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.