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Ultrasonic level meter is widely used in liquid level measurement of various containers and open tanks in industrial production. It has the advantages of wide application range, long service life, high measurement accuracy, simple installation and maintenance and stable measurement results.
The advantages of ultrasonic level meter are outstanding, but the accuracy of liquid level measurement is easily affected by temperature, humidity, dust and chemical composition of the tested medium.
According to the working principle of ultrasonic water level meter, the speed of ultrasonic propagation in a medium is constant under specific conditions of medium pressure, temperature, density and humidity. Ultrasonic level meter measures the time required from transmitting ultrasonic waves to receiving the liquid level reflection. The ultrasonic wave passing distance can be converted to liquid level information. That is, the distance from probe to the bottom of water tank - the distance from probe to liquid level = actual level.
In actual measurement, the temperature is not fixed. That is why both accuracy and temperature should be considered. The measurement error of air temperature is 1, and its influence on the speed of sound is 0.6 m/s. At 20℃, the sound speed at 1 pressure is about 340 m/s. Therefore, the influence on the measurement error can be calculated as about 0.17%. That is to say, if the temperature measurement error is 3, the horizontal measurement error exceeds the nominal range of 0.5% of most manufacturers.
Therefore, the actual accuracy of 0.5% is generally suitable for the working environment under normal temperature and pressure. However, for high and low temperatures, the measurement accuracy may exceed 0.5%. In addition, if the temperature changes or changes quickly, the measurement error will also increase.
Because the temperature has a great influence on the speed of sound, engineers embedded the temperature sensor into the probe to measure the ambient temperature, and used the temperature correction method to improve the measurement accuracy of the system to meet the requirements of industrial application. This is why the ultrasonic water level meter should be temperature compensated.
However, when the ultrasonic water level meter is set outdoors, especially when it is exposed to sunlight, the measurement is inaccurate and the accuracy sometimes decreases.
This is because the temperature sensor is sealed by a protective material as a part of the probe. The thermal conductivity of this protective material is low, so the probe temperature will not change in real time according to the ambient temperature.
If the ultrasonic water level meter is not blocked and set outdoors under sunlight, the temperature of the probe may be much higher than the ambient temperature under long-time sunlight. The temperature detected by the temperature sensor of the probe is actually not the ambient temperature, but the real-time temperature of the probe.
The huge difference between probe temperature and actual ambient temperature leads to inaccurate measurement and lower accuracy. In order to reduce the influence of sunlight on the measurement accuracy of ultrasonic level meter set outdoors, engineers adopted the measures of setting a shading protective film on the ultrasonic level meter. Practice has proved that the method of using sun-shading protective film has the following advantages:
(1) Avoiding the problem of inaccurate liquid level measurement and accuracy reduction caused by temperature change resulting from sunlight;
(2) Avoiding the aging problem caused by long-term exposure, preventing the probe of ultrasonic water level meter from being damaged, and making the instrument work normally;
(3) Protecting the ultrasonic water level meter from rain leakage and preventing the instrument from being damaged.
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