Updated: Nov 15, 2020

In this article, you will learn some important definition used in flow measurement.


In the flow metering, the nature of flow can be described by a number – Reynolds’s number which is the average velocity x density x internal diameter of pipe divided by viscosity.

Reynolds’s number has no dimensions of its own. From Reynold’s number, it can be determined whether the flow is laminar or turbulent.

Reynold’s number < 2000, the flow is laminar.

Reynold’s number > 4000, the flow is turbulent.

4000 < Reynold’s number > 2000, the flow is transitional.

Between these two values, the nature of the flow is unpredictable. In most industrial applications, the flow is turbulent.

Although measurement can be made without consideration of Reynolds’s number, greater accuracy is possible when a correction based upon it is made.

Re = Velocity X Density X Internal diameter of pipe



Industrial flow measurement includes measuring the flow rate of solids, liquids, and gases. There are two basic ways of measuring flow on a volumetric basis and the other on mass (weight) basis.

Solid materials are measured in terms of either weight per unit time or mass per unit time. Liquids are measured either in volume rate or in mass (weight) rate. Gases are normally measured in volume rate.

Fluids are classified into two types, namely incompressible and compressible. Fluids in the liquid phase are incompressible whereas fluids in gaseous phase are compressible.

Liquid occupies the same volume at different pressure whereas gases occupy different volume at different pressures.


The units used to describe the flow measurement can be several types depending on how the specific process needs the information.

Solids → normally expressed in Wight rate like Tons/hrs, Kg/minute etc.

Liquids → Expressed both in weight rate and in volume. Examples: Tons / hour, Kg/minute, liters/hour, m3/hour etc.

Gases → Expressed in volume rate at NTP or STP like stdm3/hour, Nm3/hour etc.

Steam →Expressed in weight rate like Tons/hour/kg/minute etc.

Steam density at different temperature and pressure vary. Hence the measurement is converted into a weight rate of water which is used to produce steam at the point of measurement.

Flow can be generally classified into two:

Flow in a closed pipe or conduit

Flow in an open channel

Selection of type of the flow measurement is based on various considerations such as cost, accuracy, size and shape of process pipe, nature of the fluid, the amount of allowable pressure drop, type of output required etc.

Flow measuring devices may be

· Flow rate meters

· Volumetric flow rate meters

· Mass flow rate meters

· Compound flowmeters


The measurement unit to express the rate of flow actually refers to the velocity of the flow or how rapidly the substance moves. Flow rate is a measure of distance moved by a particle in a given period of time.

Commonly used units are feet/second, kilometre/hr, miles/hr etc.


The method of measurement used to indicate the volume of fluid that passes a point over a period of time is volumetric flow rate.

Usual volumetric flow units are gallons/minute (GPM), cubic feet/second, cubic meter/hr, liter/hr etc.


Mass flow rate determines the amount of mass that passes a specific point over a period of time. Mass flow rate application determines the weight or mass of the substance flowing through the system. Mass flow measurement is independent of temperature and pressure.

Common units are tons/hr, kilogram/hr etc.


Meters that have the feature of both positive displacement and velocity are known as compound meters. The unit of measurement is usually in litres, but sometimes in cubic meters.

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