ULM radar gauges are used to measure the level of not only liquid but also granular materials such as cement, sand, break stones, coal, coal dust, black coal (smut), polypropylene, components of mineral fertilizers, apatite.

The use of noncontact radar level gauge in order to accomplish these tasks has a number of incontestable advantages as compared to other methods: in case of contact methods, sooner or later the sensor breaks down, as of laser level gauge, it's impossible to measure the product level under the dust conditions, as there's no direct visibility and the beam can't “break” through the dust to reach the product.

But the use of noncontact radar level gauge has a number of special application features.

Application features

Granular materials, unlike liquid products, have no plane surface on the air-product boundary line. That’s why measurement accuracy will be determined by the size of surface irregularities (units of centimeters), and not by the inaccuracy of the level gauge itself (units of millimeters).

As the signal reflected from the granular material is weaker than for the liquid product (due to its uneven surface, the stable operation of the radar level gauge will depend on its sensitivity. Under conditions of weak reflected signals, the continuous level gauges (FMCW-radars) show the best performance, in this case, the frequency of probing signal, that doesn't depend on the pulse height, is used to measure the level.

The formation of cone on the product surface in the process of loading, as well as conic cavity in the process of unloading are treated as the additional factors that can influence the level gauging of granular products.  It also contributes to the weaker reflected signal, the most part of the signal reflects aside, if the surface of granular product were plain, the process of measuring would be very problematic. But as the product surface is not homogenous, the signal reflects right and left, and the more sensitive the level gauge, the higher its ability to process a weak signal, and the more stable its operation.

Methods of sensitizing

As the reflectivity of granular material is small, it’s necessary to reduce the loss of reflected signal by concentrating the radiated energy on the particular section of surface, therefore, to minimize the losses connected with the natural process of the signal diffusion.   That’s why in order to measure the product level in the high bins, many producers of the radar level gauges have to use the level gauges with parabolic antenna that provides beam width of 6-10°. 

But this method has its disadvantages, firstly, the parabolic antenna is a bulky construction that requires an aperture of the large inner diameter in the hooper roof and makes the level gauge more expensive, secondly, the outer feed element, located under the antenna, gets often clogged with dust, which makes level gauging impossible. 

Sometimes the method of sensor slope installation is used. The level sensor has a movable holder on the flange that allows declining it. Thus, it’s possible to find the optimal inclination of this instrument and provide the best reflection from the product surface.

The obvious drawbacks of this method consist in the fact that selected inclination of the level gauge will be optimal only for one position of the product level, if it changes (in the process of loading or unloading), the situation changes as well, and can become absolutely non-optimal. This method has been worked out to compensate the insufficient sensitivity of the level gauge and is typical of the pulse level gauges.

ULM level gauges

ULM level gauges don’t require the parabolic antenna to measure the level of granular products. The lineup of ULM level gauges is represented by different modifications with the beam width of 4°, 15°, 22°, it allows to cover all the necessary ranges of measuring the granular product level. For instance, ULM-11 level gauge has the beam width of 4° that is the narrowest beam among all the other devices in the level gauges market. There’s no need to make the large apertures in the bin roof! There’s no need to complicate the construction of the mounting place to find the necessary level gauge inclination!

 

Thus, electromagnetic energy is fully concentrated on the certain section of the product and has a minimal “inefficient” diffusion. The reflected signal returns back to the level gauge antenna. But in case of level gauging of granular materials, all other conditions being equal, the reflected signal will be considerably weaker than in case of liquid materials. That’s why high sensitivity of level gauge is necessary to acquire the stable measurements.

This situation demonstrates another significant advantage of ULM level gauge construction: the signal reflected from the surface of granular material has a small amplitude, and if the pulse measuring procedure is used, it will be extremely difficult to gauge the level, the amplitude of reflected signal plays a key role for the normal performance of pulse level gauge. But ULM level gauges use the adaptive FMCW-method that isn’t based on the amplitude of reflected signal, it plays the minor role, as the information is carried by the frequency. That’s why if the signal is reflected from the product and it’s “visible” against the aerial noises, the level will be measured!

The unique LIMACO technologies and algorithms allow obtaining the guaranteed level gauging of products that are really difficult to be measured, such as smut, dust coal, components of mineral fertilizers. It’s only necessary to choose the appropriate model and configuration of level gauge on the basis of questionnaire.