In real-life operating conditions, situations are frequent when waves appear on the surface of the measured product. This can be disturbances caused by pouring, agitation, boiling, etc. Appeared irregularities (waves) on the product surface dramatically reduce the product reflectivity as they increase dissipation of the reflected electromagnetic signal in the space.
Level measurement at the disturbed surface is the toughest condition for radar measurements. Different level transmitters behave differently in this situation.
The high-frequency pulse level transmitter sends electromagnetic-signal pulses towards the product to measure a reflected-signal receiving delay time.
Since the electromagnetic wave propagation time is very short, while the signal processing system in such level transmitters is built as simple as possible, and the simplest direct calculation of delay time (known since times of ultrasonic level transmitters) is used rather than spectral analysis (in contrast to FMCW); the measuring-cycle time is also short. Therefore, thousands measuring cycles per unit of time are possible. That is, tracing each surface oscillation is potentially possible. But this is the ideal case...
In reality, at bubbling, the reflected signal forms a segment of the product surface rather than one point or a surface (please see the Figure). The reflected signal will be dissipated in the space to a larger extent.
Less energy therefore returns to the antenna, with the reflected pulse distorted and significantly worse (or even completely lost) than when the surface is even, which decreases the amplitude. Herewith, a structure of the classical principle of pulse level measurement avoids serious data processing in such a level transmitter, except for statistical type (various accumulation methods). That is, if a pulse returns distorted and with low amplitude, it will give either a very high error or measurement failure.
Unlike pulse level transmitters, the FMCW-based level transmitter emits and receives an electromagnetic signal continuously (continuous radar) with a linearly varying sawtooth frequency (for more details please see Operating Principle). And the final level value is resulted from spectral analysis of the transmitted and reflected signal difference frequency. This method ensures high accuracies even in weak reflection conditions because the amplitude and waveform of the reflected signal are not determinant factors anymore: the information is carried by the frequency, and reliable measurements are always possible, even in conditions of weak reflected signals.
However, this method has a number of disadvantages, one of which is unstable operation in conditions of product surface boiling or bubbling. This is caused by information accumulation for a quite long time of probing-signal sweep (100–500ms). During this period, the product surface changes many times, due to which the resultant frequency is a “mixture” of extremely large number of undesired frequencies, which, in turn, leads to either dramatic increase of the error or practically impossible level measurement.
ULM level transmitters use an intelligent parameter-tuning system. The measuring system is built in such a manner that, on a situation-dependent basis, it can radically change measurement algorithms and characteristics of measuring units, exploit special algorithms and computing systems, etc.
When measuring the level of boiling products, the ULM level transmitters use a quick adaptive hopping system. As with a classical FMCW radar, the ULM level transmitters generate a sawtooth-frequency electromagnetic signal. However, unlike the classical FMCW radar, the probing-signal characteristics are not constant, but vary and adapt to the process according to proprietary algorithms.
Particularly, when measurement the level of surface-bubbling products, a “fast measurement” principle is used: a measuring time and a processing algorithm are continuously varying, while the former can decrease to millisecond units!
During “surface variation”, the ULM level transmitter is able to measure the product level multiple times, i.e. the measuring-cycle time is simply not enough for the product level to change. Thus, level measurement of a bubbling product for the ULM level transmitter consists in measuring the product level with a fixed, but uneven surface. The level measurement reliability with an uneven surface results from the FMCW method.
The ULM level transmitters combine high measurement accuracy and reliability of FMCW radars and the speed of pulse radars!