Non-contacting radar is the favored technology for providing accurate and reliable liquid level measurement in a range of applications aboard marine vessels, such as in fuel tanks and cargo tanks. The measurements provided help to ensure both safe and efficient operations. In an article for Riviera Maritime Media, Radar antenna selection is crucial in marine tank level measurement, we describe the importance of antenna selection in radar applications and outline the factors that should influence those choices.

The article begins with an explanation of how radar instruments work – emitting microwaves to measure the distance to the liquid surface – and then describes how non-contacting radar devices use either pulse or frequency modulated continuous wave (FMCW) modulation techniques. The article states:

…A key advantage of FMCW devices is that their signal strength is much greater than in pulse transmitters. This enables them to provide superior measurement accuracy and reliability, which is why they have become the preferred solution for many challenging applications.

We discuss how frequency directly influences measurement performance, and is therefore a fundamental characteristic of radar-based level instruments. We list the three frequency bands that have historically been employed for level measurement applications (~6 GHz, ~10 GHz and ~26 GHz) and note that transmitters utilizing the ~80 GHz frequency band have recently been introduced.

The article then explains that the bandwidth of an FMCW radar level device – referring to the range of frequencies covered by the transmitted signal – is a crucial parameter. This is because a broader bandwidth typically allows for better range resolution – i.e., the device’s ability to distinguish between two closely-spaced objects or surfaces along the path of the radar beam, and detect them as separate targets.

Next, we describe how the dielectric constant of the liquid being measured can impact the performance of level measurement devices. The article says that in the case of radar technology:

…the measured media needs to provide a strong enough reflection of the transmitted signal. In general, the higher the material’s dielectric constant, the stronger the reflected signal will be. However, as the distance to the target increases, the reflected signal must be stronger to ensure an adequate return to the radar device. Additionally, turbulence or ripples on a liquid surface can lead to signal scattering, diminishing the signal received by the radar device. In cases where agitation is coupled with a low dielectric medium, unintended reflections from internal structures within the tank may surpass the intended liquid level measurement.

The article focuses on the issue of antenna selection, and we explain that signal generation is based on factors such as the frequency being utilized by the radar transmitter, and the size of the antenna. The gain of an antenna is a measure of its ability to direct or focus the signal in a particular direction, and is calculated by the equation

where gain (G) relates to the antenna diameter (d), wavelength (λ), and efficiency (η). The article explains:

…Comprehending this equation empowers users of radar-based level instruments to decide on an instrument’s suitability for different level measurement applications. For example, a comparison can be made between a relatively large cone antenna and a parabolic antenna. For a device operating at 26 GHz with a wavelength of 1.2 cm and employing a 19.5 cm parabolic antenna with an efficiency of 0.45, the gain is seven times greater than that of a device operating at 10 GHz, utilizing a 15 cm (6 in) cone antenna with an efficiency of 0.70. This implies that in applications such as measuring level in the cargo tanks of oil tankers, where the tanks may exceed 15 meters in height and the content has a low dielectric constant, a larger antenna can prove advantageous.

We explain that the overall beam width of a radar signal is inversely proportional to the frequency of the device. This means that a radar device operating at a higher frequency will have a smaller beam width compared to a lower frequency device with an equivalent antenna diameter. This characteristic greatly simplifies the installation of radar-based level measurement devices on marine cargo tanks, as tank designers do not need to keep a substantial portion of the tank free from potential obstructions, which could otherwise compromise measurement reliability. We then provide the following example:

…At a distance of 10 m and using a 100 mm antenna, a 26 GHz radar device features a beam width of 1.5 m, while a 6 GHz transmitter has a wider beam width of 7 m. The beam width of the 6 GHz device is 4.6 times larger than that of the 26 GHz transmitter with the same antenna size. Increasing the antenna size not only reduces the beam width but also effectively enhances the gain of the unit.

The article then describes the accuracy requirements for various marine level measurement applications. For example, the expected accuracy and reliability of a cargo monitoring system is usually high, as it must ensure that the contained liquid will not rise above permitted limits, thereby eliminating the risk of overfill incidents. These systems predominantly employ radar-based transmitters for measuring level on a wide range of vessels, from very large crude carriers (VLCC) to general purpose (GP) and inland barges. We then explain that:

…Historically, devices employed for measuring level in fuel tanks have been subject to relatively modest accuracy standards, with a notable reliance on pressure-based measurements in many installations. Nevertheless, the landscape is evolving due to heightened environmental standards, leading to the increased adoption of radar-based devices, which is particularly evident in alternative fuel applications.

Visit here to learn more about Emerson’s solutions for marine tank level gauging systems, and here for marine cargo monitoring systems.

The Rosemount™ TGU 68 Tank Radar Gauge with a parabolic antenna is a non-contacting device that is integrated in the Rosemount Cargo Monitoring System. It is designed for use in harsh conditions on marine tankers and offshore installations.