Dive duration is governed by the CNS oxygen toxicity clock, the life of the CO2 Scrubber and the gas supply.
CNS oxygen toxicity clock
Oxygen exposure limits are laid down by NOAA and according to their Exposure Limits dive duration at the default setpoint of 1.3 bar is limited to 3 hours (or 3 1/2 hours per day). This will vary if a different setpoint is selected.
Third party tested for CE certification: the Inspiration xpd and Inspiration evp's 2.5kg Scrubber stated duration is 3 hours. The Inspiration evo’s 2kg Scrubber is 2 hours *
[* tested with 797 Sofnolime in cold water (4ºC) at a continual breathing rate of 40 litres per minute RMV and a CO2 production rate of 1.6 lpm]
These times will vary as scrubber duration depends upon factors such as the individual diver’s CO2 production rate, water temperature and work rate. In practice, with an AP CO2 Scrubber Monitor (TempStik) fitted (an optional upgrade) which takes account of actual conditions, scrubber life can be accurately tracked with a visual display on the handset and these nominal durations extended considerably with confidence. Note: Without a scrubber monitor, divers should adhere strictly to these duration guide times.
The Inspiration xpd and evp's 3 litre cylinder will give up to 10 hours of diving. The smaller Inspiration evo ’s 2 litre cylinder (2 x 200 bar = 400 litres) offers over 6 1/2 hours duration.
Even if you had to work really hard on a series of dives and doubled your usual metabolic rate (to about 2 litres/min) – you would still get 5 hours or so from the one 3 litre, or 3 hours+ from the 2 litre cylinder of the smaller unit.