Air temperature has the greatest impact on the measuring accuracy of an ultrasonic sensor. 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. However, as the air temperature changes, the speed of sound changes by 0.17% per degree Kelvin. Almost all Pepperl+Fuchs ultrasonic sensors have a temperature probe to compensate for this effect. This probe measures the ambient temperature and the sensor corrects the temperature-related distortion of the measured values (see temperature compensation).
Humidity has negligible influence on the speed of sound at room temperature and at lower temperatures. However, at higher air temperatures, the speed of sound increases as humidity increases.
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.
If the object has the reflective properties of the standard reflector, regular air currents (wind) have no effect on ultrasonic measurement to speeds of 7 Bft (50-61.5 km/h). Stormy weather or hurricanes can cause unstable measurements (with loss of signal). Regarding changes to the speed of sound, no general conclusions can be drawn. This is because air current direction and air current speeds constantly change. For example, 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 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 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.
Types of gas
Ultrasonic sensors by Pepperl+Fuchs 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.