Chapter08 02 Analog Level Sensor

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FAB10402– SENSORS TECHNOLOGY

Chapter_10-2-Others Sensors : Level sensor

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INDUSTRIAL AUTOMATION AND NETWORKING SECTION

2. ANALOGUE LEVEL SENSORS

These sensors provide a continuous reading of levels from minimum to maximum.

This means that they provide an analogue signal (0-10Vdc, or 4-20mA).

Types of analogue level sensors:

� RF admittance (capacitance)

� Sonic

� Sounding tape and reel-type

� Differential pressure

2.1 RF Admittance (Capacitance) Level Sensor

An AC power source is used to provide the radio frequency for the probe. An op amp is

used in conjunction with a transformer to provide a series of pulses whose frequency is in

the radio wave range.

The secondary of the transformer s center tapped and connected to an op amp that

measures the amount of power used by the probe to transmit the energy wave.

When the probe is in free air, and no material or liquid is close to it; the amount of energy

will be minimal. When the material or liquid begins to rise and changes the dielectric

between the probe and the tank, the amount of energy to transmit the energy wave will

change.

The amount of change will be proportional to the level of material or liquid. The output of

the op amp will provide a 4-20mA signal that is proportional to the level of the liquid.

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Fig 17: Electrical diagram of an RF admittance level sensor.

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2.2 Sonic Level Sensor

Fig 18: Ultra sonic level sensor. The sensor has a transmitter and receiver in its

head. Sound waves are transmitted to the liquid and the time it takes for them

to be reflected and received by the sensor is detected. This time is converted

to distance so the sensor can determine how far the level is from the probe.

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2.3 Sounding Tape and Reel-Type Level Sensor

A weight is attached to a line and the line is lowered from top of a tank. The weight will

cause the line to have tension as long as it is suspended in air.

When the weight comes into contact with the level of material it is measuring, the amount of

tension will be reduced, and a sensor can detect this change.

When the tension changes, a mechanism stops the line from lowering and begins to

retrieve the line. A spool is used to reel the line in and the number of rotations the spool

uses to reel the line completely to the top is measured and converted to a level

measurement.

The transducer signal is converted to a useable 4-20mA output signal.

Fig 19: An application of a sounding-

type level sensor that uses a weight

and line to measure the level of

granular material in a tank.

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Fig 20: An application of a sounding-

type level sensor that uses a weight

and line to measure the level of liquid in

a tank. The weight in the application will

float on the liquid when it reaches its

surface.

Fig 21: Example of a sounding

type level indicator shown mounted

in place to indicate the level of a

tank.

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Fig 22: An example of the spool mechanism used to measure the amount of line that

has been lowered to determine the level material. The output of this amplifier

is 4-20 mA.

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Fig 23: An example of the output of the level sensor sent to a display.

Fig 24: An example of display indicating position of gate.

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Fig 25: An example of a level sensor connected to a floating roof for a tank, and the

output signal is sent to a transmitter and a display.

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2.4 Differential Pressure Level Sensor

Fig 26: A pressure differential sensor that

determines the level of liquid by using the

pressure of a column of liquid to calculate

the level.

From pressure conversion tables is known

that water exerts 0.434 psi per each foot of

water column. This means that if the water

is 1 foot deep in the tank, the pressure at

the bottom of the tank would be 0.434 psi.

Formula to be used:

P = depth (ft) x pressure of 1 ft of water

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Example 1:

Find the pressure at the bottom of a tank that is 40 ft deep.

Ptotal = D x P at 1 ft

= 40 x 0.434

= 17.36 psi

Example 2:

Determine the level of water in tank if the pressure at the bottom of a tank of water is

24.5 psi.

Ptotal = D x P at 1 ft

D = Ptotal / P at 1 ft

= 24.5 psi / 0.434 psi

= 56.45 ft

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Fig 27: A differential pressure sensor used to measure the level in a buried tank.

Fig 28: An application where

differential pressure sensor can be

mounted in the top of a tank or in the

side of a tank to determine the level

of liquid in the tank.

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AUT 0112 – SENSOR TECHNOLOGY.

Chapter_06-Cylinder sensor

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Chapter08 02 Analog Level Sensor