Guided Wave Radar
Level Measurement: How to Make the Best of GWR
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.