Level sensor Contents - Introduction.

-Types of level measurements.

- Types of level Sensor .

- Conclusion .

- References.

Introduction The process tanks are widely used for many purposes such as mixing

processes and Interim storage of working fluid, as well as treatment

and avoiding instability errors of flowing fluid.

so

The controlling of the tank level is an important function in these

applications.

Firstly that we need an equipment to represent the liquid level in the

control system These equipment are called sensor

In the language word sensor means “ device which is used to record

something that has changes with time ".

In industrial side sensor means a device used to convert physical

behavior or phenomena to electrical signal

level sensor

It is a sensor used to convert the level of fluid in tank or reservoir to

electrical signal (analogue signal or digital signal)

Types of level measurements.

i)- open tank measurement .

it is the level measurement when the tank without roof(open to

atmospheric).

Pm= Lm * S.G (inch water column )

Where Pm: maximum pressure. Lm :maximum head. S.G: liquid specific gravity

ii)- close tank measurement.

it is the measurement when the tank has roof , in this cause there are

two pressures , one caused by weight of fluid and the other form the

gas between the fluid service and roof.

Pm=Pgas + Lm * S.G (in w.c)

Where

Pm: maximum pressure. Lm :maximum head. S.G: liquid specific gravity

Pgas: gas pressure

iii)-open tank with sealed fluid.

In this cause the level sensor is supplied with seal fluid that its level is

proportional to the level of the liquid .

Span=( P.seal - P.liq_min) - (P.seal - P.liq_max )

P.liq = Lm * S.G(liquid)

P.seal = Lm * S.G(seal)

Where

Lm: max liquid level. L.seal : max seal level.

P.liq_max: max pressure of liquid . P.seal: max pressure of seal.

Types of Level sensor - Float operated type.

- Solid probe type.

- Ultrasonic type.

- Microwave radar type.

- Hydrostatic type.

- Float operated level sensor type:-

There are two types of float level sensor

1)- float switch level sensor ( digital work principle “ low or full ”)

This kind is used for alarming

applications which gives two

signals, signal that indicates low

level ( switch is off ) when level is

falling and signal indicates high

level ( switch is on )when level is

rising by

assuming that the switch is

normally open

2)- continues float level sensor (analogue work principle )

This type measure the level to determine the exact amount of

substance in a continuous manner

which gives the level of fluid in every state by converting the

displacement of floating part to equivalent electrical signal .

This type consist of floating part attached to

metal rod and there is variable resistor

which its knob is mounted on this rod .

As the floating part displaced the position of

the resistor knob will change which in turn

change the resistor value.

This change is sensed by proper circuit such

as whetstone bridge and then translated to

electrical signal .

R= K*L Where :

R - resistance.

L- length of displacement.

K - calibration constant.

Advantages :-

Has simple construction which easy to maintain

Long life stand

Disadvantages :-

The sensor must be has length of a tank (the sensor and the tank are the same in

length ), so its used in small tanks.

- Solid probe level sensor type(capacitive type):

The principle of capacitive level measurement is based on change of capacitance. An insulated electrode acts as one plate of capacitor and the tank wall (or reference electrode in a non-metallic vessel) acts as the other plate the capacitance depends on the fluid level so the empty tank has a lower capacitance while a filled tank has a higher capacitance.

A simple capacitor consists of two electrode plate separated by a small thickness of an insulator such as solid, liquid, gas, or vacuum. This insulator is also called as dielectric. Value of capacitance depends on dielectric which is used, also depends on the area of the plate and distance between the plates. When electrical signal is applied between the conductive probe and the vessel wall. The signal caused very low current flow through the dielectric (process material in the tank ) When the level in the tank drops, the dielectric constant drops causing a drop in the capacitance reading and then drops in current flow. the capacitance is (from the probe to the vessel wall)

This change of level is detected by internal circuit and translated into electrical signal .

C=K*A*€ /D

Where :

C- capacitance .

A-area of plat and the wall of tank.

€-permittivity of air

K-dielectric constant

A-area of plats

Advantages :-

-Has high accuracy range.

-Can be used in fluids of high viscosity.

Disadvantages :-

-difficult to maintains

-expensive

-the fluid must be in steady sate.

- Ultrasonic level sensor type:

ultrasonic level sensors consists of a transmitted ,receiver ,gate control unit, clock pulse generator, counter down, timing and control unit.

To measure the level with an ultrasonic sensor the module should be mounted above the tank.

When the ultrasonic signal is transmitted ,the module outputs a pulse.

When the echo is received another pulse is generated using an external counter you can measure the time between two pulses and then the distance of the liquid through this equation.

D= 0.5*V * t

Where;

D=distance of the level

V=velocity of sound 331.5 m/s

t = total time ( go and back )

Advantages :-

-Ultrasonic level sensors do not make any contact with the process fluid under

level detection.

- They consist of fixed components only hence require less maintenance

-They are usually mounted at the top of the vessel due to which they are less

likely to offer leakage problems as compared to entirely (wetted) kind.

Disadvantages :-

-Ultra sonic sensors can not be used with materials like powders and Foam.

-Also can not be used in cause of heavy vapors and unstable liquid Surfaces

-The roof must be covered by insulated material to avoid the echo signal form

repeating itself many times .

- Microwave radar level sensor type:

Radar level measurement is based on the principle of measuring the

time required for the microwave pulse and its reflected echo to make

a complete return trip between the non-contacting sensor and the

sensed material level.

Then internal circuit converts this plus signal to electrical analogue

signal which will indicates the level of fluid .

Radar level transmitters range between 5.8 GHz and 26 GHz

D=c*N/2*¥f Where

C=3 ×10^8 (light speed) , N=number of repeats

¥f =the difference between two frequencies

Advantages of this type is the same as ultra sonic type

Disadvantages

-Expensive

-Not accurate if the liquid is not steady state .

Hydrostatic level sensor type:

For many years hydrostatic pressure measurement has been the most important measuring principle in continuous level measurement.

What is meant by hydrostatics?

Hydrostatic is the weight (force) of fluid acting downwards with earth gravity on specific area

Where it means pressure

This pressure is direct proportional to the acting force (weight of fluid )and indirect with the area of the tank

P=F/A

Which can be written as

P= ρ.g.h (open tank)

Where :

ρ- liquid density

h-height of liquid

g-gravity

So as the height increase the hydrostatic pressure increase

In case of closed tank the hydrostatic pressure is

P2= ρ.g.h -P1

p2 = hydrostatic pressure .

p1= pressure of the enclosed gas in the tank .

ρ = density of the fluid .

g = gravity ( earth acceleration ) .

h = height of the liquid column.

Types of hydrostatic pressure level sensors

the hydrostatic pressure sensor in to main types

-external hydrostatic pressure sensor.

-internal hydrostatic pressure sensor.

Classification by structure :

1)- ON/OFF level sensor type :

in this type we can obtain a simple digital signal with two level conditions as maximum level or minimum level for opening and closing valve or for alarming indication.

Sensor response to the pressure variation by the linear mechanical spring

The mathematical expression of spring

F=K x

Where

F - The applied force on the piston

X- the displacement of piston K- spring constant

2)- Modulating hydrostatic pressure level sensor

in this type we can obtain an analog signal expresses Instantaneous value of tank liquid level using variable impedance of the sensitive member which responses the hydrostatic pressure by variation of flexible resisting diaphragm .

construction :

a)- diaphragm

b)-pressure sensor

(strain gauge and Wheatstone bridge ).

c)-amplifier

d)-V to I converter

e)- the body ( cover )

Conclusion : Level sensor : It is a sensor used to convert the level of fluid in tank or reservoir to

electrical signal

There are many types of level sensors which its ideal of operation depending on different

principles but the most common and the best types are:

Ultrasonic because it is modern and accurate

Hydrostatic level sensor type because it can be used in many applications and because

is has long using life .

Level sensors give us the ability to know how much the fluid level in the tank to let the

storage processing under control.

By using level sensor(on /off type) we can close or open fluid flow line or pump in such

way as closed loop system .

Hydrostatic pressure type level sensor. Advantages

-widely using in industrial side

-relatively accurate

-long life stand

-easy to calibrate Disadvantages

-the tank must be empty for maintenance or replacing the sensor

-casing leakage errors

References :- 1)-Wikipedia website .

2)-process control instrumentation technology ( book).