How easy is it to measure flow?

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Can solids flow?

It is usually said that flow can occur only when the substance has fluidity. Solids like powder, coal stacks, etc. do not have fluidity flow and cannot occur for such substances. Still, there are some cases where the flow of solid substances is also required to be measured, e.g., coal is fed to the boiler, and bagasse is also used to feed the boiler as fuel. There are certain methods to measure such stuff while it passes over conveyor belts, but measurement of the flow of such items is not the scope of this article. But we will be covering problems while handling the measurement of slurries and pastes.

Fluids are always either in liquid form or in gaseous form. In these forms, they have fluidity. When the fluid is passing from one place to another, it is called a flow of that fluid. Flow can occur only when a pressure differential occurs across two given points. If there is no differential pressure, there cannot be any flow, and fluids are stagnant in such cases.

Measuring the flow:

When the fluid is abundant, no one is interested in measuring it, but whenever it is rare, costly, or required for processing or collecting some data on its usage, the flow rate needs to be measured, and thereby the total quantity. There are various ways and means to measure the flow rate of various types of fluids.
There are two types of fluids:

  • Liquids
  • Gaseous fluids
Is fluid measurement essential?

Whenever expensive fluids are used in the process, e.g., gases like LPG, CNG, and argon, or liquids like furnace oil or diesel, etc., it is extremely important to measure and control their utilization. Measurement helps not only in maintaining the preciseness of the process but also in finding the right cost of the final product.

Process industries need to add many times hazardous liquids like acids, alkalis, or other organic liquids to the main vessel or reactor in a certain proportion. Here,  it is extremely essential to measure them accurately before adding them for control purposes.

In industry, the fluid is either entering or leaving process equipment. Its amount per unit time, which means the flow rate, must be known, as every piece of equipment is designed to handle a specific amount of fluid.

The effluent generated in all the processing industries is to be treated before being disposed of to nature, as toxic water cannot be disposed of from natural sources. This is mandatory. In this case, unless the effluent water is measured with fairly good accuracy, it cannot be treated properly. The same is the case with sewage.

These days, even irrigation water is becoming a scarce commodity. The government bodies are well aware of this fact and have started measuring domestic as well as industrial water consumption. This helps with the analysis of consumption. The government can then balance the water supply for drinking, agriculture, and industry.


Here we are considering the fluids passing through a closed conduit only. Usually, one of the two types of flow parameters is measured. Volumetric Flow or Mass Flow.

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