise and dressings remain as an emulsion and unchanged throughout the transfer.

Shear Thinning and Thickening Example

In this video we demonstrate how substances act different according to the speed and force applied when moving through them.

Viscosity Chart

Below you can find a helpful chart that matches the given viscosity and temperature for specific substances.