LEVEL MEASURMENT

Introductions Level measurement is one of the oldest measurements. The measure of industrial process level parameters is of great importance in the industrial fieldTypes of instruments Level gauge 1. Tubular type 2. Reflex types 3. Float type 1TUBULAR TypeA Generated tubular glass to is mounted on the side of the vessel by coupling flanges

Reflex typeIn reflex type level gauge the gauge glass has 90* prismatic grooves running lengthwise on the inside face.

Reflected glass

closed

type

protective case

Float type:In float operated level indicators , the primary device is a float, which by reason of its buoyancy flows the changing level of the liquid .the movement of the float is transferred to the pointer through some suitable mechanism .various type of floats are such as hollow metals spheres cylindrical shaped floats and disk shaped floats

INDICATOR

FLOAT

SCALE

Differential pressure type

RADAR TYPE FMCW (frequency modulated coundinary wave) radar principle and FFT signal analysis A radar signal is entitled from the antenna is reflected from the target and received back cat led a delay I the well. The distance of the reflecting products surface is measured by transit `T`of the micro wave signal for every meter from a target the waves travel a distance of 2mm,for which they require a time of approx,6.7as in general, the measured distance is A=c*T/2 Where c=the speed of light The FMCW radar system uses a linear frequency-modulated high- frequent signal. Transmitter frequency increase linearly with in a time interval (Frequency sweep) Since the transmission frequency change due to the time delay during signal proportion, above-frequency signals (typically, up to a few kHz) the frequency for which is proportional to the reflector distance as obtained from the difference between the current transmission frequency and the received frequency. The product level is then compound from the difference between tank height and distance.

Displacer type level gauge The gauge on Archimedess buoyancy principle which states that a body which fully or particularly immersed in fluid gets buoyant up by a force which is equal to the weight of the liquid displaced. Which is usually cylindrical in shape a it is waited by lead shorts as to skin in the liquid.

Working As liquid level changes, they amount of immersion of the displacer in the liquid changes. Change in displacer immersion causes in buoyant force acting on it. More the displacer is submerged; larger is the buoyant force and vise versa. Thus for every particular liquid level, there is certain fixed buoyant force acting of the displacer. This change in buoyant force with liquid level causes rotation of torque tube and pointer. This rotation can be used for pneumatic transmitter Electrical capacitance methodPrinciple The electrical capacitance of the sensing probe varies with the level of material and hence level changes can be recoded in terms of changes an be recoded in terms of changes in electrical capacitance of the sensing probe Construction In capacitance level detectors is an insulated capacitance probe fixed near parallel to tank wall such that the probe and metal tank wall act as the conductors of the condenser with conducting tank liquid as the dielectric medium .these two conductors are connected to capacitance detecting element.

Electric wires

Capacitance

measuring

Insulatedinstrument

capacitance

probeinsulator

liquid

Working: As liquid level changes, the dielectric constant changes due to which capacitance changes. Thus any changes in liquid level can be measured in terms of change in the capacitance Radiation method

Principle The extent of adsorption of radioactive rays depends upon the level of liquid inside the vessel; hence any changes in the liquid level can be measured in terms of change in intensity of radioactive radiations. Construction .The radiation receiving element like Geiger-Muller counter is fixed to the side of the vessel density across the source location along with the indicator

Radiation

receiver

Indicator

Radioactive

source

Working A liquid or solid level inside the vessel changes ,the amount and intensity of radioactive radiations received by the receiver changes .the intensity of radiations various inversely with the thickness of the vessel walls and the medium inside the vessel .thus larger is the level of liquid inside the vessel smaller is the intensity of radiation and vice-versa. Ultrasonic Level Detectors

This method is based upon utilization of law of reflection of ultrasonic obligations from solid or liquid surface or from the interface between two immiscible liquids. The time taken by the wave for its travel from transmitter to the liquid surface to the receiver can be taken as the measure of liquid level

Construction& working: Transmitter is the source of ultrasonic oscillations such as Piezo-element like quartz, which is positioned at top or bottom of the vessel. The ultrasonic waves from the transmitter reach the material surface from where they get reflected back and those reflected waves are received from the receiver. The time interval from the instant of an emission of the waves to the reception of the reflected rays is measured, which varies with liquid level.

change in

liquid level

TransmitterReceiver

LEVEL SWITCHES When the level increases a set limit. One of the most common methods is the operation of a magnetic level switch. The float when reaches the set level operates a switch to change contact between card no, or NC.Displacer type level switch The basic sensing element consists of a weight (Displacer) heavies than the liquid, suspended from a spring. When the liquid contacts the displacer a buoyant force is produced which causes the effective weight of the displacer to change in turn causing the spring to a new balance position which moves the magnetic contact switch. This principle allows desired level switching point to the adjusted by moving the displacer up or down. Another advantage of the system is can be used for high pressure application. Because the displacer are made out of solid material.