Level sensor
Substances that flow become essentially horizontal in their containers (or other physical boundaries) because of gravity whereas most bulk solids pile at an angle of repose to a peak.
Also important are the application constraints: price, accuracy, appearance, response rate, ease of calibration or programming, physical size and mounting of the instrument, monitoring or control of continuous or discrete (point) levels.
These include vibrating, rotating paddle, mechanical (diaphragm), microwave (radar), capacitance, optical, pulsed-ultrasonic and ultrasonic level sensors.
With proper selection of vibration frequency and suitable sensitivity adjustments, they can also sense the level of highly fluidized powders and electrostatic materials.
Vibrating level sensors are not affected by dust, static charge build-up from dielectric powders, or changes in conductivity, temperature, pressure, humidity or moisture content.
For materials with very low weight per unit volume such as Perlite, Bentonite or fly ash, special paddle designs and low-torque motors are used.
Fine particles or dust must be prevented from penetrating the shaft bearings and motor by proper placement of the paddle in the hopper or bin and using appropriate seals.
[3] Pneumatic level sensors are used where hazardous conditions exist, where there is no electric power or its use is restricted, or in applications involving heavy sludge or slurry.
For those conductive liquids that are corrosive, the sensor's electrodes need to be constructed from titanium, Hastelloy B or C, or 316 stainless steel and insulated with spacers, separators or holders of ceramic, polyethylene and Teflon-based materials.
Since the current and voltage used is inherently small, for personal safety reasons, the technique is also capable of being made intrinsically safe to meet international standards for hazardous locations.
A microprocessor controlled frequency state change detection method uses a low amplitude signal generated on multiple sensor probes of differing lengths.
Use of multiple sensing rods of different length allows the user to intuitively set up control switches at various water heights.
Turbulence, foam, steam, chemical mists (vapors), and changes in the concentration of the process material also affect the ultrasonic sensor's response.
In addition, the hopper, bin, or tank should be relatively free of obstacles such as weldments, brackets, or ladders to minimise false returns and the resulting erroneous response, although most modern systems have sufficiently "intelligent" echo processing to make engineering changes largely unnecessary except where an intrusion blocks the line of sight of the transducer to the target.
Capacitance level sensors excel in sensing the presence of a wide variety of solids, aqueous and organic liquids, and slurries.
The sensor contains no moving parts, is rugged, simple to use, and easy to clean, and can be designed for high temperature and pressure applications.
By using pulse modulation techniques and a high power infrared diode, one can eliminate interference from ambient light, operate the LED at a higher gain, and lessen the effects of build-up on the probe.
Alternately, they are absorbed in various degrees by 'low dielectric' or insulating mediums such as plastics, glass, paper, many powders and food stuffs and other solids.
Two basic signal processing techniques are applied, each offering its own advantages: pulsed or time-domain reflectometry (TDR) which is a measurement of time of flight divided by the speed of electromagnetic waves in the medium (speed of light divided by the square root of the dielectric constant of the medium [11]), similar to ultrasonic level sensors, and Doppler systems employing FMCW techniques.
In the latter technique, performance generally improves in powders and low dielectric media that are not good reflectors of electromagnetic energy transmitted through a void (as in non-contact microwave sensors).
Microwave transmitters also offer the same key advantage of ultrasonics: the presence of a microprocessor to process the signal, provide numerous monitoring, controls, communications, setup and diagnostic capabilities and are independent of changing density, viscosity and electrical properties.
Applications such as inside stilling tubes or external bridles or cages, offer an excellent alternative to float or displacement devices, as they remove any moving parts or linkages and are unaffected by density changes or build up.
On bulk solids and powders, GWR offers a great alternative to radar or ultrasonic sensors, but some care needs to be taken over cable wear and roof loading by the product movement.
One perceived major disadvantage of microwave or radar techniques for level monitoring is the relatively high price of such sensors and complex set up.
However, price has reduced significantly over the last few years, to match those of longer range ultrasonics, with simplified set up of both techniques also improving ease of use.
The choice of float and stem materials depends on the liquid in terms of chemical compatibility as well as specific gravity and other factors that affect buoyancy.
With the proper choice of two floats, resistive chain level sensors can also be used to monitor for the presence of an interface between two immiscible liquids whose specific gravities are more than 0.6, but differ by as little as 0.1 unit.
Due to the presence of a microprocessor and low power consumption, there is also capability for serial communication from to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal.
These sensors can be designed to keep the diaphragm free of contamination or build-up, thus ensuring proper operation and accurate hydrostatic pressure level measurements.
Nucleonic level sensors are often used in mineral crushing circuits, where an increase in gamma ray detection indicates a void, compared to being filled with ore.[15]
