Non-Contact: A Story of Radar Level

InTech, Jul 2006

Any user who wants complete, automated control within a plant site wants continuous level monitoring. More than 20 technologies measure continuous level in vessels or silos, but users need reliable measurement of the contents of these containers, so they'll be sure not to overfill the container or lower the contents to a critical point of jeopardizing the process.

Radar technology has advanced in the last 10 years and even more so in the last five years to provide solutions in a variety of applications. Radar level measurement sees over 60,000 users in the chemical, pharmaceutical, oil and gas, food, dairy, steel, beverage, paper, automotive, aggregates, water and waste water, and marine industries.

Non-contact level measurement systems include laser, non-contact radar, and ultrasonic level measurement technologies. In general, these systems consist of a level sensor located above the material surface that emits a signal and processes the returns (reflections of that signal). Materials that reflect better produce stronger returns that allow measurement over larger distances. In general, solids do not reflect as well as liquids, so a given sensor and transmitter will typically be rated to measure over a shorter distance in solids service.

Advantages

The advantages of this technology? It makes no contact with the liquid or solid material within the container. Radar technology is unaffected by airspace conditions like humidity, heavy vapors, and vapor layer stratification due to temperature fluctuations, along with dealing with heavy dust in the airspace throughout distances of 150 to 200 feet. There are now two-wire, loop-powered designs, which offer low cost of wiring and ownership, and prices have come down considerably.

You can use non-contact radar gauges for tank and vessel level measurement on almost any liquid and many solids whether in storage or during processing. It is versatile because it is unaffected by changes to product or process-temperature, pressure, density, conductivity, vapors, or dust. Radar transmitters are non-contact, making them versatile and reliable. With built-in linearization software, some can calculate the level versus shape and provide a direct output of volume on:

* Radar tank gauging in storage vessels

* Level measurement in processing vessels, reactors, blenders, scrubbers, and mixers

* Hygienic radar systems for distillation and purified water vessels (even with water sprays)

* Interfaces on some special applications/liquids

* Monitoring handling of high temperature molten products, metals, minerals, and coke products

There are different techniques of propagating radar such as pulsed and frequency modulated continuous wave (FMCW). Pulsed wave radar is similar to ultrasonic non-contact technology with the exception that the radar signal is traveling at the speed of light versus the speed of sound.

The FMCW technique for radar propagates at the speed of light sending millions of signals in a short duration of time at different frequencies and views as many signals in frequencies coming back and compares those frequencies from transmit to reception. The differential in frequency is proportional to the level based upon Fast Fourier mathematics. Microwave echoes are evaluated due to the transmission at the speed of light by a method of sequential sampling, which essentially builds a profile by expanding the time.

Limitations

While not in contact with the material, these sensors are exposed to the material's vapors and general environment. They can come in direct contact with the materials when the level of the material is excessively high. While these technologies measure level continuously, they only do it at one point in the vessel. This probably won't pose a problem for liquid applications where the gas or liquid interface is horizontal.

However, in solids applications, material entering and leaving the vessel affect the solid/gas interface. As such, this interface is typically not horizontal, so a single level measurement may not be representative of the amount of material in the vessel. In these applications, the level measurement reflects the level at one point in the vessel. Level measurements can change rapidly when the material level that is sensed changes rapidly. In some applications, multiple level measurements may be needed to provide a more accurate indication of the inventory of material in the vessel.

A rat hole may form as solid material leaves the vessel. If the level measurement reflects a point in the rat hole, the measured level will decrease, as expected. However, if the material remaining on the sides of the vessel falls and fills in the rat hole, the level will abruptly and unexpectedly increase. Sensors should be located such that the measured level represents the actual level while avoiding rat-hole effects. Multiple sensors may be needed if an appropriate location cannot be found.

Most non-contact level sensors cannot accurately measure distances that are close to the sensor itself. Sensors are typically installed to allow the transmitter to disregard measurements at these distances. This region is often called the blanking distance.