Ultrasonic level sensor, also known as ultrasonic liquid level gauges, are non-contact level measurement instruments. They feature high accuracy, easy installation, and minimal maintenance. Commonly used to measure liquid levels in various containers, as well as water levels in canals, pools, reservoirs, rivers, lakes, and seas, they can also measure interface levels and level differences. They are particularly suitable for wastewater and corrosive environments. When used in conjunction with tanks and weirs to form open channel flow meters, they can also measure flow rate. Therefore, they are increasingly widely used in many industries and fields such as steel, petrochemicals, water treatment, and water conservancy.
Measurement principle of ultrasonic level sensor
Installed on the upper part of a container, the ultrasonic level gauge, under the control of an electronic unit, emits a beam of ultrasonic pulses towards the object being measured. The sound waves are reflected by the object's surface, and part of the reflected echo is received by the probe and converted into an electrical signal. The time from the emission of the ultrasonic wave to its reception is proportional to the distance between the probe and the object being measured. The electronic unit detects this time and calculates the measured distance based on the known speed of sound. The distance from the probe to the bottom of the tank minus the distance from the probe to the liquid level equals the actual liquid level or level height. This is used to convert the liquid level height into a 4~20 mA current signal or a 1~5 V voltage signal for output. Alternatively, it can be transmitted to the control center via RS485, HART, or GPRS communication. Since temperature has a significant impact on the speed of sound, the instrument should measure the ambient temperature to correct for the speed of sound.
Advantages and disadvantages of ultrasonic level gauges
Compared to other types of level gauges, ultrasonic level sensors have the following advantages:
(1) Non-contact measurement: The ultrasonic transducer is installed above the liquid surface and does not contact the measured medium. This allows for convenient measurement of corrosive, viscous, or toxic liquids, avoiding corrosion or contamination by the measured liquid and eliminating maintenance requirements.
(2) Good versatility: The level gauge can measure the liquid level in open channels as well as in large storage tanks.
Easy installation and disassembly.
(3) Strong adaptability: It has a wide range of applications and is not affected by the density, dielectric constant, or conductivity of the medium. It is highly adaptable to the physicochemical properties of the measured liquid. (4) Suitable for measuring the level of toxic, corrosive, and high-viscosity liquids, overcoming the shortcomings of other level gauges in such harsh measurement environments.
(5) Almost no moving mechanical parts, no wear, long service life, and light weight. The piezoelectric element inside the transducer vibrates at acoustic frequencies, with small amplitude, long life, and good stability.
Disadvantages mainly include: when the measured liquid is volatile, uneven air density above the liquid surface can lead to larger measurement errors; when the measured liquid surface has large undulations, it can easily cause chaotic sound wave reflections, resulting in errors. Additionally, ultrasonic level gauges have unavoidable blind zones when measuring liquid levels, making short-distance measurements difficult.
Difference between integrated and split-type ultrasonic level gauges
Integrated ultrasonic level gauges integrate the ultrasonic probe, signal processing unit, display, etc., into one device. All measurement and control functions are completed within the same housing.
Easy installation: Due to its compact structure, only one device needs to be fixed during installation, eliminating complex wiring and installation procedures. Comprehensive Functionality: Suitable for harsh industrial environments, with protection ratings up to IP66/IP67. Multiple models are available, such as corrosion-resistant and explosion-proof types.
Simplified Calibration: The integrated design simplifies and simplifies the calibration process, typically requiring calibration of the entire system rather than multiple separate components.
Integrated ultrasonic level gauges are commonly used in storage tanks, industrial wastewater treatment, chemical reaction vessels, and shallow water pools.
Split-Type Ultrasonic Level Gauges: Split-type ultrasonic level gauges consist of a separate probe and signal processing unit. The probe is installed at the measurement point, while the signal processing unit can be installed in a control room or other convenient location away from the measurement point. The two are connected by a cable.
Strong Anti-interference Capability: With the probe and signal processing unit separated, the signal processing unit can be kept away from areas with high temperature, high pressure, or strong electromagnetic interference, thereby improving measurement accuracy and reliability.
Easy Operation: The signal processing unit typically features a larger display screen and more operating interfaces, facilitating adjustments and monitoring by operators.
High adaptability: The split design allows it to adapt to harsh measurement environments, such as high-temperature, corrosive gas or liquid environments. The probe can be made of special materials to withstand these conditions.
Split-type ultrasonic level gauges are suitable for level measurement in large storage tanks, complex processes, and high-temperature or highly corrosive environments. They are especially suitable for applications requiring remote operation or protection from environmental interference.
Key Technical Specifications of Ultrasonic Level Gauges
1. Measuring Range and Dead Zone
Measuring range and dead zone are two important indicators of ultrasonic level gauges.
The measuring range represents the maximum range that the level gauge can measure, reflecting the transducer's sensitivity. In other words, the larger the measuring range, the higher the sensitivity. Most manufacturers specify the measuring range for a smooth liquid surface, but in actual measurement, liquid level fluctuations, floating objects on the surface, and the presence of dust or steam in the measured solid material can all cause the measuring range to fall short of the nominal value.
The dead zone, also known as the blind zone, is the distance that the ultrasonic level gauge cannot measure due to the aftershocks of the ultrasonic transducer. For example, a dead zone of 30cm means that when the distance between the liquid surface and the probe is less than 30cm, measurement will not be possible. Therefore, for products with the same measuring range, a smaller dead zone indicates a better transducer design; it also makes installation easier for measurements in closed tanks or with short measuring ranges.
2. Temperature and Accuracy
The temperature range is mostly specified as -20~60℃. Because most level gauges using LCD displays have an operating temperature range limited to a certain limit for the LCD screen; exceeding this range will cause malfunctions. If the limitations of the LCD display are disregarded, the operating temperature range is generally -40 to 80℃. Under normal circumstances, the operating temperature of ultrasonic transducers rarely exceeds 150℃: exceeding 150℃ can easily damage the piezoelectric ceramics inside, therefore 150℃ can be considered an absolute destructive temperature. Furthermore, some materials used in the manufacturing process of ultrasonic transducers cannot operate for extended periods at temperatures above 100℃; therefore, the maximum temperature limit for most transducers is 100℃.
Why consider accuracy and temperature together? Because in air, a 1℃ temperature measurement error affects the speed of sound by 0.6 m/s. At 20℃ and 1 atmosphere, the speed of sound is approximately 340 m/s. Therefore, the impact on measurement error is calculated to be 0.17%; if the temperature measurement error exceeds 3℃, the level measurement error will exceed the nominal range of 0.5% for most manufacturers. In reality, the 0.5% accuracy is for normal temperature and pressure conditions. At higher or lower temperatures, the measurement accuracy may exceed 0.5%. Measurement errors also increase in environments with temperature gradients or rapid temperature changes. Furthermore, gas composition has the greatest impact on measurement accuracy. For example, in the presence of volatile liquids, the evaporation of the liquid changes the air composition, which in turn changes the gas sound velocity, ultimately causing measurement errors.
3. Pressure
Under negative pressure, ultrasonic measurement is generally not recommended because ultrasound propagation is achieved through gas. Negative pressure means the air inside is rarefied. Ultrasound propagation in rarefied air causes two problems: firstly, the sound velocity changes, causing measurement errors; secondly, sound wave attenuation increases in rarefied air, leading to a reduced measurement range or even preventing measurement altogether.
4. Corrosivity
The corrosivity of level gauges primarily tests the material of the probe. In weakly acidic or alkaline environments, ordinary plastic shells are sufficient. Polytetrafluoroethylene (PTFE) shells can withstand most strong acids and alkalis. It is worth noting that if the substance being measured is highly corrosive and volatile, it is best to apply adhesive to the circuit board when using an integrated level gauge. This is because most waterproof housings are not gas-proof; once gas enters the equipment, it will corrode the circuit board.
