The BIPM is running an international programme to ensure the equivalence of national ozone standards underpinning the international comparability of ground-level ozone measurements. It is now coordinating the ongoing key comparison BIPM.QM-K1 for ozone at ambient level, following the successful pilot study CCQM-P28
The BIPM currently maintains three NIST Standard Reference Photometers (SRP27, SRP28 and SRP31) as ozone reference standards to underpin its international comparison programme. The BIPM and the NIST are cooperating to transfer the international responsibility for the comparison of national ozone standards to the BIPM.
The BIPM also maintains two standard reference photometers (SRP32 and SRP33) for its research and development programme. The BIPM and the NIST have been cooperating to study systematic biases and measurement uncertainties in the SRPs. SPR33 was constructed at the BIPM as part of the NIST/BIPM collaboration programme.
Studies of gas temperature within the instrument have led to the construction of a thermo-electric cooling device that is installed on all BIPM SRPs. With this system, the gas temperature is constant to 0.1 °C inside the instrument's gas cells. Improvements in the optical design of the SRP have also been completed.