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| Home >Level Instrumentation | |||
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Contents |
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| 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. |
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| 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. |
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| 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 |
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| 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 |
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| 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 |
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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. |
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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 |
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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 |
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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. |
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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. |
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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. |
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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 |
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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. |
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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 |
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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 |
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| Glossary | |||
Glossary of Terms - Burkert Fluid Control - Level and other instrumentation terms |
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| Level Calculators, Tables, & Tools | |||
Tank
Capacity Calculators - Water
& Wastewater
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Lasers Come to Level Measurement
- D. Spitzer - ControlGlobal.com
Laser Level Applications - Hitech
Optech Level Monitors - Optech
Silo Level Measurement - Dimetix
Level Monitors - Applications - Optech
Laser Scanners offer volumetric measurement of bulk solids - Bnet Business Network
Laser Level Transmitters - K-Tek
See also Laser Displacement Sensors
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).
Non-Contact Radar
Please support our contributors
For a complete product and manufacturer listing visit the ISA directory
Copyright © 2004 by Controls Weekly | Legal | Last Modified: April 4, 2008
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