Level Measurements

 Level Measurements




Level Switches

  • Float switch
  • Capacitance
  • Optical
  • Conductive
  • Tuning fork
  • Magnetic




Float Switch















When product is applied or removed (Float switch) will open or close circuit.

Application:  Float used in  liquid storage tanks for high or low level.

 


it can be fixed by external weight or with out weight in a fixed support

Capacitance Switch 




The probe and the tank wall form a capacitor whose capacitance is dependent on the amount of product in the tank:

An empty tank has a lower, a filled tank a higher capacitance

 





Optical Switch




uses an infra-red LED and phototransistor, When the sensor's tip is immersed in liquid, the infra-red-light escapes from the tip causing a change in the amount of light at the photo- transistor which makes the output change state

application: They can be used with Oil and hydraulics



Conductivity Switch




Uses a probe to read conductivity. The probe has a pair of electrodes and applies alternating current to them. When a liquid covers causing current to flow which signals a high or low level.

 


Tuning Fork





Use fork with two arms vibrating , when liquid come over them vibration changes and liquid detected

Applications: Used in (mining ,food industries and chemical processes)

 



Magnetic Switch

 Level in the measuring chamber will be the same as the level in the vessel.

 The measuring chamber is fitted with a float, which has a magnet inside magnetic switch will act when reached by float

Application : Liquid storage tank

 





Level Transmitters













  • Ultrasonic
  • Radar
  • Capacitance
  • Displacer
  • Differential pressure
Ultrasonic Level Transmitter

It works by the “Time of flight” Work by transmit and receive Ultrasonic waves 

the time it takes for the wave to reflect and get back refer to the distance measured (Level)

Time of travel is proportional to the level of product.

Ultrasonic frequency is above 20kHz




 Radar Level transmitter

A radar transmitter should be mounted on the top of a tank, 

  • Non  contact
  • Guided wave radar  (Liquid interface)

 

The transmitter sends out microwaves via the antenna, which then travel down to the product surface, the time it takes for the wave to reflect ang get back refer to the distance measured(Level), time of travel is proportional to the level of product.

applications: Hot liquid storage tank


Capacitance Level Transmitter

The probe and the tank wall form a capacitor whose capacitance is dependent on the amount of product in the tank:

An empty tank has a lower, a filled tank a higher capacitance

 




Displacer Level Transmitter


Transmission lever and torque tube transmit movement of the displacer and convert it rotor motion RVDT transfer it to electrical signal (4:20mA).

 






Hydrostatic Level Transmitter

Assume 2 m level tank contains water

P= ρgH

 = 1000*9.8*2 

=196 mbar

• Highly accurate (± 0.25% Full Span)
• Ideal for wide range of liquids
• Ranges up to 30 m H20
• Sea water, oil and chemical resistant

 


Differential Pressure Transmitter

High pressure and Low pressure are applied to the transmitter and it calculate the difference between them representing the level measured

Open vessel


assume open vessel 10.2 m level contains water

Apply High pressure port is connected to zero meters tank point

 And the low port to atmosphere.

Δ p =PH - PL

           = ( ρgH + P atm )-(P atm)

           = ρgH

          = ρ(1000kg/m3)*g(9.81 m/ s²)*H(10.2m)

          = 100,000 pa = 1 bar

So:

               “10.2 m H2O need 1 bar Δ p transmitter”

 


Closed vessel

  • Dry leg




10.2 m water

Apply High pressure port to tank bottom

 And the low port to tank highest point

Δ p =PH - PL

           = ( ρgH + P gas)-(P gas)

           = ρgH

         = ρ(1000kg/m3)*g(9.81 m/ s²)*H(10.2m)

          = 100,000 pa = 1 bar

So:

               “10.2 m H2O need 1 bar Δ p transmitter



  •   Wet leg

When the liquid ,gas or steam has an negative effect on the Transmitter champers.

Closed 1m tank fill of water and has steam above 

Δ p =PH - PL

Full tank

URV (20mA)span : ( 0 mbar)

Empty tank

LRV (4mA)zero trim : ( -98 mbar)


Point Level measurements (Switches)
Continuous Level measurements (Transmitters)





Level Indicator

Magnetic Indicator used a float which has a magnet inside magnetic indicator will turn to red color when reached by float

 


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