Level is one of the many variables encountered in industries such as temperature (temperature), pressure (pressure) and flow (flow). Therefore, the level of measurement is one of the important things in relation to the overall sustainability of the process. Failure level measurements may result in failure of a process or may even cause harmful factor for safety. Some examples of instruments relating to the measurement of the level shown in Figure 1.

Basic principles.
In general, the level of measurement is always based on the determination of boundary (interface) of two different fluid. For example between a fluid liquid with another liquid fluid, the fluid is a liquid with gas / vapor or gas between the gas fluid. By knowing where the limit is, then the level of fluid in question will be known.

Level measurement method.
There are several ways that can be used to measure the level of which is:

1. Buoy (Floating device).
Buoy sedehana a tool that can be used in level measurement. This measurement is based on an object (buoy) is capable of floating on a particular fluid. Thus the location of boundary / interface of the two types of fluid, which is the level of fluid, can be identified based on the location of the buoy. General level measurement using a float is limited to the interface between the liquid – gas.
Figure 2 shows the use of buoys to measure the level of a tank. One drawback of this instrument is a float can not be used for fluid that changes (like changing the density or temperature).

2. Displacer.
Displacer is a level measuring instrument placed in the vicinity of the fluid to be measured and is also associated with the fluid in question. By using the rules associated vessel, the fluid level can be identified based on the boundary / interface fluid contained in the displacer has been given a specific unit scale.

3. Hydrostatic head
Level measurement using this method is based on the difference in pressure between the bottom and the surface of the fluid in a vessel / tank. The relationship between level and pressure used in this method is as follows:
P = MHD (1)
H = MP / d (2)
H: level
P: pressure
d: density
m: constant

Density of a fluid is dependent on the size of the corresponding fluid temperature, so for the precision measurement density levels must be compensated for or associated with the measured fluid temperature.

4. Ultrasonic Waves
Level measurement method using ultrasonic waves or other sound waves have the same working principle. An ultrasonic wave source is placed at the top of a vessel / tank. This ultasonic wave will propagate and reflected back to when the boundary / interface of two different fluid. Travel time used by the ultrasonic waves can be dikonvesikan to know or measure the level of fluid.

5. Radio Frequency (RF) Capacitance
RF method using electrical characteristics of a capacitor for the measurement level. Electrical capacitance between two conductors separated a certain distance (d) is an important trait in these instruments. The amount of capacitance that is owned by the two conductors can be expressed dala relationship as follows:
C: capacitance
: electric permeability
K: dielectric constant
A: area
d: distance of the two conductors

Figure 6 shows the capacitance of a capacitor. An electric probe is inserted in the tank and serves as a conductor slah, while the tank wall serves as the other conductors. When the tank was empty, the material that serves as a dielectric between two conductors is air. When the fluid has started to fill the tank there is a material change of dielectric fluid from the air into the process. This dielectric material changes cause changes in capacitance of the conductor. Further changes in capacitance is used as information to determine the level of fluid process.

6. Radio Frequency (RF) Impedance
Another method that can be used in measuring the level of the use of electrical impedance characteristics. Impedance in an AC circuit expressed by the equation:
Z: impedance
R: resistance
j: complex numbers
C: capacitance

Level switches are functioning instrument for the condition of the state ‘alive / on’ to ‘not normal’ based on certain criteria. Level switch is very useful for application in the PLC, alarm, Interlock, relay and control on-off.

Basic principles.
The basic principle of level switches are based on the level of change that will result in changes in the force created by the fluid. These lifestyle changes will lead to an attractive force on a plate to activate a switch in the condition on. This situation also applies to state the opposite.

There are several types of level switches berbasarkan used sensing elements include:
-. Level using a float switch.
-. Level switches using displacer.

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