Understanding the Echo Curve in Guide Wave Radar Transmitters

Guided Wave Radar (GWR) level transmitters are highly precise instruments designed to measure levels in liquids, slurries, and solids with exceptional accuracy (±0.1% theoretically). Utilizing GWR technology enables the dual measurement of liquid surface and interfaces within tanks, such as separators, without the need for additional penetrations. These transmitters excel in identifying crude oil interfaces, making them especially valuable in various industrial settings. Through the analysis of echo curves, they establish initial configurations, theoretically minimizing noise readings by leveraging tank/vessel design and probe data.

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Basic Ideas

Echo-based level instruments like radar type might encounter challenges when faced with layers of foam atop the liquid, potentially leading to inaccuracies in measurement. Models designed to detect liquid-to-liquid interfaces may struggle with identifying non-distinct interfaces like:

• Emulsions,
• Within the vapor space of a vessel,
• Irregular structures such as access portals or mixer paddles can interfere with echo-based instruments, generating false echoes,
• Liquid streams entering the vessel through the vapor space,
• Dead zones where the liquid level is too close to the transceiver, making accurate measurement or detection difficult due to the short echo time-of-flight.

However, this issue can be addressed by implementing guide tubes for wave transmission or utilizing wave probes, as seen in guided-wave radar instruments. Measurement using GWR relies on radar signal pulses being bounced back by the product surface and the interface between two fluid phases. Signal amplitude thresholds are employed to distinguish the measurement signal from interfering echoes. The transmitter employs specific criteria to determine the type of pulse detected. For instance, starting from the tank’s top, the initial echo encountered above the Surface Threshold is identified as the product surface, while subsequent pulses beyond this threshold are disregarded. Once the surface echo is identified, the subsequent pulse below the product surface, with a signal strength surpassing the Interface Threshold, is recognized as the interface.

Then, what is the physical theory behind that idea, exactly?

Each phase of liquid or fluid possesses its unique relative permittivity, also known as the dielectric constant, influencing its ability to reflect or transmit radio waves. The velocity of radio waves through a medium is contingent upon the medium’s dielectric permittivity, represented by the Greek letter “epsilon” (ε). When a radio wave encounters a sudden alteration in dielectric permittivity, a portion of its energy is reflected as another wave traveling in the opposite direction. Essentially, the wave exhibits an “echo” upon reaching a discontinuity. This phenomenon forms the fundamental principle underlying all radar devices.

The power reflection factor (R), which signifies the proportion of reflected power to incident (transmitted) power at any material interface, is a crucial metric. It can be represented either as a unitless ratio or, more commonly, as a decibel value. The correlation between dielectric permittivity and reflection factor is defined as follows:

It’s inherent that the proportion of incident power passing through the interface (P_forward / P_incident) inversely corresponds to the power reflection factor, expressed as 1 − R. For example, in Figure 3, with a scenario where two media were involved, air (ε_r ≅ 1) and water (ε_r ≅ 80), an ideal setup for significant signal reflection was established. With these relative permittivity values, the power reflection factor was calculated to be 0.638 (63.8%), equivalent to -1.95 dB. This implies that well over half of the incident power reflects off the air-water interface, leaving only 0.362 (36.2%) of the wave’s power to propagate into water. However, if the liquid were gasoline, characterized by a much lower relative permittivity value (ε_r ≅ 2), the power reflection ratio would decrease significantly to 0.0294 (2.94%), or -15.3 dB. In this case, the majority of the wave’s power would effectively penetrate the air-gasoline interface.

Why We Need an Echo Curve?

Similar to other level measurement instruments, GWR also requires precise configuration prior to operate in order to ensure optimal accuracy as guaranteed by the manufacturer. Just as incorrect input of upper range value (URV) or lower range value (LRV) can compromise the accuracy of a differential pressure (dp)-type level transmitter, errors in recording the echo curve can yield similar outcomes, the reading would be noisy and seems incorrect. To address this, initial parameters need to be specified before recording the echo curve, and fine-tuning the threshold amplitude ensures proper functionality of the GWR.

Typically, the essential steps for recording or generating the echo curve include:

1. Perform Basic Configurations: This involves setting up measurement output, inputting tank geometry, defining environmental conditions, defining volume and define probe construction data. This step would acquire from the provided datasheet and level-sketches.
2. Perform Echo Tuning: This step mainly to adjust the Surface Threshold (ATC, Amplitude Threshold Curve) to optimize performance.

Anatomy of Echo Curve

Based on what we understood in basic ideas of echo curve, now we need to dig dive into the details for what a good echo curve should have.
See below Figure 4 & 5.

Echo curve will give us information as below:

• Reference Measurement, as well as Upper Null Zone (UNZ) threshold
• Surface Measurement
• Interface Measurement (if any)
• Surface Threshold, as well as Amplitude Threshold Curve (ATC) which will filter-out single disturbing echos.

During the pre-commissioning phase, particularly when conducting loop checks for GWR systems, it’s imperative to include the echo curve as an essential attachment to the loop check-sheet. This report serves to fulfill all the necessary checklists, ensuring that the GWR level transmitter has been installed and powered up according to the design specifications.

Echo Curve from Real Applications

I will share one example of echo curve which taken from real application at field. This LIT installed in water separator with below probe data:

The comprehensive guided setup instructions are detailed in the Radar Master software manual. During the pre-commissioning phase, as the tank was empty, no interface peak could be visualized in the report. However, the measurement output is still provided, indicating the distance to the tank bottom. As long as the dielectric constants of the fluids are accurately recorded to match the actual filling fluids (not significantly different), everything will be fine.

As you can see from Figure 8, P1 surface seems near the probe end peaks which means the tank is fully empty, the “ullage” showing almost the same of C-to-C length as specified in datasheet. The ATC could be acquired automatically or manually tune.

References

• Bela G. Liptak, Chapter 3.14 Radar, Contact Level Sensors (TDR, GWR, PDS)
• PAControl, Lessons in Industrial Instrumentation
• Rosemount 5300 Level Transmitter Manual, 00809-0100-4530