Adjustment of the Flame Ionisation Detector (FID. (a) Optimization of the detector response If hydrocarbons are measured, the FID shall be adjusted as specified by the instrument manufacturer by following paragraph 5.4.1. of Annex B5 to UN Regulation No. 154 on WLTP. A propane-in-air or propane- in-nitrogen span gas shall be used to optimize the response in the most common operating range.