 Most trimix divers limit their working oxygen partial pressure [PO2] to 1.4 bar and may reduce the PO2 further to 1.3 bar or 1.2 bar depending on the depth, the duration and the kind of breathing system used.A maximum oxygen partial pressure of 1.4 bar for the active sectors of the dive, and 1.6 bar for decompression stops is recommended by several recreational and technical diving certification agencies for open circuit, and 1.2 bar or 1.3 bar as maximum for the active sectors of a dive on closed circuit rebreather.Retaining nitrogen in trimix can contribute to the prevention of High Pressure Nervous Syndrome, a problem that can occur when breathing heliox at depths beyond about .