Ultrasonic level gauge: principle, selection

1, Measurement principle: the core logic of time difference method for distance measurement
       The working principle of ultrasonic level gauge can be summarized as "echo ranging method". The instrument is controlled by a microprocessor and emits high-frequency ultrasonic pulses to the measured liquid surface through a transducer (probe). After the sound waves are reflected by the liquid surface, the echoes are received by the same transducer and converted into electrical signals. The system calculates the distance from the sensor to the liquid surface based on the time difference between the emission and reception of sound waves, combined with the propagation speed of sound waves in the medium. The mathematical expression is: S=C × T/2, where S is the distance, C is the speed of sound, and T is the propagation time.
       It is worth noting that the pulses emitted by the ultrasonic transducer have a certain width, which causes the emitted waves and reflected waves to overlap and be unrecognizable in the area closer to the probe, which is called the "measurement blind spot". Blind spot is a hard indicator that must be carefully considered when selecting - the installation height must be greater than the blind spot, otherwise it will not be possible to measure normally. In addition, the speed of sound is affected by factors such as temperature and medium density. Modern intelligent instruments usually have built-in temperature sensors for speed compensation to ensure measurement accuracy.

2, Selection key points: comprehensive consideration based on working conditions
       The scientific selection of ultrasonic level gauges should revolve around the following core dimensions:
Measurement range and blind spots: The range of conventional products covers 0.1m to 30m, and a redundant range of 10% -20% should be reserved when selecting. At the same time, it is necessary to ensure that the highest liquid level is not higher than the height of the blind spot of the instrument, which is the basic prerequisite for measurement.
Characteristics of the medium: It is necessary to determine whether the tested liquid is corrosive, volatile, and contains a lot of foam or suspended solids. Ultrasonic level gauges are non-contact measurements, and corrosive media do not require special treatment, but probes made of corrosion-resistant materials need to be selected; However, a large number of foam will absorb and scatter ultrasonic waves, resulting in unstable echo and even inability to measure.
Environmental conditions: The working temperature range of conventional models is -20 ℃ to 60 ℃, and the extreme working condition can reach -40 ℃ to 85 ℃; The pressure usually needs to be below 0.3MPa. Targeted selection is required for environments such as high temperature, high pressure, strong dust, and strong electromagnetic interference.
Installation space: The probe should be vertically aligned with the liquid level, avoiding obstacles such as tank walls, mixers, and pipelines, and the distance from the tank wall should generally be greater than 0.3 meters.

3, Applicable and Not Applicable Scenarios
Applicable scenarios: Ultrasonic level gauges are most suitable for use at room temperature, atmospheric pressure, calm liquid level, and without strong interference. Typical applications include sewage treatment tanks, water treatment plants, water conservancy and hydrological monitoring, acid-base storage tanks, food and beverage tanks, etc. For highly corrosive and high viscosity media, their non-contact characteristics have natural advantages.
Not applicable scenario: The limitations are also clear. Firstly, a vacuum environment cannot be used - ultrasound is a mechanical wave that requires a medium for propagation. Secondly, high temperature and high pressure are not applicable. Generally, the upper limit of temperature tolerance is 80 ℃, and caution should be taken when the pressure exceeds 0.4MPa. Third, in the case of strong volatile media (such as fuming sulfuric acid and nitric acid) and a large number of foam and dust, the sound wave will be seriously attenuated or absorbed, and the measurement error will be large or even invalid. Fourthly, large storage tanks such as internal floating roof tanks have a large range, and ultrasonic level gauges typically have a range of 3-15 meters, which is difficult to handle. Fifthly, frequent temperature changes can also affect accuracy due to fluctuations in sound velocity.