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The DHE-RD series of radar level gauges can be widely used for measuring the level of liquids and solids. It can continuously measure the level of various media even under high pressure, highly corrosive conditions, strict hygiene requirements, or extreme operating conditions. The DHE-RD series of radar level gauges are divided into non-contact radar level gauges and contact radar (guide wave) level gauges, depending on the measurement purpose, properties of the material, and conditions of the storage tank. Currently, the automatic tank level measurement systems used for trade handover measurements at home and abroad are typically hybrid tank measurement systems (HTMS). The most commonly used in HTMS is the radar level gauge. Radar level gauges have no moving parts, so there is no mechanical wear. In practical use, their operational life typically reaches 10 years; they do not pollute the environment and are easy to install. It is highly suitable when the medium in the storage tank is viscous and highly corrosive. The radar level gauge features high accuracy, a wide operating temperature range, a maximum operating pressure of up to 4 MPa, and a measurement range of 0–40 m. By using different antenna configurations and waveguides, it is possible to meet the requirements for measuring the liquid level in most storage tanks. 1. Working principle and classification: Radar level gauges that meet the measurement accuracy requirements for trade handovers (±1mm) are all non-contact radar level gauges. From a measurement principle perspective, they are basically divided into two categories: (1) Based on the time-of-flight method: The antenna emits electromagnetic waves that travel at the speed of light; after passing a certain delay, these waves hit the liquid surface and are reflected back. Distance is proportional to the lag time; by measuring the lag time between transmission and reception, the instrument determines the distance between the setting surface and the reflecting surface (altitude). (2) Composite Frequency-Modulated Continuous Wave principle (FMCW). The amplitude-modulated microwave signal with linear frequency modulation is transmitted through the antenna; when it encounters an obstacle, it is reflected back. After a delay, the instrument receives this reflected microwave signal and mixes it with part of the transmitted wave. The frequency of the mixed output signal is proportional to the distance being measured. When the modulation frequency and frequency offset of the solid-state source are fixed, as long as the difference frequency is measured, the distance (altitude) can be calculated using signal processing techniques. 2. Factors affecting level measurement 2.1 Influence of temperature inside the tank on the measurement accuracy of the instrument. The electromagnetic waves emitted by the radar antenna can also propagate in a vacuum. When propagating in air, temperature changes have little effect on its propagation rate. When the temperature of the medium under test changes, the resulting change in reflection time is very small. 2.2 Influence of pressure inside the tank on the measurement accuracy of instruments: Microwave propagation is hardly affected by changes in air density; therefore, radar level gauges can operate properly in vacuum or under pressure conditions. In a vacuum environment, the speed of microwave propagation changes by only 0.029% compared to when there is air present ； However, when the operating pressure reaches a certain level, the error introduced by pressure in the measurements becomes significant. 2.3 Influence of medium properties on measurements Factors such as the relative dielectric constant of the liquid medium, the stability of the liquid surface, and bubbles can all affect the reflection of radar wave signals, which may prevent the level gauge from functioning properly. When using a radar level gauge to measure the liquid level in a tank, it is best to have a stable and regular liquid surface. However, when liquid flows in and out, waves are generated that cause the liquid surface to fluctuate. During the production process, boiling or foaming may also occur, making the liquid surface blurry. When electromagnetic waves propagate through a medium, the lower the dielectric constant of the medium, the greater the attenuation rate; and as the attenuation rate increases, the reflectivity decreases. Therefore, the dielectric constant is low, the reflectivity is weak, and the signal strength is low. When the relative dielectric constant of the medium being measured is low enough, the effective reflected signal of the radar waves attenuates significantly; as a result, the amount of energy in the reflected waves received by the instrument is too small. Combined with interference waves, this prevents the level gauge from functioning properly. Different companies’ radar level gauges have varying requirements regarding the minimum relative dielectric constant, as the value of the dielectric constant is influenced by temperature and electromagnetic fields. However, through other measures to reduce the energy loss due to electromagnetic wave reflection or by improving the signal processing technology within the instrument, the strict requirements regarding the minimum value of the relative dielectric constant are becoming increasingly less stringent. Currently, the products of major manufacturers (such as Honeywell and Saab) can measure dielectrics with a relative dielectric constant of only 1.2. For boiling or highly turbulent liquid surfaces, or when the dielectric constant of the medium being measured is low, or to eliminate interference caused by the structural shape of storage tanks or containers, guided wave tubes and other measures should be used to enhance the echo signal and ensure measurement accuracy. 3. Antenna type and design selection: The structural form and material of the antenna should be determined based on the characteristics of the medium to be measured, as well as factors such as the type of storage tank, its openings, and the temperature and pressure inside the tank. Common forms include horn (conical) antennas, parabolic antennas, waveguide array (planar) antennas, and rod antennas, among others. For conventional dome tanks, radar level gauges should have a horn-shaped (conical) antenna design, which eliminates the need for guide tubes and allows them to be installed close to the tank wall while ensuring measurement accuracy. For dome tanks that are relatively viscous and require heating, parabolic antenna radar with anti-adhesion capabilities is very suitable, and it is usually not necessary to use waveguides. For floating roof tanks, radar level gauges should feature a planar antenna design to facilitate installation on the tank’s guide wave tube. For high-pressure spherical tanks, the antenna consists of an extended flared antenna coupled with a waveguide, and an integrated ball valve is also provided to facilitate the maintenance of the radar level gauge while the tank is under pressure. To perform measurement calibration under the condition of pressurized operation of the storage tank, reference equipment must be installed: a reference needle inside the guided wave tube opening, as well as a reflector with a reference ring at the end of the pipe below, can provide reference echoes at fixed distances and known distances. 4. Installation notes: The radar level gauge should not be installed at the center of the tank top, as the electromagnetic waves emitted by the antenna spread throughout the entire interior of the tank. The electromagnetic waves reflected off the tank walls and bottom converge at the center, creating significant interference that overpowers the electromagnetic waves reflected from the surface of the medium. A horn antenna radar level gauge should be used, with the antenna extending at least 10 mm beyond the pipe connection. If the length of the pipe connection on the container does not meet this requirement, an antenna extension tube can be used, or the manufacturer can be consulted to design a suitable pipe connection length. Radar level gauges without waveguide requirements vary in the distance between the gauge’s installation connection and the inner wall of the tank, depending on the type of antenna used. Generally, the distance between the center of the liquid level gauge connection tube and the inner wall of the tube should not be less than 1/2 of the beam width. Beam width = 2X measurement distance X tan(beam angle/2), where the beam angle is the angle at which the intensity of the radar waves reaches half of its maximum value. When the microwave frequency emitted by the antenna is fixed, the larger the size, the smaller the beam angle. Compared to horn antennas, parabolic antennas have a much narrower beam width.