Ultrasonic liquid level gauge is a non-contact continuous liquid level measuring instrument controlled by microprocessor.Simple installation, convenient measurement, not affected by liquid viscosity and density.It is widely used in liquid level measurement due to its absence of mechanical moving parts, higher reliability and less maintenance.However, the measurement accuracy is relatively low, and there is a blind area in the measurement, so it cannot be measured in a pressure vessel, nor can it be used to measure volatile media.It is precisely because of the limitations of the ultrasonic level gauge itself that if no attention is paid during use, some failures are prone to occur.In order to help users use the ultrasonic liquid level gauge better, this article focuses on the working principle and failure causes of the ultrasonic liquid level gauge.
1. Measuring principle of ultrasonic liquid level gauge
In the measurement of the ultrasonic liquid level gauge, the ultrasonic pulse is sent out by the sensor (transducer), and after being reflected by the surface of the object, it is received by the same sensor (transducer) and converted into an electrical signal.The distance between the sensor (sensor) and the measured liquid level is calculated by the time between ultrasonic emission and reception.
Therefore, the ultrasonic liquid level gauge uses the principle of measuring the echo distance to measure according to the emission and reception of ultrasonic waves.
Since the transmission and reception of ultrasonic waves is a round trip, the actual measured distance value is expressed by the formula: S=CT/2.Where s is the measured distance value, c is the speed of sound, and t is the propagation time.
The propagation speed of ultrasonic waves in the air is fixed, and the depth of the pool or the height of the water tank is also fixed.The actual liquid level is the depth of a container such as a pool or tank minus the distance measured by ultrasonic waves, that is, the liquid level or the height of the liquid level.Thus the ultrasonic measurement of the liquid level is realized.
Second, the cause of the failure of the ultrasonic liquid level gauge
1. Inaccurate measurement caused by ultrasonic propagation speed C
Because the propagation speed C of ultrasonic waves in different media is different, for example, at 0, the propagation speed of sound waves in air is about 331 m/s, and the propagation speed in water vapor is 403 m/s, and the propagation speed C will also vary with changes with changes in pressure and temperature.When the temperature is 100, the propagation speed of the sound wave in the air increases from 331 m/s at 0 to 387 m/s, and the closer the probe is to the liquid surface, the propagation speed is measured.In this way, measurement errors due to variations and uncertainties in the propagation velocity C must be reduced by physical calibration or pressure and temperature compensation.There are now many measuring probes with pressure and temperature compensation.It is also possible to directly compensate by setting the pressure and temperature values to the current values when the pressure and temperature are stable.
2. Power supply voltage fluctuations lead to inaccurate measurement
For a single-probe measurement device, both the transmitter and the receiver use the same measurement probe, so when a high excitation voltage is applied to the probe when transmitting ultrasonic waves, the distance of the blind zone will increase and may exceed the measurement range; when the probe on the probe When the voltage is too low, due to the long measurement distance, the return signal may not be received, resulting in abnormal operation or measurement error.At this time, the power supply voltage needs to be adjusted to the rated value to make the probe work normally.
3. The selected measurement range is not suitable
Due to the existence of blind distance and attenuation in the process of ultrasonic transmission, ultrasonic measurement of liquid level has a certain measurement range, but there is no limit, so the appropriate probe and working mode should be selected.
4. Measure the failure caused by the propagation medium
In measurement, when the propagating medium contains large droplet particles, ultrasonic waves will scatter and reflect when encountering these liquid droplets, and these liquid droplets will also absorb ultrasonic waves, resulting in attenuation of ultrasonic waves, resulting in measurement errors or making the measuring probe unable to work .This phenomenon is more common in northern China, especially in winter.The hazard is even greater when these droplets adhere to the surface of the measuring probe.On the one hand, the measuring probe will not work; on the other hand, if the time is too long, the liquid droplets may corrode the surface of the probe, making the measuring probe unusable.Therefore, it should be ventilated and wiped in time, and a filter device should be installed, and a protective cover should be installed on the measuring probe.When the droplet particles are small and not attached to the probe surface, it can also be solved by using a high-power probe or a dual-probe measurement method.
5. Faults caused by measuring liquid level
When the measured liquid level is attached to other impurities or the liquid level is very irregular, the ultrasonic wave encounters diffuse reflection or the beam is reflected to other directions, but the ultrasonic wave returning to the measuring probe is very little, which exceeds the sensitivity range of the measuring probe, causing the measuring probe to fail The measuring beam, which cannot be reflected, will not work properly.At this time, iron wire surrounding wells and other methods should be placed under the measuring probe.
6. Failure caused by installation method
Ultrasonic level gauges have blind spots, just like the human eye.A blind spot is when an object is close to the eye but cannot be seen clearly.This distance is called the blind spot.
The blind zone of the ultrasonic level gauge varies according to the range.The smaller the range, the smaller the dead zone, and the larger the range, the bigger the dead zone.But generally between 30cm and 50cm.Therefore, blind spots must be considered when installing ultrasonic liquid level gauges.
The measuring probe (sensor) of the ultrasonic level gauge should be installed vertically in a suitable position.When the installation is inclined, the ultrasonic beam emitted by the measuring probe is reflected in other directions.At this time, the measuring probe cannot work normally without receiving the ultrasonic beam, so it should be corrected and reinstalled.Measurements can also be affected when the measuring probe is too close or too far from the liquid level.Therefore, the measuring probe should be installed at a suitable position from the liquid level, so that the range of the liquid level height change is within the measuring range of the measuring probe, and the highest liquid level should be in the blind area.
