Understanding the Factors Influencing the Accuracy of Ultrasonic Level Measurement

Introduction to Ultrasonic Accuracy

  • The accuracy of ultrasonic distance and level sensors is crucial, since it provides a measure of the error band as a percentage of a specified scale. 
  • Despite being conceptually straightforward, achieving accuracy in practical applications is difficult.  
  • This specification is often determined in controlled environments, free from external influences such as temperature variations, wind, or air movement.

What factors can affect the accuracy of ultrasonic level measurement?

Key Concepts and Challenges in Accuracy Specifications

  • The accuracy principles apply to all ultrasonic level sensors, regardless of the measured parameter. 
  • Many published accuracy specifications are used to compare different sensors, however discrepancies and confusion make comparisons difficult.  
  • Some sensors incorporate features that enhance overall accuracy.

Absolute Accuracy and Its Calculation

  • Absolute accuracy, which is stated as the difference between the measured output value and the actual target distance, provides a clear picture of the performance of a sensor because it expresses the difference between the two values. 
  • For instance, a water level sensor with a full-scale range of 12 feet might have an accuracy of 0.144 inches when tested in a controlled environment. 
  • This accuracy percentage remains consistent across all of the distance readings recorded.

What are the external factors influencing on accuracy of distance measurement by use of ultrasonic sensors?

Physical Parameters Influencing Accuracy

Air Temperature

  • The temperature of the surrounding air emerges as the key element that affects the accuracy of ultrasonic sensors. 
  • Changes in temperature have an effect on the speed of sound waves, which causes variations in the distances that targets appear to be. 
  • Due to the fact that the speed of sound varies with temperature by approximately 0.17% for every degree Celsius, the distances that are estimated are subject to distortion. 
  • Even while the vast majority of sensors have compensating mechanisms and internal temperature sensors built in, there are still some limits, particularly in environments where conditions are constantly shifting.
  • During the installation and commissioning of the ultrasonic level transmitter in process industries, our team enabled the internal temperature sensors with compensation mechanisms parameter in the ultrasonic level sensor. We then verified the function of the ultrasonic level sensor with different environmental temperatures and compared the actual levels of the substance being measured.

Humidity

  • At ambient temperature, the effect of humidity on the speed of sound is insignificant; however, this effect becomes significant at higher temperatures. 
  • An increase in humidity will change the composition of the air, which will in turn impact the molecular weight of the air and, as a result, the speed of sound. Because of this, the accuracy may be affected negatively under certain environmental conditions.

Air Pressure

  • Although changes in air pressure have only a minor impact on the speed of sound at a given point, the speed of sound does drop significantly with increasing altitude. The variations in the atmosphere continue to be insignificant, which guarantees steady readings in applicable circumstances.

Air Currents and External Disturbances

  • Stormy weather with strong winds, air currents, or hurricanes can disrupt ultrasonic measurements, causing a loss of signal. 
  • Additionally, hot objects generating air turbulence may deflect ultrasound, leading to unstable readings.

Paint Mist and External Noise

  • Paint mist has no discernible impact on ultrasonic sensor operation, but settling on transducer surfaces should be avoided. 
  • External noise, if at the same frequency as the sensor, must not exceed the level of target echoes to prevent interference.

Types of Gas

  • Ultrasonic sensors are designed for operation in atmospheric air. Operation in other gasses, such as carbon dioxide, can lead to substantial measurement errors or complete loss of function due to variations in the speed of sound and attenuation.

Tank Configurations and Dimensions

  • There is a considerable correlation between accuracy and tank types, shapes, and dimensions.
  •  Calculating accuracy and capacity is made simpler with tanks that have a flat bottom and straight sides. This is because there is a linear relationship between the level of the tank and the volume. 
  • Tanks of a non-standard shape present additional challenges.

External Reference Targets for Enhanced Accuracy

  • A system has been developed to address rapid changes in air temperature, providing a quick response without the need for a built-in sensor. This system includes an accessory known as Reference Target Temperature Compensation (RTTC).
  • The RTTC accessory incorporates an external reference target placed at the sensor’s forefront in the measurement path. Alongside the latest ultrasonic level sensor software, the sensor takes two readings for each measurement: one to identify the reference target and another for the distant object. Changes in the speed of sound affect both readings. During calibration, the reference target’s position is fixed, and any perceived shift is proportionally applied to correct the distant object’s apparent location. This results in more accurate measurements unaffected by ambient air temperature, diurnal temperature fluctuations, sensor self-heating, sunlight warming, cold ambient temperatures, or vibration. Field calibrations can be performed at any time or temperature.
  • With the RTTC accessory, diurnal temperature swings can be disregarded, and external heating or cooling of the sensor will not lead to incorrect measurements. The system allows for on-site sensor exchange and calibration at any temperature, improving performance in conditions with significant temperature changes due to diurnal effects. The ultimate performance is influenced by the intensity of diurnal changes and the sensor configuration.

Summary on Accuracy and Sensor Selection

  • Accuracy specifications vary among ultrasonic sensors, ranging from 0.1% to 3% under controlled conditions. When accuracy is crucial, consideration of temperature compensation features and external reference targets, especially for outdoor use, is vital. 
  • Selecting a sensor involves not only examining published accuracy specifications but also ensuring the effectiveness of built-in temperature compensation mechanisms.
  • In conclusion, understanding the multifaceted factors influencing the accuracy of ultrasonic level measurement is essential for deploying sensors effectively across diverse environmental conditions.

FAQs for Accuracy of Ultrasonic Measurements

What is the Accuracy of Ultrasonic Measurements?

  • Under conditions that are properly regulated, the most accurate ultrasonic sensors are able to reach an accuracy of between 0.1 and 0.2% of the range being detected, while the majority of good ultrasonic sensors can normally achieve an accuracy of between 1% and 3%.
  • Ultrasonic level measurements typically offer high accuracy, with precision ranging from a few millimeters to centimeters, depending on the sensor design and application requirements.

What are the Problems with Ultrasonic Level Sensors?

  • Chemicals that outgas can disrupt ultrasonic level sensor performance by interfering with sound waves. 
  • This interference may lead to inaccurate or absent readings. Additionally, obstacles, temperature fluctuations, humidity, and acoustic impedance changes pose challenges, affecting accuracy and reliability.

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