Product category:
Level and tank contents instrumentation
News Release from: Krohne | Subject: Optiwave FMCW
Edited by the Processingtalk Editorial
Team on 16 January 2006
Two-wire FMCW radar level measurement
New FMCW devices not only provide unparalleled accuracy and reliability for reactor vessel level measurement, they offer an easy user interface, low power 2-wire connectivity, and much lower prices
Radar technology has evolved into the method of choice for level measurement in many demanding reactor vessel applications These types of difficult level measurement applications are common in the chemical and pharmaceutical industries
This article was originally published on Processingtalk on 14 Aug 2008 at 8.00am (UK)
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The latest generation of Frequency Modulated Continuous Wave (FMCW) radar level gauges provide highly-accurate non-contact measurement, with mapping capabilities to compensate for and ignore the presence of internal tank obstructions such as agitators, and provide precise level measurements, independent of the material being measured.
With no moving parts and no drift in performance, they are quickly gaining favour over the traditional hydrostatic differential pressure (DP) methods for tank gauging.
Unlike DP methods, radar is particularly well-suited to level measurement in reactor vessels, because it is not affected by changes in process temperature and pressure/vacuum, vapours, steam, dust, or heavy agitation, nor by variations in product properties such as density, conductivity, and dielectric constant.
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However, despite these theoretical advantages, the first implementations of radar level measurement for reactor vessels proved less than satisfactory and the technology developed a negative reputation in the early 1980s.
Users drawn to the theoretical advantages of radar were disappointed by the limited performance and complex user interface of the expensive first generation products.
After a quarter century of continuous improvement, today's state-of-the-art radar level gauge technology has fully overcome these shortcomings.
The radical improvements in radar measurement are largely built on the advances in microprocessor technology.
Ever faster, smaller, and inexpensive microprocessors enable new possibilities that were unfathomable a few decades ago.
We have seen clunky DOS monochrome desktop PCs evolve into portable laptops and handheld computers that pack enormous processing power behind simple and engaging graphical user interfaces.
A paradox of microprocessor evolution is that faster processing power has the potential to increase both functional capabilities and usability, at the same time.
A second paradox is that the increased intelligence of newer technology comes at a lower price.
And yet a third paradox is that the more powerful processors are smaller and require less electric power.
Faster microprocessors have enabled dramatic improvements in the accuracy of Frequency Modulated Continuous Wave (FMCW) radar level gauging, a highly accurate technique that uses extensive sophisticated signal processing to map out obstructions and accurately and reliably detect even the smallest level changes, even in the most demanding applications.
FMCW radar transmits a high frequency microwave signal whose frequency increases linearly during the measurement phase (called the frequency sweep).
As the signal is emitted through the antenna and into the vessel, it will remain at the same frequency as it travels through the atmosphere, is reflected off the product surface, travels back up through the atmosphere and received by the antennae with a time delay (t).
This "time of flight" is directly proportional to the distance travelled, since the microwave signal travels at the speed of light, independent of the atmosphere conditions (ie vacuum, vapour, dust, steam, etc).
Unlike pulse radar systems, which must accurately and precisely measure the transit time directly with received pulses that originated at an undetermined time, the FMCW radar is able to determine the transit time much more accurately and reliably through direct comparison of the reflected received frequencies to the current transmit frequencies.
Knowing the time it takes to sweep to the higher frequency, this difference between the received and transmitted frequencies is directly proportional to the distance travelled by the reflected signal.
Sophisticated mathematical equations utilising Fast Fourier Transforms (FFT) translate this information into spectrum lines, providing the precise location of the reflecting surface as well as its strength or magnitude.
The level calculation results from the difference between the configured tank height and the measured distance.
During commissoning, the FMCW device is able to map out all internal structures in the empty vessel; or if emptying the vessel isn't possible, partial mapping just above the product can be performed.
While monitoring this empty tank spectrum, the magnitude and location of all tank internals to be ignored are stored into memory.
During normal level measurement, the signal processing algorithms subtract out the empty tank spectrum from the measured spectrum to eliminate the possibility of interpreting a parasitic reflection as the level in the vessel, and lock onto only the actual product level in the vessel.
All of this sophisticated FMCW device functionality is transparent to the commissioning personnel, hidden behind remarkably simple set-up wizards.
More powerful microprocessors not only improve signal processing accuracy and repeatability, but also enable user-friendly, graphical wizards that guide the user through commissioning, process analysis, process control and, if necessary, troubleshooting.
Because today's microprocessors offer high speed computing capabilities while consuming less electric power, it is now possible to take advantage of the accuracy of sophisticated FMCW radar level measurement in a low-power 2-wire configuration that is intrinsically safe, as required by most chemical and petrochemical industry applications.
In fact, the Optiwave device from Krohne is the first FMCW Radar Level Gauge available in a low power 2-wire configuration.
Prior to the Optiwave, environments requiring 2-wire intrinsically safe devices needed to settle for radar devices that utilise the less accurate pulse radar method.
Because pulse systems do not utilise FFT algorithms, they do not require as much processing power as FMCW.
This had been an advantage of pulse radar technology, prior to the availability of a 2-wire FMCW device.
FMCW technology is significantly more accurate than the pulse method.
FMCW achieves a signal-to-noise ratio up to 1,000 times better than pulse radar.
For example, the Optiwave 7300 C has a measuring range of 131 feet (40 meters), with an accuracy of 0.12" (3 mm) at ranges of less than 33' (10 m): and an accuracy of 0.03% of measured distances when the range is greater than 33' (10 m).
With FMCW, very weak signals can be identified and reliably tracked, even in extremely noisy environments.
Together with appropriate signal evaluation, significantly higher application reliability is obtained.
This is of particular benefit for difficult applications such as horizontal cylindrical vessels, agitated tanks, where there are layers of foam, vapour or steam, or in dusty atmospheres.
The latest FMCW radar devices transmit at frequency sweeps over the range of 24-26 GHz.
The higher bandwidth of the microwave signal, compared to previous generations and other radar technologies, provides better reflection separation and more reliable reduction of noise.
Moreover, the higher transmitting frequency means a more focused beam at a narrower angle, which results in fewer disturbing reflections.
Another advantage of the higher transmitting frequency is that a smaller antenna diameter can be employed for the same measuring range, allowing installation into smaller process connections.
Frequency Modulated Continuous Wave radar meets the stringent demands of reactor vessel level measurement applications.
The chemical, petrochemical, and pharmaceutical industries are driven by the need for ever-tighter process control, pushing process engineers to seek more precise and reliable level measurement systems.
The highly accurate FMCW method makes it possible to reduce chemical-process variability, which improves product quality, reduces costly waste, and improves safety.
Regulations, especially those governing electronic records, set stringent requirements for accuracy, reliability and electronic reporting.
CONCLUSION.
Riding the wave of new microprocessor technology, the latest generation of FMCW devices not only provide unparalleled accuracy and reliability for reactor vessel level measurement, they also offer an easy user interface and low power 2-wire connectivity.
Moreover, like many microprocessor-based technologies, FMCW radar devices have benefited from dramatic reductions in price - with purchase prices dropping nearly 75% compared to earlier generations of FMCW radar devices.
The lower purchase price, combined with the maintenance-free operation, makes FMCW not only the most accurate and reliable level measurement method, but also one of the most cost-effective methods as well.
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