Nuclear density measurement instrumentation is the only practical option for some applications from a standpoint of non-invasive and low-maintenance operation. But the question of calibration and performance is difficult since there is not a practical, physical material to use as a standard. You cannot easily “feel” if the density reading is correct.
The quality of a density measurement is composed of two parts: repeatability and accuracy. However, the accuracy of any nuclear density system is only obtained after it is installed and calibrated on live process material since it is an inferred, rather than direct, measurement technology.
The level measurement market in Canada has seen considerable growth in the usage of radar level devices. One type of radar level device is known as “non contact” or "through air radar". These devices use, and emit, very low levels of microwave energy. A small fraction of what a cell phone emits.
However as these devices are “intentional radiators” they fall under the review of Industry Canada, and the FCC in the United States. These government bodies use the term “ "Tank Level Probing Radar" (TLPR) for these devices. They are required to ensure that these devices will not interfere with any other devices. The traditional approach in Canada has been to require these devices be installed inside a vessel that would help contain this small amount of microwave energy.