Level measurement is one of the used measurements in industry. In the most industrial applications level can be defined as the height of a liquid or solid above a reference line. If the dimensions of a vessel are known, then the volume or mass of the contents can be determined by measuring the level. Hence the contents can be directly displayed in units of level (meters), volumes (liters) or mass (kilogram).
Industrial plants must store raw materials and finished materials, process and mix many materials and buffer some materials for intermediate processing. Moreover, some plants want to know how much material they bought or possess in terms of volume or mass. Some wants to know how much they will mix and some wants to know how much finished product they have or will sell. For all of these reasons an accurate level measurement is become necessary.
Different types and shapes of tanks exist in industry. They include storage tanks, buffer tanks and other process tanks. Tank shape as well as physical and electrical properties of the liquid, presence of foam and material agitation are some of the reasons why different types of level measurement device exist.
A wide variety of technologies exist to measure the level of substances in a vessel, each exploiting a different principle of physics. The major level measurement technologies in current use can be divided as following.
1. Continuous level measurement
2. Point to point level measurement
3. Direct level measurement
4. Indirect level measurement
1. CONTINUOUS LEVEL MEASUREMENT
In much processes continuous level measurement is required because it is necessary to know at all times the exact position of the level in relation to one or more specific reference points. A gauge or sight glass can be used to continuously observe the position of the interface.
2. POINT-TO-POINT LEVEL MEASUREMENT
Certain process requires only that the level of a substance be maintained between two points. Frequently these points are a high level and a low level. When this is required, a point-to-point level measurement system is used. Such a system activates control device only when predetermined levels are reached.
3. DIRECT LEVEL MEASUREMENT
Level can be measured directly by using the varying level of the liquid as a means of obtaining the measurement. Direct level measurement does not require compensation for changes in level caused by changes in temperature. Direct level measurement shows the actual level of the liquid.
4. INDIRECT LEVEL MEASUREMENT
Level can be measured indirectly by using a variable, which change with the liquid level to actuate the measuring mechanism. Indirect level measurement converting measurements of some other quantity such as pressure to level by determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above the measuring point can also be determined.