Flow measurement is an important parameter in process industries. The plant material balances, quality control, operation etc. of continuous as well as batch process become impossible without flow measurement. To understand basic principles of flow measurement one should be familiar with the relationship between fluid flow and pressure, temperature, viscosity, density.Two basic properties – Density and Viscosity play an important role in flow measurement.© Density applies to fluids in static phase© Viscosity applies to fluids in motion.

DENSITY : In simple terms, density is a measure of closeness of molecules in a substance. Density is defined as mass per unit volume.

d = m / v,

Where, d = density, m = mass, v = volume.

SPECIFIC GRAVITY : Another term commonly used to express density of fluid is specific gravity. Specific gravity of a liquid is a reference number that compares the density of liquid to the density of water at standard conditions. Specific gravity of a gas is a reference number that compares the density of gas to the density of air at standard conditions.·

PRESSURE : The flow of fluid which is caused by a force, which shall be a change or a difference in pressure. There are different types of pressure in flow we consider it as static pressure and differential pressure. Static pressure is defined as the pressure of the fluid at rest. It is independent of the kinetic energy of the fluid. Dynamic pressure is the increase in pressure above static pressure that results from the transformation of the fluid kinetic energy into potential energy. Differential pressure is the pressure difference between the two related pressures. Differential pressure is used to determine the fluid flow rate.·

VISCOSITY : Viscosity is the property that determines how freely fluid flow. The viscosity of a fluid refers to its physical resistance to flow. Fluids have various degree of viscosity. Such variations results from internal friction between particles of the substance. A substance with a higher viscosity has a higher resistance to flow. For example, two

substances with different viscosities are oil and water. Water pours freely while oil pours more slowly. Oil is more viscous than water and water much more viscous than gas.

Viscosity reduces with increase in temperature. For example, when molasses is heated its viscosity will decrease. There are several viscosity units; the most widely used being the centipoises. The viscosity of water at 680F is 1.0centipoises. The viscosity of kerosene at 680F is 2.0centipoises.


Incompressible flow is fluid flow under conditions of constant density.



Compressible flow is fluid flow under conditions that cause significant change in density.


Mass flow is the amount of fluid measured in mass unit that passes a given location per unit time.


Steady flow is a flow in which the flow rate in a measuring section does not vary significantly with time.


Pulsating flow is a flow rate characterized by irregular or repeating variations.


Static pressure is the pressure of a fluid which is independent of its kinetic energy.


It is a theoretical pressure that could be developed if a flowing fluid could be brought to rest without loss of energy.


It is the maximum allowable operating pressure for an internally pressurized vessel, tank or piping system.


With respect to flow differential pressure is the pressure drop across a restriction.


It is the decrease in pressure of a fluid as it passes through a restriction.

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