Principle and Application of Ultrasonic Level Gauge

Ultrasonic level gauge is also called ultrasonic level meter. Ultrasonic water level meter is a non-contact level measuring instrument with high measurement accuracy, easy installation and basically maintenance-free. It is often used to measure the liquid level in various containers, as well as the water level of canals, pools, reservoirs, rivers, lakes and seas. It can also measure the interface level and liquid level difference. It is especially suitable for sewage and corrosive occasions. It can also measure the flow rate when combined with troughs and weirs to form an open channel flow meter. Therefore, it is more and more widely used in many industries or fields such as steel, petrochemicals, water treatment and water conservancy.

Principle of ultrasonic level gauge

The ultrasonic level gauge is installed on the upper part of the container. Under the control of the electronic unit, the probe emits a beam of ultrasonic pulses to the object to be measured. The sound wave is reflected by the surface of the object, and part of the reflected echo is received by the probe and converted into an electrical signal. The time from the ultrasonic emission to the re-reception is proportional to the distance from the probe to the object to be measured. The electronic unit detects this time and calculates the measured distance based on the known sound speed. Use the distance from the probe to the bottom of the tank - the distance from the probe to the liquid level = the actual liquid level or the level height, and convert the liquid level height into a 4~20 mA current signal, a 1~5 V voltage signal output: or transmit it to the control center through RS485, Hart, GPRS communication. Since temperature has a great influence on the sound speed, the instrument should measure the ambient temperature to correct the sound speed.

Main technical indicators of ultrasonic level gauge

1. Range and blind area

Range and blind area are two important indicators of ultrasonic level gauge.

The range represents the maximum range that the level meter can measure and reflects the sensitivity of the transducer. In other words, the larger the range, the higher the sensitivity. Most manufacturers' nominal ranges are for flat liquid surfaces, but in actual measurement, liquid level fluctuations, floating objects on the surface, measured solid material levels, dust, and steam, all of which may cause the range to fail to reach the nominal value.

Blind area is also called dead area, a distance that the ultrasonic level gauge cannot measure, which is caused by the aftershock of the ultrasonic transducer. For example, the blind area is 30cm, that is, when the distance between the liquid surface and the probe is less than 30cm, there will be a situation where the measurement cannot be made. Then for products with the same range, the smaller the blind area, the better the design of the transducer; for some closed tanks or short-range measurements, the more convenient the installation will be.

2. Temperature and accuracy

Most of the temperature range is -20~60℃. Because most of the ultrasonic level gauge with liquid crystal display, the working temperature of the liquid crystal screen can only be in this range, beyond the range, the liquid crystal display will appear abnormal phenomenon. If you do not take into account the limitations of the liquid crystal display, generally can do -40 ~ 80 ℃ range. In general, the operating temperature of the ultrasonic transducer is difficult to exceed 150 ℃: when more than 150 ℃, easy to damage the piezoelectric ceramics inside, so 150 ℃ can be regarded as an absolute destruction temperature. In addition, some of the materials used in the manufacturing process of ultrasonic transducers can not work for a long time at temperatures above 100 ℃: therefore, the limiting temperature of most transducers is 100 ℃.

Why are accuracy and temperature considered together? Because in the air, the temperature measurement error of 1 ℃, the impact on the speed of sound is 0.6 m / s, 20 ℃, 1 atmospheric pressure under the speed of sound is about 340 m / s. From this we can calculate that the impact on the measurement error is 0.17%; if the temperature measurement error of more than 3 ℃, the level measurement error of more than 0.5% of the nominal range of most manufacturers. In fact, the 0.5% accuracy is for normal temperature and pressure, on the high or low temperature, may lead to the measurement accuracy of more than 0.5% of the situation. In the temperature gradient, the temperature changes quickly, the measurement error will also increase. In addition to the measurement accuracy of the largest impact is the gas composition, such as in some volatile liquid occasions, the volatilization of the liquid leads to changes in the composition of the air, and then lead to changes in the gas speed of sound, and finally caused by the measurement error.

3. Pressure 

In the case of negative pressure, ultrasound measurement is generally not recommended, because ultrasound propagation is realized through the gas. Negative pressure means that the air inside the thin air, ultrasound propagation in thin air, one is the speed of sound will change, resulting in measurement errors, and secondly, thin air, sound wave attenuation increases, resulting in reduced measurement range or even can not be measured.

4. Corrosivity 

The corrosivity of the level meter is mainly tested by the material of the probe. In the weak acid and alkali environment, ordinary plastic shell can be. Teflon shell, can be resistant to most of the strong acids and alkalis. It is worth noting that, if the measured material has a strong corrosive and volatile, with a one-piece ultrasonic level meter, it is best to the circuit board for the rubber peak. subsnce Because most of the shell can be waterproof, can not prevent gas; gas but into the device inside, will corrode the circuit board.