Working principle of ultrasonic level gauge

Ultrasonic level gauge, Chuchen Technology is a professional manufacturer of ultrasonic level gauges, and ultrasonic level gaugesJ grid discountLarge quantities are preferred! This instrument contains multiple self-developed componentsZ LiTechnology, with new signal processing technology, has the characteristics of safety, cleanliness, high precision, long service life, stability and reliability, easy installation and maintenance, and simple reading. It is widely used in industries such as petroleum, chemical, water treatment, water conservancy, steel, coal mining, electricity, and food processing, and is suitable for various fields such as acid, alkali, salt, anti-corrosion, high temperature, and explosion prevention. This instrument can be connected to various DCS systems through 4-20mA or RS485 (Modbus protocol), providing real-time liquid level data for industrial automation operation.

Working principle of ultrasonic level gauge: complete analysis from acoustic characteristics to level calculation

1、 The basic characteristics of ultrasound: why choose ultrasound?

• Strong directionalityHigh frequency ultrasound has a short wavelength (such as a 20kHz sound wave with a wavelength of about 1.7cm and a 200kHz wave with a wavelength of about 1.7mm), and has a small divergence angle during propagation. It can be directionally focused on the liquid surface, reducing stray reflection interference.

• Media dependenceUltrasonic waves need to propagate through elastic media (such as air, liquid, solid) and cannot propagate in vacuum, which is also the reason why their measurement relies on air media.

• Reflection characteristicsWhen ultrasound enters another medium (such as air → liquid) from one medium, significant reflection occurs due to the large difference in acoustic impedance (medium density × sound velocity) between the two media (reflectivity can reach over 90%), which is the source of liquid level echo signals.

• Stable propagation speedAt standard atmospheric pressure, the propagation speed of ultrasound in air is about 340m/s, and the speed is linearly related to temperature (the sound velocity increases by about 0.6m/s for every 1 ℃ increase in temperature). The deviation can be corrected by temperature compensation.

2、 Core Measurement Principle: The Complete Process of Echo Ranging Method

1. Ultrasonic emission: from electrical signals to mechanical waves

• The liquid level gaugeMain control circuit(Usually a microprocessor) generates high-frequency electrical pulse signals (frequency 20kHz~200kHz, selected according to the range: use high frequency for small ranges to reduce divergence; Using a large amount of low frequency to reduce attenuation, transmit toTransducers (probes).

• The transducer is a core component that operates based on the "piezoelectric effect": its internal piezoelectric crystals (such as piezoelectric ceramics) undergo mechanical vibration under electrical pulse excitation, converting electrical energy into ultrasonic mechanical energy and emitting it directionally towards the measured liquid surface.

2. Sound wave propagation: Path and attenuation in air

• Ultrasonic waves start from the probe and propagate in a straight line in the air, and the propagation speed is affected by environmental factors

• temperatureThe core influencing factor, as mentioned earlier, is that the sound velocity varies significantly with temperature (formula: v=331.4+0.607 × T, where T is the ambient temperature ℃).

• atmospheric pressureThe impact is relatively small and can be ignored under standard atmospheric pressure. It only needs to be fine tuned in low-pressure environments such as plateaus.

• humidityAn increase in humidity will cause a slight change in air density, resulting in a deviation of about 0.1% in sound velocity, which is usually negligible.

• During the propagation process, ultrasonic waves will attenuate due to air absorption and scattering (such as dust and water vapor), and the larger the range and frequency, the more obvious the attenuation (therefore, low-frequency sound waves are often used in large range level gauges).

3. Liquid level reflection: generation of echo signals

• When ultrasound reaches the surface of a liquid, due to the significant difference in acoustic impedance between air (with an acoustic impedance of about 415 Rayl) and liquid (such as water with an acoustic impedance of about 1.48 × 10 ⁶ Rayl), sound waves will occurspecular reflectionForm an echo.

• If there is foam, wave or suspended solids on the liquid surface, the reflectivity will decrease (the reflectivity of foam may be lower than 30%), and the echo signal will be weakened, which needs to be optimized through probe gain adjustment or filtering algorithm.

4. Echo reception: from mechanical waves to electrical signals

• The reflected echo is captured by the same transducer (or an independent receiving transducer), and the piezoelectric crystal converts the mechanical vibration into a weak electrical signal. After amplification and filtering by a preamplifier (to remove environmental noise), it is transmitted to the main control circuit.

• The main control circuit needs to distinguish between "effective echoes" and "interference echoes" (such as container wall reflections and bubble reflections), usually through the following methods:

• settingblind spotEchoes within a certain distance (such as 0.3~1m) below the probe are not recognized to avoid strong reflection interference at close range.

• Echo characteristic analysis: The amplitude and frequency of effective echoes are consistent with the transmitted wave, while interference echoes usually have low amplitude and disordered frequency.

5. Liquid level calculation: conversion from time difference to height

• The main control circuit records the time difference between the "emission time" and "reception echo time" of ultrasonic waves (i.eRound trip propagation time t）.

• Calculate the vertical distance (d) from the probe to the liquid surface: Due to the back and forth propagation of sound waves, the actual one-way distance is half the product of the sound velocity (v) and time (t), that is:

• Calculate the actual liquid level height (h): Given the installation height of the probe (H, which is the vertical distance from the probe to the bottom of the container), the liquid level height is:

• Finally, the liquid level gauge outputs measurement results through methods such as 4-20mA current signal, RS485 communication, or digital display.

3、 Key component: Core hardware that ensures the implementation of principles

1. Transducers (probes)

• Polytetrafluoroethylene: acid and alkali resistant, suitable for chemical applications;

• Stainless steel: wear-resistant, suitable for dust and particle environments;

• Polypropylene: lightweight, low-cost, suitable for clean water and sewage scenarios.

• Function: To achieve bidirectional conversion between electrical signals and ultrasonic waves, it is divided into "transmission/reception integrated type" (low cost, suitable for most scenarios) and "separation type" (strong anti-interference ability, suitable for complex working conditions).

• Material: The surface of the probe should be resistant to environmental corrosion, and commonly used materials include:

2. Main control circuit

• The core is the microprocessor (such as MCU, DSP), responsible for generating transmission signals, capturing echo time, executing temperature compensation algorithms, calculating liquid level, and outputting data.

• Key performance: Time measurement accuracy (needs to reach nanosecond level, such as 1ns accuracy corresponding to a distance error of about 0.17mm), anti-interference ability (achieved through electromagnetic shielding and filtering circuits).

3. temperature sensor

• Built in NTC thermistor or PT1000 sensor, real-time monitoring of ambient temperature, used to correct sound velocity (v), is the core compensation method to ensure measurement accuracy (without temperature compensation, a temperature change of 10 ℃ can cause an error of ± 3%).

4. Power supply and output module

• Power supply: Usually supports 24V DC industrial power supply, and some models are compatible with 110-220V AC.

• Output: Analog quantity (4~20mA), digital quantity (RS485/MODBUS), relay switch quantity (used for high and low liquid level alarm), to meet the needs of different control systems.

4、 Error Sources and Compensation: Key Technologies for Improving Accuracy

1. temperature error

• When not compensated, for every 10 ℃ deviation in temperature, the deviation in sound velocity is about 6m/s, and the corresponding liquid level error for a 10m range can reach ± 8.8cm.

• Compensation method: Real time temperature is collected through a built-in temperature sensor, and then corrected using the sound velocity formula (v=331.4+0.607 × T) to control the error within ± 0.1%.

2. Liquid level fluctuation error

• Severe shaking of the liquid surface can cause unstable echo signals and fluctuations in time difference (t).

• Compensation method: Adopt "taking the average of multiple measurements" (such as taking the average of 10 measurements), set "damping coefficient" (delaying the output change speed), or use a waveguide (limiting the range of liquid level fluctuations).

3. Environmental interference error

• Choose a "high-temperature probe" (capable of withstanding temperatures above 150 ℃) to cope with steam environments;

• Increase probe power (increase the amplitude of emitted sound waves) to counteract attenuation;

• Install dust and splash covers to reduce physical interference.

• Steam, dust, and strong airflow can cause sound wave scattering or attenuation, resulting in weakened or even lost echo signals.

• Optimization method:

4. installation error

• The tilted installation of the probe will cause the measurement distance (d) to be a "slant distance" rather than a vertical distance, resulting in deviation.

• Avoidance method: During installation, ensure that the probe is perpendicular to the liquid surface (tilt angle ≤ 3 °) and away from the container wall (distance ≥ 1/6 of the container diameter) to reduce wall reflection interference.

5、 Principle limitations: Boundary of applicable scenarios

• Cannot be used in a vacuum or inert gas environmentUltrasonic waves need to propagate through an air medium, and there is no signal in a vacuum environment.

• Not suitable for high viscosity or easily crystallizable mediaIf the liquid surface is covered with thick layers of crystals, the reflected echoes will be absorbed, resulting in measurement failure (requiring a cleaning device).

• Range limitedUltrasonic waves attenuate rapidly in the air, and the maximum range of conventional models is about 50m (low-frequency probe). If it exceeds this range, a radar level gauge should be used.

• Accuracy lower than contact instrumentsThe precision of conventional industrial grade ultrasonic level gauges is ± 0.5%~± 1% FS, which is lower than that of input level gauges (± 0.1% FS) and is not suitable for high-precision measurement scenarios.

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