The BIPM primary standard for x-radiation in the range of generating potentials from 100 kV to 250 kV is a parallel-plate free-air ionization chamber. The dimensions of the standard are given below. The measuring volume is 4.6554 cm³. The normal polarizing potential is 4000 V.

Primary standards for medium-energy x-rays are not easily transported and comparisons are normally made indirectly using one or more transfer ionization chambers calibrated at the BIPM and at the NMI both before and after transportation to the BIPM. Calibrations are made at a series of reference radiation qualities and, at the BIPM, at a distance of 1.2 m and a beam diameter of 10.5 cm.