Conductivity and capacitive level sensors serve as a continuous and point-level sensors by measuring the impedance between two electrodes immersed in the liquid or between one electrode and the electroconductive tank's wall.
Capacitive level sensors with two (a) or one (b) electrodes. L = level, Z = impedance, 1 = tank, 2 = liquid, 3 and 4 = electrodes.
| Float Type Sensor ^ |
In a float-type level sensor the buoyancy force holds the float on the surface of the liquid. The float carries a member having a magnetic coupling with a transduction element (coil, magnetic reed, or Hall-effect switch), that is mounted on the outside wall of the tank and can be actuated by the proximity of the float. In some designs, the float mechanically links the switching mechanism through the sealing in the wall (e.g., bellows). The switching system can respond to the restraining force developed by a spring element connected to the float or by an actuator of a force-balance servo system.
Float-type sensors with magnetic coupling (a) or mechanical link (b). L = level,
1 = tank, 2 = liquid, 3 = float, 4 = magnet, 5 = magnetic armature, 6 = contacts,
7 = bellows, 8 = lever.
Heat-transfer level sensors are built from a heated (usually self-heated) wire, thermistor, or thermocouple, whose heat transfer undergoes a step change when the transition from gas to liquid takes place. This change causes the change in the element's resistance or electromotive force.
Heat-transfer level sensor. L = level, R = resistance, 1 = tank, 2 = liquid, 3 = resistive heated element.
An inductive-level transducer finds its application in the measurement of the level of liquid metals and other electroconductive liquids. In one of the designs, a coil is wound around a tube containing the liquid. The inductance of this coil changes rapidly as the liquid moves and approaches the coil. In another design, the transducer is introduced by a transformer with a primary coil wound on one limb of a twin-limbed iron core. The other limb is enclosed by a tube containing the liquid and forming one turn of the secondary winding. The effective resistance of this turn is inversely proportional to the height of the liquid column in the tube. The change in the height can be sensed by measuring the power consumption at the primary coil.
Variable-inductance level transducer (a), and transformer-type level transducer (b).
L = level, Z = impedance, 1 = tank, 2 = liquid, 3 = coil, 4 = core.
| Photoelectric ^ |
Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mode, a sensing system, including a light beam source and a photodetector, responds to the interruption or the attenuation of the light beam when the liquid breaks the beam path from the source to the detector. In the reflection mode, an optical prism mounted inside a tank changes the reflectance of the light when it is immersed in the liquid. The construction of the transducer is arranged so that a light source and photodetector for sensing the change in the light's intensity are mounted on the outside wall of the tank. The light beam passes through and is reflected from the faces of the prism.
Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tank, 2 = liquid, 3 = light source, 4 = photodetector, 5 = prism.
Photoelectric level sensors operate in transmittance or reflection modes. In the transmittance mode, a sensing system, including a light beam source and a photodetector, responds to the interruption or the attenuation of the light beam when the liquid breaks the beam path from the source to the detector. In the reflection mode, an optical prism mounted inside a tank changes the reflectance of the light when it is immersed in the liquid. The construction of the transducer is arranged so that a light source and photodetector for sensing the change in the light's intensity are mounted on the outside wall of the tank. The light beam passes through and is reflected from the faces of the prism.
Transmittance-mode (a) and reflectance-mode (b) photoelectric level sensors. L = level, 1 = tank, 2 = liquid, 3 = light source, 4 = photodetector, 5 = prism.
A pressure-type level sensing system contains a pressure transducer mounted at the bottom of a liquid-filled tank. The transducer responds to the pressure developed by the weight of the liquid's column. This pressure is directly proportional to the measured height.
Pressure-type level sensing system. L = level, 1 = tank, 2 = liquid, 3 = pressure transducer. Several sensing techniques are used in ultrasound-level sensors, including:
- Oscillations of quartz, ceramic or magnetostrictive elements at an ultrasound frequency have a greater amplitude in gas than in liquid. Wetting the elements causes a decrease in the amplitude, providing the detection of the liquid level.
- Point-level or continuous-level sensing is provided by measuring the time lapse between the transmission and reception of the ultrasound pulses generated by ceramic crystals at the bottom of the tank. Usually one crystal acts, alternately transmitting and receiving pulses that pass along the liquid height and are reflected from the surface back to the tank bottom. Some constructions contain separate elements for generating and receiving the pulses.
- A point-level detection is also performed by two piezoceramic crystals oriented toward each other across the inside of a tank. One of the crystals transmits ultrasonic waves and the other one receives them. The transmission is intensified when the liquid wets the crystals. The increase in the output voltage of the receiving crystal indicates that the level has reached the specific point.
Ultrasound-level sensors, a, b, and c = level-sensing systems with one crystal at side (a), bottom (b), and two crystals at side (c) of tank; L = level, 1 = tank, 2 = liquid,
3 = piezoelectric crystal, 4 = pulse generator, 5 = pulse receiver.
In a vibrating-element level sensor, the oscillations of a member (paddle) are damped when it is immersed in the liquid. The attenuation of oscillations indicates that the liquid has reached the measured level. The oscillations are stimulated and sensed by electronic means.
Vibrating-element level sensor. L = level, 1 = tank, 2 = liquid, 3 = vibrating paddle, 4 = excitation coil.
A weighing sensing system for measuring level determines the level with load cells placed underneath the bottom of the tank or connected to the tank by a mechanical link. If the tank's weight and liquid's density are known, the level is readily calculated using data obtained with the cells.
Weighing sensing system for measuring level. L = level, 1 = tank, 2 = liquid, 3 = load cell.
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