When using radar level gauging to determine the level of certain products, in particular products which are relatively transparent to microwaves, such as liquid gases, the bottom of the tank, which may be of a highly reflective material such as metal, may cause a relatively strong reflection, interfering with the surface reflection. In some situations, e.g. when the product surface is close to the bottom, this bottom reflection can make it difficult to determine the product level correctly.
A solution to this problem is presented in WO01/29523, disclosing an absorbing element to be arranged at the bottom of the tank, below the opening of a pipe used as a wave guide. Such a pipe is sometimes used to avoid reflection of the waves in structures present in the tank. The pipe also serves to provide an area of the product surface with less wave motion and turbulence, thus improving the quality of the surface reflection. Such pipe wave guides are used in various implementations where the surface reflection from the product is weak, e.g. in tanks containing liquid gases. The damping element disclosed in WO 01/29523 will serve to reduce or even eliminate any reflection in the bottom of the tank.
However, in some situations, it may be advantageous to acquire a reflection from a known position in the tank, e.g. to verify the RLG system. It may thus be undesirable to completely eliminate the bottom reflection.