Radar and ultrasonic level measurements are both widely used in industrial and environmental applications for measuring the level of liquids and solids in lift stations, tanks, and open channel flows. Both technologies work on the principle of measuring the time of flight between the transmission of the signal and its reflection from the surface of the media being measured, ultimately providing a level value based on that time. However, there are some fundamental differences between the two technologies that can affect their performance, and a few reasons as to why radar becomes the obvious technology of choice:
- Radar is not affected by ambient conditions
Radar signals are electromagnetic waves that travel at the speed of light, while ultrasonic signals are sound waves that travel at the speed of sound. In essence, this means that radar signals can penetrate through dust, vapor, or changing air properties such as temperature variance and wind. Ultrasonic signals on the other hand, can be affected by all these ambient conditions as the changes in air properties cause changes in signal attenuation, reflection, or refraction. Changes in air temperature, windy conditions, or condensation can in other words all cause unreliable measurement when using an ultrasonic level transmitter.
- Radar is not affected by process conditions
One example is surface turbulence. This occurs when the surface of the liquid or solid being measured is agitated or disturbed, which causes waves and ripples on the surface. These waves and ripples can reflect and scatter pulsed radar or ultrasonic signals, making it difficult to accurately measure the level of the material. In applications such as pump stations, open channel flows, filter beds and aeration basins, this form of turbulence cannot be avoided.
When dealing with frequency modulated continuous wave (FMCW) radar level measurement, the impact of surface turbulence is minimized as the radar sweeps the surface with varying frequency, gaining more values to calculate an average level value from. Paired with Fast Sweep Technology, you gain even more sweeps, providing an accurate and reliable measurement even in turbulent applications.
In contrast, ultrasonic level measurement is more sensitive to surface turbulence because the waves and ripples on the surface can cause the ultrasonic signals to scatter and reflect in unpredictable ways, leading to inaccurate and unreliable measurements. The same issues with ultrasonic level measurement is also true for the presence of gases in air, spiderwebs, or internal obstructions, where ultrasonic sound waves reflect and refract, causing unreliable level measurements, as opposed to FMCW radar.
- Radar is easier to setup
When setting up a radar level device, you need to set the reference height value, which is the distance from the bottom of the sensor lens to the bottom of the application, and you are done. The algorithms in modern radar level devices are designed to track moving surfaces and not let go of it. This is especially effective in water-based applications, where the high dielectric constant of water creates a highly reflective surface for the radar, removing the need for echo-tuning, thus creating a plug-and-play solution.
Setting up an ultrasonic level device can prove to be more demanding. The wide beam angle of ultrasonic sensors will force you to precisely determine your mounting location, avoiding floors, pipes, walls, etc. You will need to make sure that you install it a bit above your application so that the ultrasonic level sensors blind zone does not become an issue. You will then need to set the range, echo tune, and configure the damping value to account for the various reflected signals in your application. Lastly, since the speed of sound varies with temperature, any changes in temperature can affect the time it takes for the ultrasonic signal to travel back and forth to the media, consequently affecting our measured level value. Hence, you will often need to calibrate the transmitter to account for temperature variance. Some ultrasonic level transmitters even require an external temperature sensor to compensate for temperature variations.
All things considered; all factors above were previously deemed tolerable due to ultrasonic level sensors being more price competitive than radar. However, with the introduction of new, compact and cost-effective radar devices such as the Rosemount™ 1208 Level and Flow Transmitter from Emerson, this is no longer the case. Hence, the choice of level measurement technology becomes obvious, especially in water-based applications.
Learn more about the Rosemount 1208 from Emerson.