Some circumstances would require a two person, two compartment compression chamber to be on site ready for immediate use. This would be the case for dives with in-water stops that exceed 20 minutes.
Other scenarios would call for there to be a compression chamber within six hours of the dive site. This includes:
Dives shallower than 10m with in-water decompression that is less than 20 minutes.
Dives between 10m - 50m with in-water decompression that is less than 20 minutes, and a reduced risk of decompression illness (DCI). These risks factor in things like water temperature and number of dives.
Dives of up to 50m without in-water decompression have no requirement for a chamber, unless specified by the risk assessment or client.
Visit Diving at Work Regulations 1997 Approved Code of Practice (ACOP) for more information.