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Home >Level Instrumentation
     
 

Contents

 
   
   
Summary of Commonly Used Level Technologies
 
Load Cells

Operating Principle
: Measure tank weight with load cells to infer volume
Advantages:
   Provides actual mass reading
   Doesn't depend on knowing actual tank volume including internal members and machinery
Disadvantages:
   Ensure attachment (piping, supports, ladders, machinery) don't interfere with loading on load cells.

 
Floats

Operating Principle
: Utilizes movement of a float on the liquid surface.
Advantages:
a) Simple
b) Various float configurations are available to provide either a single point measurement or multiple readings over a limited range.
Disadvantages:
a) Tank contents must not build up on float
b) Density of tank contents must remain constant or recalibration is required.
 
Magnetostrictive level instruments

Magnetostrictive level instruments are a specialized float devices.
Operating principle: A donut shaped magnet mounted on the float. The center of the magnet is installed over a waveguide and an electric pulse is sent down the waveguide. The magnet interferes with the pulse to enable sensing liquid level.  
Advantages:
a) Provides precise continuous level measurement over a large range
b) Requires little recalibration
c) Low power consumption so useful for wireless application
d) Can be provided with multiple floats to sense multiple liquid levels such as oil on water
 
Vibration

Operating Principle: A piezoelectric crystal causes a tuning fork assembly in the tank to vibrate freely in air. If the sensor becomes immersed in the tank contents, the vibrations are impeded to provide a point level indication of tank contents.
Disadvantages:
a) Subject to damage from tank contents

 
Paddlewheel

Operating Principle
: A small electric motor rotates a flag or paddlewheel within the tank. If the sensor becomes immersed in the tank contents, free rotation of the motor is impeded to provide a point level indication of tank contents.
Disadvantages:
a) Subject to damage from tank contents
 

Differential Pressure


Operating Principle:
The head or pressure exerted on the bottom of the tank by the tank contents is sensed to provide continuous tank level. If the tank contents are maintained at atmospheric pressure a simple pressure gauge will indicate pressure. If the pressure in the tank varies, the high pressure side of a differential pressure sensor is connected to the tank bottom while the low pressure side is connected to the vapor space at the top of the tank.
Disadvantages:
a) May not be suitable for liquids that clog instrumentation.
b) Liquid density must remain constant or recalibration is required.
 

Capacitance Probe


Operating Principle:
The capacitance of the probe changes when immersed in the tank contents. The change in capacitance is detected by an electronic circuit to provide a point level indication of tank contents.
Advantages:
a) No moving parts
b) Wide temperature and pressure operating range
c) Generally requires a single tank penetration and that is at the top.
d) Some models are capable of distinguishing between tank contents (Example - oil & water
Disadvantages:
a) Must be chemically compatible with tank contents
b) Changes in dielectric constant of tank contents may necessitate recalibration
c) Build up of tank contents on probe may necessitate recalibration or cleaning
 

Conductivity Probes


Operating Principle
: Uses the conductivity of the tank contents to provide a point level indication of tank contents.
Advantages:
a) Simple
Disadvantages:
a) Only suitable for conductive liquids
 

Thermal Probes


Operating Principle
: A small electric heater in the probe tip causes the temperature of the tip to rise. If a liquid surrounds the tip, the heat is dissipated by the liquid and the temperature rise is small. If no liquid is present at the tip, the temperature rise is much higher.
 

Light Beam


Operating Principle: Liquid in the tank obscures the transmission of light between the transmitter and receiver to provide a point level indication of tank contents.
Disadvantages:
a) Restricted to liquids that reduce the transmission of light
b) Restricted to liquids that drain away from the sensor rather than collect on it.
 

Ultrasonic


Operating Principle:
A sound wave is transmitted in free air from the top of the tank down to the tank contents and the time for the echo to return to the sensor at the top is recorded to provide a continuous tank level. Alternatively some models are mounted outside the tank wall. The reflection of sound by the tank contents, when present, provide a point level indication of tank contents.
Advantages:
a) Works well for many applications
b) Doesn't require contact with tank contents
c) For continuous level applications access to top of tank only is generally required.
Disadvantage:
a) May be adversely affected by dust and foam in some applications
b) Limited temperature and pressure range.
 

Radar


Operating Principle:
A microwave signal is transmitted in free air from the top of the tank down to the tank contents and the time for the echo to return to the sensor at the top is recorded to provide a continuous tank level.
Advantages:
a) Generally more precise than ultrasonic
b) Can see through dust and foam better than ultrasonic
c) Operates over a higher temperature and pressure range than ultrasonic
Disadvantages:
a) May not work well with tank contents having low dielectric constant.
b) Can't measure interface between two liquids that don't mix such as oil and water
 

Guided Wave Radar


Operating Principle: Similar to radar except instead of propagating through free air, the radar signal is directed down a wave guide which is in contact with the tank contents.
Advantages:
a) Suitable for contents having low dielectric constants
b) Calibration can be done without the tank contents being present
Disadvantages:
a) Waveguide must be compatible with tank contents.
b) Adequate headspace above tank is required for installation of rigid type waveguides
c) Waveguide is subject to damage by equipment such as agitator in the tank
d) If the waveguide comes loose, damage to tank equipment agitation and conveying systems may result.
 

Nuclear


Operating Principle:
A gamma ray source is mounted on one side of the tank and detectors are mounted on the opposite side of the tank. The attenuation of the gamma ray radiation by the tank contents is detected at the detectors to provide point level indication
Advantages:
a) No tank penetrations are required
b) No contact with tank contents so compatibility probes are eliminated
Disadvantages:
a) Expensive
b) Requires special training and licensing


 
The above table is based on information compiled from the following articles. Please see them for a more detailed evaluation:
Choosing Level Sensors - P. Welander - Control Engineering
Keep Measurements on the Level - A. Sloley - Chemical Processing

 
Glossary
 
 
Glossary of Terms - Burkert Fluid Control - Level and other instrumentation terms

 
   
Level Calculators, Tables, & Tools
 

Tank Capacity Calculators - Water & Wastewater
Specific Gravity Lookup Calculator
- Magnetrol Orion
Dielectric Constant Lookup - Magnetrol Orion
Corrosion Resistance - Magnetrol Orion
Technical Handbook - Chemical Processing/Magnetrol


Density Concentration Tables - Mettler Toledo
Dielectric Constant Chart
- ASI Instruments
Bulk Density and Specific Gravity Chart - ASI Instruments
Dielectric Constants - K-Tek
Typical Specific Gravities - K-Tek
Density Unit Converter: - Eesiflo - Convert between different density units



 
     
 

General

 
 

Sight glass - Wikipedia
Liquid level measurement – Basics 101 Part 2
- ControlGlobal/ Endress + Hauser
In Control Part III - Liquid Level Measurement
- D. Capano - DTS/Water & Wastewater

Level Continuum Chart - ControlGlobal
What is Capacitance - Arjay
Choosing Level Sensors - P. Welander - Control Engineering
Keep measurements on the level - J. Boisvert - Siemens/Chemical Processing
K-Tek Intro K-Tek
Differential Type Level Detector - Engineer's Edge
Magnetic Bond Type Level Detector - Engineer's Edge
Conductivity Type Level Detector - Engineer's Edge
Chain Float Type Level Detector - Engineer's Edge
Ball Float Type Level Detector - Engineer's Edge

Glass Guage Type Level Detectors - Engineer's Edge
Design Tips for Sensing Probes - Chemical Processing
Pressure Vessel Level Control - Babbitt
What's the Right Level Technology for Your Application? - Lesman Instrument Company
How to Select an Ultrasonic Level System - Lesman Instrument Company


A Dozen Ways to Measure Fluid Level and How They Work -K. Hambrice/H. Hopper - Sensors Online
Keep measurements on the level
- J. Boisvert - Siemens/Chemical Processing
Introduction to Level Measurement
-SearchEng
Level Measurement and Inventory Tank Gauging - J. Taylor - Control Engineering

Non-Contact: A Story of Radar Level - Intech / July 2006 - Advantages and disadvantages of radar level measurement.
Magnetostriction and Its Application to Liquid Level Measurement - W. Williams - IDC/K-Tek
Level Sensing - Illawarra
What is Capacitance (Applied to level control) - Arjay Engineering

Transactions in Measurement & Control, Flow & Level - Omega
Leveling the Playing Field (Overview of level sensing technologies) - M. Cartier - Global Spec/George Fischer

Electrode Design - Quantum Research

Thermal Technology, Another Level Option - G. Fishman & G. Ochs - FCI/Intech Online

An Introduction to Ultrasonic Sensing - P. Shirley - Massa/Sensors
Ultrasonic Level Measurement - Pulsar Process Measurement
RF Admittance Probe Operating Theory - Delta Controls Corp
Guided Reflex Microwave Operating Principle - HiTech

Penetrating Pulse Technology - HiTech
Capacitance Level Measurement vs. Guided Wave Radar - Magnetrol
Direct Reading and Pneumatic Signal Reading - Ueling Instrument
Fundamentals of Radar Technology for Level Gauging - Krohne
Flow and Level Measurement - Omega

About Level Controllers - Global Spec
Non-contact Radar Measurement -Magnetrol
Level Control - Burkert Fluid Control
Doing Your Level Best - Plant Services - N. Battikha, P.E.
Jan/05



 
     
  Manufacturers  
 

Following are some popular manufacturers.

* Indicates manufacturer received Control Magazine's Reader's Choice Award. Please refer to the complete survey in Control Magazine, Jan 2005).

 
 
   
Electrical Properties
* Ametek Drexelbrook
* Endress + Hauser
* Emerson Process-Rosemount
  * Magnetrol  
 
  Float/Displacer Type
 
  * Emerson Process - Fischer Controls  
  * Magnetrol Orion  
  * Danaher Gems Sensors  
  * K-Tek  

* Dresser Masoneilan

     
TDR Radar
  * Ohmart Vega  
  * Emerson Process-Rosemount  
  * Endress + Hauser  

* Magnetrol

  * Krohne  
  * Krohne  
 
 

Non-Contact Radar

 
  * Ohmart Vega  
* Emerson Process- SAAB Rosemount
  * Endress + Hauser  
* Siemens-Milltronics
 
Ultrasonic
* Siemens-Milltronics
* Endress + Hauser
* Magnetrol
* Ohmart Vega
* Emerson Process- SAAB Rosemount
* Flowline
 
Point

* Endress + Hauser

* AMETEK Drexelbrook

* Magnetrol

* SOR
* Thermoelectron (Sesall)
 
Visual Level Indicator
* Magnetrol-Orion
* K-Tek
* Penberthy
* Clark-Reliance (Jerguson)
* Emerson Process Brooks
 
Level Switches
Kayden
Sika
Harwil
 
 

Please support our contributors

For a complete product and manufacturer listing visit the ISA directory

 

 

 

Copyright © 2004 by Controls Weekly | Legal | Last Modified: February 29, 2008

 

 
 
 
 
 
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