The CLD and the HCLD shall be calibrated in the most common
operating range following the manufacturer’s specifications
using zero and span gas (the NO content of which must amount to about
80% of the operating range and the NO2 concentration of
the gas mixture to less than 5% of the NO concentration). The NOx analyser
must be in the NO mode so that the span gas does not pass through
the converter. The indicated concentration shall be recorded.