Hyperbaric calibration of condenser microphones in helium/oxygen/nitrogen mixture in the context of Manned Saturation Diving Facilities

In the context of Manned Saturation Diving Facilities (MSDF) used in offshore drilling activities, organisations concerned about Health and Safety aspects, like the Norwegian Petroleum Directorate stipulate noise level limits in chambers serving as accommodation for personnel and in control rooms: sleeping accommodation 60 dB (A); living accommodation 65 dB(A); control room 65 dB(A) in normal operations. To perform measurements for assessing these noise levels, the measurement microphones have to be calibrated at various pressure levels. These measurements help the operating organisation of such MSDFs to comply with ISO 9001 quality assurance requirements. The measurement microphones used for monitoring the noise are usually condenser microphones. The description of the measurement system used, the results obtained and the effects of gas composition are presented in this paper. The noise curves were compensated for pressure as well as for gas composition. The problems associated with the vent in the condenser microphone and its influence on the calibration results are discussed