Guided Wave Radar Level Measurement: How to Make the Best of GWR
Guided Wave Radar (GWR) technology has transformed the way process companies measure level in challenging applications: Providing accurate and reliable results, GWR modules have no moving parts, reducing maintenance to a minimum. Critical for a GWR application is, nevertheless, the correct installation.
In a Guided Wave Radar installation, the GWR is usually mounted on top of the tank or chamber with a probe extending to the full depth of the vessel. A low energy pulse of microwaves is sent down the probe that is reflected back when the pulse reaches the media surface. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back with an on-board microprocessor that accurately calculates the distance to the media surface using ‘time-of-flight’ principles.
GWR level transmitters are accurate and reliable: They are not density dependent and are relatively unaffected by high turbulence or vibrations. Since there are no moving parts to stick or wear, maintenance costs are reduced. As this setup reduces the problems of false readings, possible hazardous situations can often be avoided.
Level Measurement in a Wide Range of applications
Guided Wave Radar measurement technology is suitable for a wide range of applications, but as with any measurement technology, installation good practices are important and there are some situations where the mechanical installation, the process media itself, as well as the effects of electromechanical noise may interfere with the basic measurement.
One Ping Only: The Echo Curve
To simplify the commissioning process, GWR level transmitters can be pre-configured at the factory to include parameters such as tank height and media properties. Some applications can however be quite challenging — this is where being able to view the echo curve can be a valuable tool for troubleshooting difficult applications. The echo curve represents the tank, as seen by the radar transmitter. Each peak corresponds to a reflection of the radar signal (e.g. the surface of the level or interface, an obstacle, or something else).
By viewing single instances or movies of the echo curve, the transmitter configuration can be adjusted to achieve a reliable measurement. Additionally, the echo curve gives insight into transmitter functionality and changes in application conditions. The software includes functions for viewing and recording the echo curve, and advanced functions for configuring the amplitude thresholds. Emerson’s Rosemount 3300/5300 and the new wireless 3308 Series level transmitters for example allow the user to adjust threshold settings and suppress or block out noise that may be present at the top of the measuring range.
Different amplitude thresholds are used to filter out unwanted signals and to pick up the different pulses. The transmitter uses certain criteria to decide which type of pulse is detected. Some transmitters have a ‘Measure and Learn’ function, which creates an Amplitude Threshold Curve (ATC) that filters out all disturbing echoes. The ATC can also be manually edited (for example to remove random peaks) if further fine tuning is needed.
Overcoming Weak Signals
In some applications, for example those with a long measuring range or if the products have very poor reflectivity (low dielectric constant), the surface pulse is too weak to be detected. To overcome this problem, some GWR manufacturers have developed enhanced electronics that can carry out reliable measurements even in these circumstances.
For example, the Probe End Projection function in Emerson’s Rosemount 5300 transmitter is based on the principle that when the surface level is too weak to be detected, the echo from the probe end can be utilised because it (as displayed on the echo curve) will appear to be further away than if the tank were empty.
This is because the speed of the measurement signal through the product is less than the speed through air. The product surface level can be determined by comparing the actual probe end position, as given by the Probe Length value, with the apparent position of the Probe End pulse. The difference is related to the properties of the product, i.e. the Dielectric Constant, and the distance D travelled by the measurement signal through the product.
Sources of Mechanical Noise
GWR transmitters can be mounted in nozzles using an appropriate flange. However, nozzles introduce noise that may affect measurement. Most manufacturers publish a table of recommended nozzle dimensions that should be followed where possible. Noise can be generated by long narrow nozzles, very small or very large nozzles, or installations where the probe touches the nozzle. Other sources of noise include nearby metallic objects and bent probes. Long narrow nozzles cause noise because the impedance change as the transition to the open tank causes a large negative echo. If the nozzles are less than 15” long it is sometimes possible to suppress some of the noise using features like Emerson’s Trim Near Zone function, but there is energy loss due to the disturbance.
Trim Near Zone is a firmware function that optimises performance near the upper portion of the probe, reducing some of the impact of the compromised installation. Very large diameter nozzles (greater than 10”) can also cause noise through resonance along the entire measuring range. In these situations an insert can be used to eliminate or reduce the noise level.
Bottle-neck chambers aren’t recommended for GWR applications as they can compromise functionality and cause false readings. Other mechanical problems can include the proximity to metallic objects, such as the electrical grid inside a desalter vessel. This may require relocating the transmitter or installing a stilling well. Centering disks can be used along the length of a long flexible probe to prevent it from touching the side of the well. Using a larger diameter chamber will allow more room for the movement of flexible probes.
Protection Against Electrical Surges
There are several ways that transient energy can enter a level transmitter: Transients can be caused by natural events such as lightning, RFI generated by nearby machinery or static build up during the measurement of solids such as plastic pellets. GWR manufacturers have designed the electronic circuitry to withstand these surges. However, if the surge is large enough, such as a lightning induced transient, the discharge may be sufficient to damage the GWR. To achieve optimum transient protection, a transient terminal block should be used where these conditions could occur.
For example, Emerson’s transient terminal block is designed to provide a high degree of protection against transients and works in conjunction with a good earth ground to direct the surge away from the electronics. Good earth grounding for externally induced transients can be achieved using a conductive wire between the flange and a ground rod or grid.
Electrical noise from nearby equipment, for example variable speed drives, can be a particular problem with non-metallic silos as it may be picked up by the probe. This can be addressed by using filters to remove most common EMI from rotating equipment, motor controllers and other sources. There is no issue with metal silos as the metal walls shield the probe from the noise.
Why an Adequate Grounding is Needed for Level Measurement
Inadequate grounding can result in signal noise that makes it difficult to detect the actual peak; it can also lead to electronic failure since the transient protection relies on good grounding in order to function properly. With metal tanks, it is important to ensure a ‘metal to metal’ connection between probe and tank. Also the size of the grounding wire is important for proper installation since it provides a direct path to earth. Larger diameter cables and short distances give the best results and users should refer to the installation manual to ensure that the manufacturer’s recommendations are being followed. For a vessel with possible internal surges, such as static discharge, one or more internal ground wires between flanges provide an alternative path.
Non-metallic tanks pose special problems for GWR level transmitters as surges will often enter the transmitter through the probe. It is therefore important that an external ground wire is used in these applications. It is also important that the contents of a non-metallic tank are effectively grounded. If a tank is filled from the bottom, this can usually be achieved by grounding through the piping. If the tank is filled from the top, the contents may not be properly grounded and it may be necessary to install a grounding rod covering the entire height of the tank.
Guided Wave Radar: Correct Installation is the Key
GWR is easy to install and provides an accurate and reliable measurement for both level and interface. It is suitable for a wide variety of applications and provides a top-down, direct measurement. These features make GWR level transmitters an ideal choice for replacing traditional level devices as they can be used with liquids, sludge’s, slurries and some solids. A key advantage of radar is that no compensation is necessary for changes in density, dielectric, or conductivity of the fluid. Even more, changes in pressure, temperature, and vapour space conditions generally have little impact on accuracy. In addition, radar devices have no moving parts, so maintenance is minimal.
To achieve the best results and protect the GWR against damage, correct installation is important and dealing with the possible problems caused by noise is a vital aspect of the installation process. Providing the manufacturer’s recommendations are followed, correctly installed GWR level measurement devices will enable higher performance, lower maintenance and higher reliability. Tighter process control means that GWR will also help plants meet their environmental and safety requirements.