Viscosity And Density

Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow.
Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.
The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), which is identical to 1 kg·m−1·s−1
The cgs physical unit for dynamic viscosity is the poise

1 P = 1 g·cm−1·s−1

The relation between poise and pascal-seconds is:

10 P = 1 kg·m−1·s−1 = 1 Pa·s
1 cP = 0.001 Pa·s = 1 mPa·s
CentiPoises (cp) = CentiStokes (cSt) × SG (Specific Gravity)


Viscosity and Reference Temperatures

The viscosity of a fluid is highly temperature dependent and for either dynamic or kinematic viscosity to be meaningful, the reference temperature must be quoted. In ISO 8217 the reference temperature for a residual fluid is 100oC. For a distillate fluid the reference temperature is 40oC.

• For a liquid - the kinematic viscosity will decrease with higher temperature
• For a gas - the kinematic viscosity will increase with higher temperature
  

Density
In physics the density (ρ) of a body is the ratio of its mass (m) to its volume (V), a measure of how tightly the matter within it is packed together[1]. Its SI units are kilograms per cubic metre (kg/m³). It is also sometimes given in the cgs units of grams per cubic centimetre (g/cm³).

Density is defined by:

rho = m/V

Various substances have different densities, and it is this quantity that determines how they interact when mixed together. For example, in SI units the density of lead is 11.35 x 103, that of water is 1 x 103, and that of cork is 0.24 x 103. The lead has a greater density than water so it sinks; the cork has a smaller density so it floats.

In some cases the density is expressed as a specific gravity or relative density, in which case it is expressed in multiples of the density of some other standard material, usually water or air.

For a homogeneous object, the formula mass/volume may be used. The mass is normally measured with an appropriate scale; the volume may be measured directly (from the geometry of the object) or by the displacement of a liquid. A very common instrument for the direct measurement of the density of a liquid is the hydrometer. A less common device for measuring fluid density is a pycnometer, a similar device for measuring the absolute density of a solid is a gas pycnometer.

units for density are:

• kilograms per cubic metre (kg/m³)
• grams per cubic centimetre (g/cm³)
  

Units outside the SI

• kilograms per litre (kg/L). Water generally has a density around 1 kg/L, making this a convenient unit.
• grams per millilitre (g/mL), which is equivalent to (g/cm³).
  

They also happen to be numerically equivalent to kg/L (1 kg/L = 1 g/cm³ = 1 g/mL).

In U.S. customary units or Imperial units, the units of density include:

• ounces per cubic inch (oz/in3)
• pounds per cubic inch (lb/in3)
• pounds per cubic foot (lb/ft3)
• pounds per cubic yard (lb/yd3)
• pounds per gallon (for U.S. or imperial gallons) (lb/gal)
• pounds per U.S. bushel (lb/bu)
• slugs per cubic foot.