Guided wave radar

Guided wave radar or time domain reflectometry (TDR) works very similarly to pulse-generated free space radar. The main difference is the addition of a cable or rod from the radar unit to the process surface to guide and focus the radar signal (figure). Guided radar also operates on a lower frequency of approximately 1.2 GHz.

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Figure. The signal from a guided wave radar transmitter follows a cable or rod to the process surface for improved performance.

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The advantage of guided wave radar technology is the signal is very concentrated on the cable or rod. When it encounters foam, the radar signal does a better job of going through the foam to reflect off the liquid surface. Neither condensation nor dust has any effect on TDR.

Another major advantage in solids level measurement is that the angle of repose can be accounted for with careful placement of the rod or cable. The angle of repose results from the way solids pile up in a vessel, creating an angle on the side of the pile. When using guided wave radar, the point at which the rod or cable contacts the product will determine the signal reflecting back to the transmitter.

Installation considerations include material compatibility, possible excessive pull force on the cable in solids applications when installed in tall silos, and avoidance of interferences between the cable or rod and items such as agitators

Free space Radar

This non contact radar technology has two different versions:

• Pulse generated
• Frequency modulated continuous wave (FMCW)

What is Pulse generated Radar technology?
An electromagnetic wave between 1 and 100 GHz is sent from the antenna toward the process surface in search of a change in impedance, which will reflect the signal back to the transmitter.

The dielectric of the product is important when selecting a radar unit because the greater the dielectric, the greater the change in impedance and the stronger the reflection.

The size of the radar horn, the dielectric of the product, and the condition of the process surface (calm or agitated) determine the maximum distance from the device to the process surface.

Free space radar sends an electronic signal that either bounces off the surface or shifts its frequency to determine level

What is FMCM?
The FMCW radar versions send out a continuous radar signal, and the frequency shifts as the distance to the process changes.
Because FMCW is a continuous wave, it never loses touch with the material, making it better for agitated vessels.


Advantages:

• It can work in vaccum.
• Relatively unaffected by environmental conditions such as different gases or vapours between the transmitting device and the process surface.
• largely impervious to variations in process temperature or pressure.

Disadvantage:

• Condensation is typically a high dielectric liquid, and the radar signal cannot penetrate this material, resulting in increased “noise” in the launch area of the signal.
• Foam is very difficult to quantify for radar as it is not readily distinguishable from the process surface

Ultrasonic level sensor

The most common technologies available for continuous level measurement are ultrasonic, free space radar, guided wave radar, capacitance, gamma, and pressure. Lets learn about ultrasonic first.

Ultrasonic

An ultrasonic transducer generates a mechanical sound pulse that is directed through air to the process. When this pulse encounters the process surface, it bounces back to the transducer. The transmitter is basically a high-tech timer, measuring the time it takes the pulse to travel to the process and back. This time is directly proportional to the distance to the process surface.This time-based technology is referred to as time of flight (ToF). 

An ultrasonic level sensor sends a sound pulse that reflects from the surface of the liquid or solid in a vessel. 

Advantage

• Ultrasonic transmitters are used on a variety of simple applications for measuring liquid or solid level in a vessel. 
• The transducers are temperature compensated to give a high level of repeatable accuracy at distances of 2 to 230 feet.

Disadvantage

• Applications with heavy dust are not suitable and defuses the signal and causing a poor return.
• High temperatures or vapours can also alter the density of the air enough to affect the speed of the pulse transmission, causing errors in measurement.
• ultrasonic devices do not work in applications operating under a vacuum. 
• Foam on top of a liquid can also disrupt an ultrasonic signal.

Vortex flowmeter

What is vortex flow meters?The vortex flowmeter is used for measuring the flow velocity of gases and liquids in pipelines flowing full. The measuring principle is based on the development of a Karman vortex shedding street in the wake of a body built into the pipeline.

Advantages of Vortex meters1 .Wide rangeability (for Reynolds numbers above l0,OOO).
2. An accuracy of 1 percent of rate.
3.A wide range of sizes.
4. Linear output.
5. Availability of pulse and analog outputs.

Limitations of Vortex meters

1. A limited range of construction materials is available.
2. Vortex meters are generally not suitable for slurries or O high-viscosity liquids.
3. Users cannot check calibration
4. Turbulent flow is required.
5. Vortex meters have over range limitations.
6. Strainers may be required.
7. Vortex meters are affected by pulsating flow

Why ISA standards are neccessary?

Imagine moving into a new house and going to plug your refrigerator into the wall - only to find that the plug doesn't match the outlet!. Electrical standards set decades ago to ensure such problems don't happen. And standards today allow you to get your product developed in a manner to be easily used anywhere in the world. You can check up almost any kind of stereo component from any electronics store.

All of these conveniences are the result of a standard, a set of characteristics or quantities that describes features of a product, process, service, interface or material.

Standards don't just make life easier, they make it safer…and they enhance companies' profitability. For instance, builders save money because construction materials are available in standard sizes. At the same time, electrical codes that builders must follow save lives.

More than 4,000 individuals cooperating with more than 140 committees, subcommittees, working groups and task forces are involved in ISA standards. They're developing standards in areas as diverse as ensuring the safety of electrical equipment used in hazardous locations to cost-savings for interfaces between industrial process control computers and subsystems.

ISA standards sometimes helps an entire industry in cost-saving and safety.