This application supports the user to estimate the O2 fraction of the gas in passive addition semi-closed rebreathers (e.g. RB2000 and RB80). In addition this app calculates the gas density at certain depth for trimix gas mixtures. The developer does not guarantee the correctness of the calculated values - always check the estimated values for correctness. Choose breathing and system parameters carefully. In case of doubt use a ppO2 meter while diving.