The BIPM primary standard for x-radiation in the range of generating potentials from 10 kV to 50 kV is a free-air ionization chamber of the conventional parallel-plate design. The dimensions of the standard are given below. The measuring volume is 1.2004 cm³. The normal polarizing potential is 1500 V.

For comparisons in low-energy x-rays, the NMI primary standard is normally transported to the BIPM for direct comparison with the BIPM standard. Comparisons are made at a series of reference radiation qualities and at a distance of 50 cm and a beam diameter of 4.5 cm. For calibrations, a larger beam diameter of 9.5 cm is used.