Flow measurement is a method of measuring how much of a material flows through a particular area of interest (Figure 1). Generally, industrial processes demand for the measurement of three types of ‘materials’ viz., solids, liquids and gases.
Measurement of Solids
Solids are usually measured on their weight-basis i.e. the flow rate of solids will be expressed as the ratio of mass of the solid per unit time or in terms of weight of the solid per unit time. Thus we have its associated units as kilogram-per-second orgram-per-second or kilogram-per-hour or gram-per-minute or tonnes-per-hour or likewise.
Measurement of Liquids
Liquids can be either measured in terms of their weight or volume. This means that the flow rate of liquids is expressed as the ratio of weight of the liquid per unit time or volume of the liquid per unit time. Accordingly, we can express their rate in units such as tonnes-per-hour, kilogram-per-second, litres-per-hour, m3-per-minute and so on.
Measurement of Gases
Gases are usually measured in terms of their volume i.e. flow rate of gases will be expressed as ratio of volume of the gas-flow per unit time. Thus the units in which the flow of gases is measured are m3-per-hour, Nm3-per-hour, etc. One more important factor to be noted while measuring the flow of gases is the fact that the gases are compressible fluids (unlike liquids) due to which their volume varies in accordance with the pressure (and also temperature). In order to account for this factor, usually the values obtained by measuring the gases at the original conditions are converted into normal temperature and pressure (NTP) base or to Standard Temperature and Pressure (STP) base.
Flow measurement is accomplished through the use of flow meters (Figure 2).
These flow meters are of different types depending on the quantity being measured and the principle on which they operate. Differential pressure meter, variable area meter, turbine flow meter, Coriolis meter, gear meter, Woltman meter, single jet meter, multiple jet meter, paddle wheel meter, current meter, Venturi meter, cone meter, linear resistance meter, Vortex flow meter, magnetic flow meter, ultrasonic flow meter, non-contact electromagnetic flow meter, etc are only a few among the numerous types of flow meters in-use.
Sensors are the devices which are used to monitor the change in a particular parameter. Accordingly, flow sensors are the devices which are used to monitor the changes in the flow-rate of a particular state of matter. These are expected to record even the slightest changes in the flow rate and are thus expected to be of high precision.
Existence of numerous kinds of flow meters and flow sensors are a direct consequence of the importance which is attributed to the concept of flow measurement in numerous industrial applications including the design of automatic control systems.