Background on changes proposed to UN R49 and UN R85 to address hydrogen fuel and heavy vehicles.

Proposal based on ECE/TRANS/WP.29/GRPE/2021/14, as amended by Annex X of the report of the 83rd GRPE (Supplement 7 to the 06 series of amendments). Some modifications were not taken onboard in the 07 series of amendments (and Supplements 1, 2 and 3). For the remaining amendments included in this proposal, the justifications are:

1. UN R49 does not define linearity requirements for humidity sensors. As the humidity content of the intake air is an essential measure for the calculation of the specific exhaust emission, it is important to add requirement for humidity sensor. Reference: ISO 16183 the accuracy of the absolute humidity shall be +- 5%.
2. Typo error: the instrument should be now in NOx mode.
3. To clarify the operation procedure, make the text easier to be understood.