Bitumen is stored in tanks at rather high temperatures, a typical temperature being 180-2600C. The overheated product sticks to all metallic parts inside the tank. Contact method of level measurement in such conditions is extremely unreliable because adhesion of hot bitumen to measurement elements soon leads to a failure of contact level transmitters.
In bitumen level measurement, the best results belong to radar level transmitters. Radar level transmitters avoid contact with the product while a high temperature in the tank and a high vapour density in the air do not affect the level-measurement accuracy and stability.
The major danger for conventional radar level transmitters is product adhesion to the antenna. Level transmitters with a horn antenna placed inside the tank are most affected by this; contaminations, which make measurement quality worse, quickly accumulate on this antenna. In this case, in order to minimize the adverse effect of deposited bitumen, most manufacturers of radar level transmitters suggest using a level transmitter with a dielectric antenna made as a fluoropolymer or PTFE rod. It is more resistant to contaminations. But it also has disadvantages: considerably lower sensitivity of the level transmitter, which, as with mazut level measurement, results in a higher level measurement error and occasional measuring failures.
ULM level transmitters use a protected version of the horn lens antenna: it is not placed inside the tank, which practically prevents contamination of the antenna. A protective radar-transparent pad ensures full insulation of the level transmitter from the inner tank space and overheat protection of the level probe. Thus, the level probe is installed fully outside the tank and avoids contact with the internal tank atmosphere.
Depending on the tank height, this pad can be made of either fluoropolymer or other radar-transparent materials.