It sounds like you got the numbers backwards. 1.5 ata is 1 1/2 atmospheric absolutes or 1.5 x 14.7 psi. = 22.05 psi. 2 ata is 2 x 14.7 = 29.4 psi. 2 ata is the pressure used for TBI/PTSD research. It is the recommended pressure.

One person chambers may or may not have the highest oxygen concentration. It depends on the system. Some use face masks to breath the oxygen. Others fill the chamber with oxygen so no mask is needed. There are large multiplace chambers that use hoods or masks to provide 100% oxygen.

Most at-home systems, especially the soft sided systems, only go to 1.5 ata.

So, as you look at systems and treatments, there are two specifications to consider. The pressure listed as atmospheres, ata, or as dive depth (10 meters or 33 feet is 2 atmospheres) and oxygen concentration as a percent. The hyperbaric industry was developed by the deep sea diving industry so some use dive depth because they were trained in a dive school.

Some systems use a face mask or hood to provide the oxygen. Others pressurize the chamber with oxygen, eventually reaching a high percentage, although expiration (exhaling) puts co2 into the chamber. They need to have air scrubbers to remove the CO2 unless they have a strong flow of oxygen flowing in so they can vent air to keep CO2 levels down. CO2 in excess of 3% is not safe for more than 15 minutes no matter how high oxygen levels are.

From the picture of Rocky Mountain, it appears they may use both. There is a breathing hood on the bed by the clear chamber.