For the following partial pressures of oxygen the limits are: 45 minutes at 1.6 bar (160 kPa), 120 minutes at 1.5 bar (150 kPa), 150 minutes at 1.4 bar (140 kPa), 180 minutes at 1.3 bar (130 kPa) and 210 minutes at 1.2 bar (120 kPa), but it is impossible to predict with any reliability whether or when toxicity symptoms will occur. Many nitrox-capable dive computers calculate an oxygen loading and can track it across multiple dives. The aim is to avoid activating the alarm by reducing the partial pressure of oxygen in the breathing gas or by reducing the time spent breathing gas of greater oxygen partial pressure. As the partial pressure of oxygen increases with the fraction of oxygen in the breathing gas and the depth of the dive, the diver obtains more time on the oxygen clock by diving at a shallower depth, by breathing a less oxygen-rich gas, or by shortening the duration of exposure to oxygen-rich gases. This function is provided by some technical diving decompression computers and rebreather control and monitoring hardware.Diving below 56 m (184 ft) on air would expose a diver to increasing danger of oxygen toxicity as the partial pressure of oxygen exceeds 1.4 bar (140 kPa), so a gas mixture must be used which contains less than 21% oxygen (a hypoxic mixture). Increasing the proportion of nitrogen is not viable, since it would produce a strongly narcotic mixture.
One of the advantages of a full-face diving mask is prevention of regulator loss in the event of a seizure. Mouthpiece retaining straps are a relatively inexpensive alternative with a similar but less effective function. As there is an increased risk of central nervous system oxygen toxicity on deep dives, long dives and dives where oxygen-rich breathing gases are used, divers are taught to calculate a maximum operating depth for oxygen-rich breathing gases, and cylinders containing such mixtures should be clearly marked with that depth.The risk of seizure appears to be a function of dose – a cumulative combination of partial pressure and duration. The threshold for oxygen partial pressure below which seizures never occur has not been established, and may depend on many variables, some of them personal. the risk to a specific person can vary considerably depending on individual sensitivity, level of exercise, and carbon dioxide retention, which is influenced by work of breathing.In some diver training courses for these types of diving, divers are taught to plan and monitor what is called the oxygen clock of their dives. This is a notional alarm clock, which ticks more quickly at increased oxygen pressure and is set to activate at the maximum single exposure limit recommended in the National Oceanic and Atmospheric Administration Diving Manual.