Operational and contingent limits on oxygen intake during diving.
From first SNSI 1995 (Scuba Nitrox school International) and immediately in the following years the PADI, SSI, NAUI and other international didactic agencies of diving training proposed a limit of 1.4 ata for dives with open circuit breathing apparatus with nitrox mixture. Since this type of diving would not expose divers continuously to that level, in practice it should be just as safe, if not safer, than the US Navy's proposed 1.3 ata limit for continuous exposure. The percentage of oxygen in the breathing mix has a direct impact on the depth that can be safely reached due to oxygen toxicity on the central nervous system. Most didactices suggest maximum oxygen martial pressures between 1.4 and 1.6 bar. In case of use with rebreather for various reasons, lower partial pressures are recommended, for example 1.3 bar. In fact, SNSI, PADI, SSI, NAUI, consider 1.4 ata
the operating limit and 1.6 as a contingent limit (ie usable ONLY in actual conditions of necessity).
CALCULATIONS WITH THE RULE TO DETERMINE THE LIMITS OF O2 TOXICITY
The REGOLO for this kind of calculation has “standard” references of oxygen enriched mixture (Nitrox 32% - 36% - 40% and 50%) and these references are recognizable by their YELLOW color which highlights them from the background. The premise is that the use of this rule is advisable as a possible purpose to check or review the planning already verified in the calculations. Due to the size of the ruler, reduced to include it in the clock, reading errors may occur. This is why it is suggested to use the slide rule only as a secondary control or back-up tool. The numerical values must be considered as absolute pressure values and therefore in BAR as indicated in the outer ring next to the number “60”. These values will have to be converted into meters by subtracting 1 atmosphere. If the resulting number is 35, it means that the value to be read will be 3.5 BAR absolute pressure and from here remove 1 BAR as earth pressure = 2.5 BAR equivalent to 25 meters
You want to know the depth where the toxicity of the breathing mix enriched with 32% oxygen reaches the OTU value of 1.4
1- Collimate the value "10" of the external scale with the number 32 of the internal scale, in this case already indicated with the% ▼ symbol in YELLOW
2- On the fixed internal scale, locate the number 14 within a circle. This indicates the toxicity value of 1.4 of the oxygen partial pressure which will be at the absolute pressure indicated with the corresponding number on the external scale at 43 i.e. 4.3 BAR or 33 meters (4.3 BAR absolute pressure - 1 BAR atmospheric pressure = 3.3 BAR or 33 meters)
If you want to know the depth that can be reached considering the OTU value of 1.6, just look at the number corresponding to the number 16 inside the circle on the hard disk. We will find about 49.5 and it will correspond with the previous sampling, at 39.5 meters. (49.5 = 4.95 BAR absolute pressure - 1 atmospheric pressure = 3.95 BAR or 39.5 meters). Aeronautical calculator | Antikythera Watches
With the same calculation principle, we can very quickly see other value relationships between oxygen enrichment% and relative OTU (1.4 or 1.6). The reading of the calculation solution is immediate, just move the number "10" of the external disk to the desired percentage (Example 40% on the internal disk) and we will immediately see, on the external disk, the corresponding OTU values collimated with the values 14 or 16 Calculator aeronautical | Antikythera Watches