The vast majority of humidity calibration is performed by commercial calibration laboratories, most of which are accredited under ISO17025 where their quality and performance is regularly assessed by independent auditors. Non-accredited laboratories will often refer to their calibration as being ‘traceable’ but this does not assure that calibration results or procedures have equivalence to accredited laboratories.
Different humidity calibration systems are used, but in most cases, a transfer standard hygrometer is applied either to provide traceability through external calibration, or for verification of generated values. All types of MBW instruments are used, depending on the range and type of humidity generator. MBW instruments are widely used because their long-term stability is superior to most other makes. Over time, this long-term stability enables the laboratory to achieve lower uncertainty of calibration (CMC).