Accuracy
Accuracy on an ultrasonic distance and level sensor is relatively easy to understand, but often a difficult specification to achieve in a real-world application. What the accuracy specifications are really telling you is the error band as a percentage of a specific scale. Understanding accuracy specifications requires an explanation in a bit more detail, along with some of the issues with this specification.
The same principles apply to all ultrasonic level sensors no matter what you are measuring. To better explain how accuracy works, all potential environmental factors should be removed (like temperature, for example). Accuracy specifications are generally determined indoors, in a controlled lab environment, and under constant temperature and fixed conditions and with no interference from factors such as wind or air movement, etc. Generally, specifications such as Accuracy are used to compare one sensor to another. Many published Accuracy specifications are not clear or consistent with each other and difficult to compare as well, and some sensors have better features to help achieve better accuracy.
Accuracy, or absolute accuracy is the difference between the output value that is measured by the Ultrasonic sensor, and the actual target distance. For example, an ultrasonic water level sensor reading a full-scale range of 12 feet or 144 inches will have an accuracy of ±0.144 inches (at ambient temperature and controlled conditions). The same sensor reading a distance of 75 inches will have an accuracy of ±0.075 inches. This 0.1% detected accuracy is applicable whether the sensor is reading in distance level/volume (gallons).
The specification of accuracy of a measurement of an ultrasonic sensor depends on several physical parameters.
Air Temperature
Air temperature has the greatest impact on the measuring accuracy of an ultrasonic sensor. Temperature fluctuation affects the speed of an ultrasonic sensors pulse or sound waves. As temperature increases, sound waves travel faster to and from the target. Even though the target has not likely moved or shifted, it will appear that the target is closer.
A more detailed explanation is that after the transit time of the reflected ultrasonic pulse has been measured, the sensor calculates the distance to the object using the speed of the sound. When sound is propagated in air, the speed of sound is about 344 m/s at room temperature. However, the speed of sound is temperature-dependent and changes by approximately 0.17% with each degree Celsius. These changes affect the transit time and can distort the calculated distance. Without temperature compensation and at a measuring distance of 100 cm, a 20° C change in temperature would cause a measurement error of -8.5 cm at 70° C and +7.65 cm at -25° C.
Most ultrasonic sensors by Senix have a working range of -40° C to +70° C, and all ToughSonic ultrasonic sensors have an internal or embedded temperature sensor to compensate for this effect. This internal sensor measures the sensor body temperature, and the sensor corrects the temperature-related distortion of the measured values (see temperature compensation). Internal temperature compensation sensors do have limits, however. An internal temperature compensation may also be affected by external heating or cooling sources as they cannot adjust to extremely rapid changes well, plus they may not be close to the temperature in the actual measurement path such as inside an enclosed tank.
The following shows a Senix ToughSonic CHEM sensor measuring outdoors with fluctuating temperature. The Orange is the temperature of the sensor that is changing by as much as 5-6 degrees Fahrenheit. The Red shows the distance measurement fluctuating with temperature by at least 3-4 inches. The Green line is the distance measurement with temperature compensation turned on in the sensor and measurements much more stable and consistent.
Humidity
Humidity has negligible influence on the speed of sound at room temperature and at lower temperatures (0.036% / 10% RH change). However, at higher air temperatures, the speed of sound increases as humidity increases and can have a negative effect on accuracy. Due to increase in relative humidity some nitrogen and oxygen molecules of air are replaced by lighter molecules of water vapor as a result, molecular weight of air gets decreased which ultimately affects the speed of sound.
Air Pressure
The speed of sound decreases by less than 1% between sea level and 3,000 m altitude. Atmospheric fluctuations at a specific location are negligible and the effects on the speed of sound are hardly measurable.
Air Currents
Stormy weather with strong winds or air currents or hurricanes can cause unstable measurements (with loss of signal). Also, particularly hot objects, such as red-hot metal, cause significant air turbulence. The ultrasound can be scattered or deflected in such a way that no evaluable echo is returned.
Paint Mist
Paint mist has no detectable effect on the operation of ultrasonic sensors. However, the mist should not be allowed to settle on the active transducer surface to avoid compromising the transducer’s sensitivity.
External noise
External noise is distinguished from the desired target echoes and generally does not cause malfunctions. If the source of disturbance has the same frequency as the ultrasonic sensor, the level of the external noise must not exceed the level of the target echoes. This can occur when filling a silo with stone, as an example. Compressed air jets also issue ultrasonic noise and can possibly interfere as well.
Types of Gas
Ultrasonic sensors by Senix are designed for operation in atmospheric air. Operation in other gases (for example in carbon dioxide) can cause serious errors of measurement or even total loss of function due to deviations in the speed of sound and attenuation.
Tank Configurations and Dimensions
There are many tank types and shapes, and the dimensions of a tank or well are very important to understand when it comes to accuracy of an ultrasonic level sensor. Flat bottom tanks or wells with straight sides are the easiest to calculate accuracy and capacity because there is a linear relationship between tank level and volume. Irregular shaped tanks are obviously more difficult.
External Reference Targets
For even more severe changes in air temperature that require an even faster response, Senix has developed the ToughSonic RTTC system that overcomes the time lag inherent with a built-in sensor and offers customers with an improved compensation option.
ToughSonic Reference Target Temperature Compensation accessory uses an external reference target at the front of the sensor located in the measurement path. Combined with the latest SenixVIEW software, for each measurement the sensor takes two readings; one to locate the reference target, and one to the distant object. Any change in the speed of sound affects both measurements. The reference target location is locked during calibration, and any change in its apparent position is applied proportionally to correct the distant object’s apparent location. The result is a more accurate measurement, unaffected by ambient air temperature, diurnal temperature swings, sensor self-heating, sunshine warming the sensor, cold ambient temperatures, or vibration. Field calibrations can be done at any time or temperature.
The two illustrations below show side by side sensors operating over a canal, one with conventional temperature compensation and one with RTTC compensation. In the conventional, notice the apparent canal depth varies with the extremes of temperature, while in the RTTC plot notice the apparent distance is unaffected by temperature diurnal swings. The temperature effects have been largely eliminated.
Diurnal temperature swings can be ignored, and external heating or cooling of the sensor will not result in incorrect distance or level measurements. The sensor could be exchanged on site and a new calibration could be done on site at any temperature.
The RTTC accessory will improve the sensor’s performance in conditions with significant changes in temperature due to diurnal affects. The intensity of the diurnal changes and how the sensor software is set up will affect the sensor’s ultimate performance.
Summary on Accuracy
Accuracy specifications on ultrasonic sensors differ in their value, and some are unclear as to the detailed conditions in which they were obtained. The more accurate ultrasonic sensors can achieve 0.1 – 0.2% of the detected range under perfectly controlled conditions, and most good ultrasonic sensors can generally achieve between 1% and 3% accuracy. When selecting an ultrasonic sensor where accuracy is important, be sure to not only look at the published accuracy specification but be sure they have built in temperature compensation and that it is truly improves performance. If using the sensor outdoors see if the manufacturer has an option for temperature compensation such as a reference target adaptor.