The BIPM standard for absorbed dose to water is identical in design to the standard for air kerma, except that when used in water the chamber is enclosed in a polyethylene housing and positioned with the reference plane at a depth of 5 g cm^–2. The water phantom is a cube of side 30 cm with a square front window of side 15 cm and thickness 4 mm.

Although the BIPM standard for absorbed dose to water in ⁶⁰Co -radiation is ionometric, all NMI primary standards are either graphite or water calorimeters, which are not easily transported. Consequently, comparisons in terms of absorbed dose to water 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.

NMI transfer chamber in a waterproof envelope inside the BIPM water phantom