Yes, the multi-place system has the Oxygen level monitor, with an alarm kicking in when Oxygen level drops bellow 19% or increases above 25-26% depending on country regulation.
The standard mono-place pressure vessel doesn’t have this requirement. In Europe it’s OK to use a mono-place air filled chamber without O2 monitor. We only had one client from UK that asked for such monitor, we added it to the chamber for 700 Euro extra.
In practice, though, there is no concern regarding O2 levels, some simple math bellow:
1. Minimum level
Chamber is pressurized with normal air and it is ventilated with 80 liters per min. The 80 cm diameter chamber has a total air volume of 1200 liters at normal air pressure. Human breath is 5-8 liters of air per minute. At this ventilation rate — 80 l/min with 1800 l/ total air volume at 1.5 ATA and 2400 liters at 2 ATA — human body volume around 100 liters, the changes of air composition inside the chamber are insignificant.
(Actually the density of the human body is very close to that of water—that’s why most of us (depending on fat content) sink when we exhale and float when we inhale. Muscle weighs about 1.06 g/ml, and fat is .9 g/ml. And we are made mostly of water. So it’s pretty accurate to say that a 95 kg person has a volume of 95 liters.)
2. Maximum level
For same reason explained above, a concentrator that puts 10l/min — 95% Oxygen in to the chamber will not influence much the O2 concentration inside the chamber.