Working Principle of Level Measurement , Application, Installation , Advantage disadvantage , Technical specification, comparison with others
Non-contacting radar level measurement there are two main modulation techniques, pulse radar and FMCW (Frequency Modulated Continuous Wave) radar techniques. Non-contacting pulse radar sends out a microwave signal that bounces off the product surface and returns to the gauge. The transmitter measures the time delay between the transmitted and received echo signal and the on-board microprocessor calculates the distance to the liquid surface using the formula:
Distance = (Speed of light x time delay) / 2
Guided wave radar (GWR) is also called time domain reflectometry (TDR) or micro-impulse radar (MIR). In a guided wave radar installation, the GWR is mounted on the top of the tank or chamber, and the probe usually extends to the full depth of the vessel. A low energy pulse of microwaves, travelling at the speed of light, is sent down the probe. At the point of the product level on the probe, a significant proportion of the microwave energy is reflected back up the probe to the transmitter. The transmitter measures the time delay between the transmitted and received echo signal,
ignal, and the on-board microprocessor calculates the distance to the liquid surface using the formula:
Distance = (Speed of light x time delay) / 2
There are two Types of radar Gauge
- Guided wave radar gauge
- Non contracting type radar gauge.
Radar technology is mainly put into use for detection of level in continuous level measurement applications. Radar level transmitters provide non contact type of level measurement in case of liquids in a metal tank. They make use of EM i.e. electromagnetic waves usually in the microwave X-band range which is near about 10 GHz. Hence, they can be also known as microwave level measurement devices.
Non-contact radar is radar that is based on microwave technology that sends the microwave signals through the air and not a medium. On the other hand, Guided Wave Radar instruments uses wave guide “probes” to guide radio waves into a process fluid. Both of these techniques are used to measure the surface level of fluids in a container. Below are some of the differences between the two radar level measurement techniques.
Ultrasonic level transmitter is continuous level measurement in liquids and solids with ultrasonic level sensors, Ultrasonic waves detect an object in the same way as Radar does it. Ultrasonic uses the sound waves, and Radar uses radio waves. When ultrasonic pulse signal is targeted towards an object, it is reflected by the object and echo returns to the sender. The time travelled by the ultrasonic pulse is calculated, and the distance of the object is found. Bats use well known method to measure the distance while travelling. Ultrasonic level measurement principle is also used to find out fish positions in ocean, locate submarines below water level, also the position of a scuba diver in sea.
Capacitance type level measurement point as well as continues level measurement . A capacitor is formed when a level sensing electrode is installed in a vessel. The metal rod of the electrode acts as one plate of the capacitor and the tank wall (or reference electrode in a non-metallic vessel) acts as the other plate. As level rises, the air or gas normally surrounding the electrode is displaced by material having a different dielectric constant. A change in the value of the capacitor takes place because the dielectric between the plates has changed.
The least-common form of echo-based level measurement is laser, which uses pulses of laser light reflected off the surface of a liquid to detect the liquid level. Perhaps the most limiting factor with laser measurement is the necessity of having a sufficiently reflective surface for the laser light to “echo” off of. Many liquids are not reflective enough for this to be a practical measurement technique, and the presence of dust or thick vapors in the space between the laser and the liquid will disperse the light, weakening the light signal and making the level more difficult to detect.
Level measurement using Differential Pressure transmitters close tank and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant. For example, the measured hydrostatic head in a CANDU boiler may be only three meters (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter.
In a wet leg system, the low-pressure impulse line is completely filled with liquid (usually the same liquid as the process) and hence the name wet leg. A level transmitter, with the associated three-valve manifold, is used in an identical manner to the dry leg system. At the top of the low pressure impulse line is a small catch tank. The gas phase or vapour will condense in the wet leg and the catch tank. The catch tank, with the inclined interconnecting line, maintains a constant hydrostatic pressure on the low-pressure side of the level transmitter. This pressure, being a constant, can easily be compensated for by calibration.
A full dry leg installation with three-valve manifold is If the gas phase is condensable, say steam, condensate will form in the low- pressure impulse line resulting in a column of liquid, which exerts extra pressure on the low-pressure side of the transmitter. A technique to solve this problem is to add a knockout pot below the transmitter in the low- pressure side as shown in Figure Periodic draining of the condensate in the knockout pot will ensure that the impulse line is free of liquid.
A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged. It also allows isolation of the transmitter from the process loop. It consists of two block valves – high pressure and low- pressure block valve – and an equalizing valve. shows in figure a three valve manifold arrangement. During normal operation, the equalizing valve is closed and the two block valves are open. When the transmitter is put into or removed from service, the valves must be operated in such a manner that very high pressure is never applied to only one side of the DP capsule.
Magnetic level gauge is a continues level measurement A Magnetic Level Indicator (MLI) is a vertical indicator made up of a chamber parallel to the process vessel and a column with visual indicators that show the level.
The MLI chamber contains magnetic floats that move up and down with the level and trigger or move the visual indicators in the column. The floats can also trigger any magneto strictive sensors, which are sensors that respond when they are exposed to a magnetic field.The chamber is constructed from non-magnetic material which is compatible with the process fluid(s), temperature, and pressure.
Servo level indicator is a Continuous level measurement type The servo powered level gauge uses a bi-directional motor attached to a displacer and cable. A displacer float is attached to the wire which is stored on a measuring drum. The servo motor is controlled by an electronic weighing balance which continuously senses the buoyancy of the partly immersed displacer. In an equilibrium condition, the apparent weight of the displacer balances against the force of the balancing springs when partly immersed in the fluid. A level rise or fall causes a variation in buoyancy. The detector controls an integration circuit in the bi-directional motor which turns the measuring drum, thus raising or lowering the displacer until the balance position is restored.
Nuclear type transmitter can be used in continuous as well as point level measurement its basic principle is Nuclear devices comprise a shielded radioisotope source attached to one side of a vessel or pipe and a detector placed on the opposite side. Gamma rays are emitted from the source and are focused to travel through the tank wall, the medium in the tank, and the far tank wall through to the detector. Nuclear level switches use radioisotope sources sized to provide measurable radiation at the detector when no product material is present between source and detector. It can be used Noncontact and nonintrusive level measurement of liquids and solids.