Guided Wave Radar Transmitters:Meeting the Challenge for Level-Detection Under Harsh Conditions
When it comes to measuring the level of bulk solids, liquids, and everything in between, guided-wave radar technology now offers more level-detection capabilities than ever before. For an ever-widening range of previously hard-to-measure products such as molten sulfur, liquid ammonia and petrochemicals, guidedwave radar transmitters provide accurate level measurements even under harsh chemical environments, wide variations in operating temperatures and pressures, and low dielectric constants. Great strides have also been taken in making these units easier to configure to a variety of process applications coupled with the simplicity of integrating these devices with most digital communication protocols. These improvements come as welcome relief to process engineers within an expanded range of level applications across several different industries that seek solutions to measuring the contents of tanks, silos, hoppers, bins, mixing basins, and vessels.
A quick examination of how guided-wave technologies compare against other time-of-flight technologies, such as through-air radar and ultrasonic —along with a discussion of application guidelines—can serve to elucidate the benefits that this new technology can bring to engineers needing to improve process operations.
Because radar transmitters have no moving parts, radar has already established a dominant niche in level measuring that quickly distances itself from mechanical means, which don't hold up as well in dirty service. Radar achieves its non-mechanical level detection capability by measuring the time of flight of the transmitted signal.
Known more accurately as Time Domain Reflectometry (TDR), the process involves sending microwave energy down into a vessel. When the pulse of radar energy reaches the product (indicated by a change in impedance), part of the pulse is reflected back toward the transmitter. A receiver measures the exact duration of time between the transmitted and reflected signal—the "time of flight." The device analyzes this time and ultimately displays the level of the product as a distance in feet, meters, or other engineering units.
Through-air technology clearly pioneered the way for radar in terms of level measurement. However, one of the major problems of non-contact (with the product to be measured) through-air radar, is the high probability of false echoes. Simply pointing a radar transmitter toward the bottom of a silo allows unguided waves to bounce off the sides of the vessel itself, returning many divergent signals that must be canceled out at the receiving end. Part of the problem stems from the wide dispersion of radar beams, which radiate away from the transmitting antenna in the shape of an ever-widening cone. A similar problem also presents itself in ultrasonic measurements where divergent angles of up to 20 degrees are routine.
These obstacles have now been overcome with the arrival of guided-wave radar transmitters. While fundamentally relying on the same conventional time-of-flight technology used in through-air radars, guidedwave radars go one step further by controlling the spread of radar beams via a "probe" that is introduced directly into the product to be measured. Typically, the wave-guide is a specially designed metal rod or cable. Since the guide concentrates the radar signal within a small-diameter (often less than 12 inches) cylinder along the probe, it doesn't disperse and reflect off of materials that are not representative of product level. This results in a higher level of performance and reliability from the guided-wave device. Furthermore, the ease of configuration eliminates wasted time, as the need no longer exists to spend time programming a unit to ignore erroneous readings from the sides of the tank.
The advantages to guided-wave radar clearly play out when it comes to meeting the real-world challenges faced by engineers in correctly determining product levels within storage and processing containers.
COMPARING GUIDED-WAVE RADAR TO THROUGH-AIR TECHNOLOGIES
Performance & Reliability
Combined with additional parameters, such as the reflectivity of the inside walls of the vessel and internal tank obstructions (i.e. piping, nozzles, ladders, etc.), the use of a laptop is often required to configure a through-air instrument in order to yield an accurate reading of product level. Additionally, non-contact radar units are very sensitive to changes in the process conditions such as product build-up and condensation. Similarly, ultrasound remains highly subject to tank conditions and vapor phases, which can affect the speed of sound. If not set-up properly at the receiver, varying and inaccurate readings can result. On the other hand, the basic theory behind guided-wave radar helps prevent false echoes in the first place. Hence, performance levels can be improved over that of through-air devices. Complicated configuration is not necessary, and any adjustments can typically be accomplished via onboard push buttons within the instrument itself.
Signal strength, and its part in improving performance
At first glance, it might seem easy to increase the signal-to-noise (S/N) ratio by simply increasing the power of the transmitted radar signal. Flying in the face of this strategy, however, is the fact that there is a very limited amount of power available to operate the electronic and sensor circuits in order to use the industry- standard analog output of 4-20 mA with a loop powered transmitter. A fundamental advantage behind guided-wave technology is that less energy is required because the microwaves are concentrated along the wave-guide.
Unguided radar has many return signals from different surfaces that affect accuracy, unlike guided radar which only has to account for one signal reflection. This allows the transmitter to measure more accurately.
Guided-wave radar allows the concentration of energy where it is needed the most, hence it can be twenty times more efficient than through-air radar—which loses most of its signal within the beam spread. Therefore, less power is required with guided waves. This in turn contributes to a higher S/N ratio.
Fast response time also improves performance
Loop powered, through-air devices must store and buildup energy to handle both the pulse transmission and the signal processing. With the averaging and signal processing involved, the return signal from a turbulent or fast moving product can easily move out of the window being analyzed. Additionally, because through-air and ultrasonic devices require more signal conditioning to weed out spurious signals, extra time is wasted. Hence, inaccurate readings stand as an increased risk. Through air radar uses "fuzzy" logic to assign each target a probability level and the signal processing required leads to a slow response time and multi-second update rate. Guided-wave radar, on the other hand, can take up to 10 readings per second, yielding an almost instantaneous response. Results can then be updated at this rate if no additional filtering is needed.
Overcoming the dielectric obstacle
Obtaining accurate readings in products with a low dielectric constant provides the greatest challenge to most radar transmitters, as it becomes increasingly difficult to obtain a reflected signal. Radar waves partially pass through non-conductive materials like liquid propane or butane, making it traditionally difficult to obtain accurate levels.
In both guided-wave and through-air radar, the coax cable that carries the signal from the transmitter to the process connection typically has an impedance of 50 ohms. Ideally, if one could maintain that same impedance along the entire length of travel of the radar beam, then all of the reflected signal would bounce off of the product. In actuality though, when going from coax to air, a large impedance change occurs leading to a substantial loss of signal.
Guided-wave radar sidesteps the challenge of measuring low dielectrics by the use of a single probe that is protected by a stainless steel tube that functions like a small concentric shield surrounding the entire probe length giving the whole assembly a coaxial structure. The tube acts as a ground plane to help channel the energy. It maintains constant impedance along the entire wave-guide. The coaxial sensor can then detect more subtle dielectric changes, and correctly indicate the level of the product. Through-air radar can also use a similar pipe arrangement to channel energy and measure lower dielectrics. However, for throughair radar this mode of operation is much more susceptible to build-up and is very dependent on pipe configuration and construction.
Guided-waver radar transmitters also allow special probe configurations to perform indirect measurements by taking into account the velocity change of the energy as it travels through the product. In this manner, very low dielectric products, typically down to 1.3, can be measured in applications where other methods have failed in the past.
Measuring long spans (distances)
Modern radar transmitters are now capable of measuring the levels of very tall silos and tanks. Some radar transmitters are designed to cover distances up to 200+ feet. Ordinarily, it is extremely difficult to obtain high resolution readings over such a wide range, especially considering that radar waves travel at a speed of approximately one-foot per nanosecond. A 100-foot silo might only contain four feet of product, for instance. That means the time differential for the reflected wave to return to the transmitter is only 8 nanoseconds faster than if the tank were empty. To obtain quarter-inch accuracy for such levels, the device has to measure in increments of pico-seconds.
Some guided-wave radar and through-air transmitters utilize specialized electronic circuitry that includes very stable ramp generators. These generate a signal waveform that duplicates the reflected waveform, albeit at a much slower time frame. Only highly stable and accurate electronics can obtain resolution levels in the tens of thousands of an inch over a whole span —sufficiently accurate to meet the needs of most any application. However, guided-wave radar offers a clear advantage over through-air since the microwave energy is focused and travels along the wave-guide, which makes this technology much more suitable for long measuring lengths particularly with low dielectric products.
APPLICATION GUIDELINES FOR GUIDED RADAR
Dirty service conditions
For industries dealing with products that cake onto everything they touch, obtaining accurate levels has long presented difficulties. In particular, paraffin wax buildup (experienced in petro-chemical processes) and molten sulfur manages to build up on everything they come into contact with, including radar transmitters.
Guided wave radar level transmitters that rely on coaxial, as well as dual-rod or dual-cable wave-guides are at a particular disadvantage here. Problems arise when product buildup bridges the gap between the two rods, rendering the unit inoperable because of the modified impedance between the two wires.
The preferred arrangement for solving this application dilemma involves the use of a single cable or rod hanging into the tank—the configuration that many guided-wave radar transmitters rely on. Buildup on a single wave-guide will have minimal effect on the operation of the transmitter. This configuration however, is limited to higher dielectrics unless the single probe/ cable assembly is inserted into an outer stilling well, making it suitable for measuring lower dielectric mediums. In fact, one guided-wave radar manufacturer recently utilized this single rod approach inside of a stilling well and achieved the goal of determining levels in low dielectric molten sulfur tanks at a plant in Hopewell, Virginia. Previous mechanical measuring units could not hold up to the repeated caking of cooled sulfur as the tanks drained. Accurate tracking of sulfur levels is now consistently maintained.
Fitting radar transmitters within high turbulence vessels
Guided-wave and through-air radars can both be configured to operate in high-turbulent environments. While guided-wave radars provide better performance (because of their narrower beam) than through-air radars under turbulent vessel conditions, special precautions must be taken. The installation of a simple stilling-well (to calm the product) around the probe or through-air signal will maintain a more constant level reading.
The stilling well, often 4" or less in diameter, will have holes drilled along its length to allow the product to remain in full contact with the radar signal so that highly accurate level measurements can be obtained. Through-air radar uses this approach but requires very precise stilling well construction with tight tolerances and special welding. In addition, installation and alignment are critical with through air radar stilling wells and improper setup can sometimes reflect erroneous readings.
The guided-wave radar on the other hand, is not nearly so dependent on critical stilling well construction and alignment. Due to the already highly efficient wave guide path, radar signals glide past any cutouts and head straight down to the product. Since, the signal is contained within the wave-guide and energy is further focused with the stilling well, turbulence has minimal impact.
AVAILABLE OPTIONS TO INSURE OPTIMAL CONFIGURATION
If any advancement in today's radar transmitters is welcomed by process engineers, easily configured systems would certainly rank among the top. Nothing is more frustrating than attempting to shoehorn a onesize- fits-all transmitter into a particular application that poses specific impediments to determining product level.
Guided-wave radar manufacturers currently offer a full range of transmitters, mountings, couplers and probe types; therefore a precise solution can be matched with almost any application. For example, the variety of probes that house either a single or coaxial cable—some of which can be bent to accommodate varying tanks, bins, or silos¾allows guidedwave radar units to arrive from the factory in the optimum configuration for the plant. The availability of both rigid & flexible probes allows installation into a variety of new and existing applications. When the need arises, some probes can even be cut to length in the field. In Secunda, South Africa, the ability to send a radar beam around a 90° corner—by using a guided-wave cable— permitted the accurate measuring of levels within a cooling-tower.
Mounting a guided-wave radar transmitter is less critical than mounting a through-air device, as the latter relies on the precise configuration of the antenna. Additionally, guided-wave radar can work with smaller openings. Some manufacturer's guided-wave radar mounts into the chamber within nothing more than a ¾" NPT opening.
In many cases, radar units must be installed within external chambers. This is often the case when previously ineffective mechanical level-detectors were installed in an existing external chamber. The ability of today's guided-wave radars to adapt to a variety of vessel shapes allows a more straightforward retrofitting process.
In some applications, special instrument enclosures may be required. Stainless steel, for example, is particularly apt for offshore uses, where saltwater will corrode even powder-coated aluminum. Some guided wave transmitters are available with stainless steel housings that are explosion-proof rated. Some guided-wave radar manufacturers even display a willingness to create custom, one-off designs to meet the demands of decidedly challenging applications. Working with bromine, for example, almost always requires a customized mounting and probe that includes hermetic seals. Tanks that hold hydrofluoric acid require the specification of probes made of Monel, as opposed to stainless steel.
Proper configuration should also include the communication capability of the radar transmitter unit. Many radar units display results "locally" with a built-in scrolling LCD display that offers measurements in a field-selected choice of feet, inches, millimeters, centimeters, meters, or even percentages. But the ability for a device to seamlessly communicate this data throughout a processing plant's business system is equally important. Guided-wave units not only offer 4-20mA output, but a variety of digital communications capabilities such as Hart and Honeywell DE.
Because of its ability to accurately measure a broad range of challenging products under adverse conditions such as low dielectric constants, caustic chemical environments, and high operating temperatures and pressures, guided-wave radar transmitters are finding increased usage. In fact GWR is becoming the measurement method of choice for specific applications such as; crude oil, butane, propane, molten sulfur, liquid ammonia, plastic pellets, powders, fly ash, slurries, sludges, acids, chlorine, and this list goes on.
Their flexibility in configuring to new and existing applications, along with a wide choice of communication protocols, also adds to their increasing popularity. Due to their straightforward design, and lower operating frequency (less than1GHz), guided-wave units are often less expensive than comparable through air units, as well.
Considering all of the above, process engineers can expect an optimistic future when it comes to meeting the challenge of accurately determining the levels of products within the vessels tanks, bins, silos and hoppers of their facilities. For those that employ guidedwave technologies, the "vessel" is definitely half FULL.
This article was written by Carl Kull, Vice President Sales at K-TEK and Kevin Hambrice, Director of Marketing at K-TEK. K-TEK provides a complete family of point and continuous level products including magnetically coupled level gauges, magnetostrictive level transmitters, laser transmitters, guided wave radar transmitters, ultrasonic transmitters, and thermal dispersion & vibrating fork switches. For more information about K-TEK, please visit their website at www.ktekonline.com.