The most common technologies available for continuous level measurement are ultrasonic, free space radar, guided wave radar, capacitance, gamma, and
pressure. Let’s take a look at each technology and some typical applications.
An ultrasonic transducer generates a mechanical sound pulse that is directed through air to the process. When this pulse encounters the process surface, it bounces back to the transducer (figure 1). The transmitter is basically a high-tech timer, measuring the time it takes the pulse to travel to the process and back. This time is directly proportional to the distance to the process surface.
This time-based technology is referred to as time of flight (ToF). Ultrasonic transmitters are used on a variety of simple applications for measuring liquid or solid level in a vessel. The transducers are temperature compensated to give a high level of repeatable accuracy at distances of 2 to 230 feet.
Applications with heavy dust or high temperatures are not well suited for ultrasonic transmitters. If there is too much dust in the air, it defuses the signal and causes a poor return. High temperatures or vapors can also alter the density of the air enough to affect the speed of the pulse transmission, causing errors in measurement.
Mechanical signals travel at the speed of sound and require an atmosphere to transmit the signal; therefore, ultrasonic devices do not work in applications operating under a vacuum. Foam on top of a liquid can also disrupt an ultrasonic signal. The acoustic signal can be absorbed by the foam, resulting in no return echo.
This non-contact radar technology has two different versions: pulse generated and frequency modulated continuous wave (FMCW). The pulse-generated version works on a ToF principle similar to an ultrasonic device. An electromagnetic wave between 1 and 100 GHz is sent from the antenna toward the process surface in search of a change in impedance, which will reflect the signal back to the transmitter. In most cases, the difference in the dielectric between air and the process material will cause the signal to be reflected back. The dielectric of the product is important when selecting a radar unit because the greater the dielectric, the greater the change in impedance and the stronger the reflection. The size of the radar horn (figure 2), the dielectric of the product, and the condition of the process surface (calm or agitated) determine the maximum distance from the device to the process surface.
The FMCW radar versions send out a continuous radar signal, and the frequency shifts as the distance to the process changes. Because FMCW is a continuous wave, it never loses touch with the material, making it better for agitated vessels.
Free space radar is relatively unaffected by environmental conditions such as different gases or vapors between the transmitting device and the process surface. It is largely impervious to variations in process temperature or pressure, and it can work in a full vacuum.
Two process conditions that can affect free space radar are condensation on the transmitter antenna and foam on a liquid surface. Condensation is typically a high dielectric liquid, and the radar signal cannot penetrate this material, resulting in increased “noise” in the launch area of the signal. Foam is very difficult to quantify for radar as it is not readily distinguishable from the process surface.
Even with these limitations, free space radar is the most universal noncontact level technology, and it will work in most liquid or solids level applications.
Guided wave radar or time domain reflectometry (TDR) works very similarly to pulse-generated free space radar. The main difference is the addition of a cable or rod from the radar unit to the process surface to guide and focus the radar signal (figure 3). Guided radar also operates on a lower frequency of approximately 1.2 GHz.
The advantage of guided wave radar technology is the signal is very concentrated on the cable or rod. When it encounters foam, the radar signal does a better job of going through the foam to reflect off the liquid surface. Neither condensation nor dust has any effect on TDR.
Another major advantage in solids level measurement is that the angle of repose can be accounted for with careful placement of the rod or cable. The angle of repose results from the way solids pile up in a vessel, creating an angle on the side of the pile. When using guided wave radar, the point at which the rod or cable contacts the product will determine the signal reflecting back to the transmitter.
Installation considerations include material compatibility, possible excessive pull force on the cable in solids applications when installed in tall silos, and avoidance of interferences between the cable or rod and items such as agitators.
Source: ISA News