Said that, back to that strange concept of CNS%.
Here please excuse my ignorance (I was never trained to "tech").
What is exactly that CNS% number? CNS stands for me as the acronym for Central Nervous System: how can it become a percentage? And why should one worry about it, when using an OC system and respecting the MOD of the mixture?
From the NOAA Dive Manual:
3.3.3.3.1 CNS: Central Nervous System
CNS oxygen toxicity can occur at the high end of PO2 levels, even after short exposures. Typically, it can develop within a few to many minutes on exposure to partial pressures of oxygen above 1.6 atm (roughly 5 to 50 min, but this is highly variable) (Lambertsen 1978). The end result may be an epileptic-like convulsion that is not damaging in itself, but can result in drowning or physical injury. The acronym CONVENTID (see Figure 3.21) is a simple way to remember all the signs and symptoms of CNS oxygen toxicity. It is important to note that these symptoms may come in any order. There are other signs and symptoms of CNS toxicity. Not onerous in themselves, they are justification to stop a dive. They include twitching of lips and facial muscles, visual or hearing disturbances, nausea, dizziness, difficulty in breathing (dyspnea), anxiety, confusion, poor coordination, and unusual fatigue. These may warn of an impending convulsion; however, a convulsion is just as likely to occur without any warning. Divers have been known to “black out” or go unconscious without a convulsion; this may be a manifestation of oxygen toxicity.
3.3.3.3.5 Concepts of Oxygen Exposure Management
The traditional method used for prevention of CNS oxygen toxicity is to stay within exposure durations that are based on the oxygen level, or PO2, to which the diver is exposed (U.S.Navy Diving Manual 1999). These limits allow a certain time at each PO2 range. Such an approach has been
practiced by the U.S. Navy and by NOAA for many years in their procedures for mixed gas and oxygen diving. As with decompression, a limit appears to be implemented as if it were a solid line dividing “no problems” from “guaranteed problems.” Actually, a limit is a solid line drawn through a wide gray area of gradually increasing risk. The limits given here and in other limit-based algorithms (such as a decompression table) are recommended guidelines for use under normal conditions. They have been proven in practice. They work for most people most of the time, but they are not guaranteed to work for all people all of the time under all circumstances. They may need to be more conservative when conditions are more stressful. Diving with procedures described in this chapter imposes a relatively low risk of oxygen toxicity. The exposures are short and outside the limits that are expected to cause problems.
3.3.3.3.6 Prevention of CNS Poisoning
With the help of experts, NOAA developed estimated oxygen exposure limits that were published in the 1991 version of the NOAA Diving Manual. These limits are shown in Table 3.4. They are intended for a diver doing dives for research, sampling, inspection, observation, and
light to moderate work at the higher PO2 levels. The lower levels can be used for heavier and more stressful types of work. For each level of oxygen, the chart shows an allowable time for a single exposure and also an accumulated time at that level over a full day. If more than one dive is made to the maximum exposure of a PO2 of 1.6 ata, a suggested surface interval of at least 90 minutes is advised between dives (three dives of 45 minutes each would theoretically be possible within the 150-minutes daily total allowed at 1.6 ata PO2). This helps lower the accumulated oxygen dose. This only applies to the exposure at 1.6 ata, because only one maximal dive can be done in a single day with lower oxygen exposure levels. If, however, one or more dives in a 24-hour period taken every 20 or 30 minutes of oxygen breathing at high PO2 levels. This avoids oxygen convulsions in all but very rare cases and also postpones pulmonary toxicity. In situahave reached or exceeded the limits for a normal single exposure, the diver should spend a minimum of two hours at a normoxic PO2 (such as on the surface breathing air) before resuming diving. If diving in a 24-hour period reaches the Maximum 24-hour Limit, the diver must spend a minimum of 12 hours at normoxic PO2 before diving again.
3.3.3.3.7 The “Oxygen Clock” or “O2 Limit Fraction”
These exposure limits are sometimes referred to as the “oxygen clock” in percentage of the allowable limit, or the “O2 limit fraction” as a decimal fraction of the limit (Hamilton 1988). For single dives to a single depth (square profile), calculating the percentage of oxygen exposure is as simple as dividing the minutes of the exposure by the maximum allowable exposure time at a given PO2. However, it is rare that a diver is ever at one
depth for the entire dive. Although the principle has not been verified experimentally, it is customary to add the percentages or fractions of exposure for different parts of the dive to calculate an estimated total oxygen exposure for a given dive. It is not necessary to have a dive computer to track these exposures if the dive can be separated into segments that have a predominant or average level. The times spent at each depth or exposure level can be assigned a fraction or percentage of the “allowable” limit, and these can simply be added together. Table 3.5 allows these segments to be determined from a chart. For multilevel dives or more than one dive of less than maximum allowed duration, it is possible to interpolate the limit values. That is to say, at any level the full limit on the oxygen clock is 100 percent of the limit, or an O2 limit fraction of 1.0. Exposures at all levels are totaled. For example, at 1.4 atm the allowable exposure time is 150 minutes (see Figure 3.22). If a diver has an exposure to that level for 75 minutes, half the allowable time, this would run the oxygen clock to 50 percent of the limit or the limit fraction to 0.5.
If there is additional exposure on the same dive, for example, 60 minutes at 1.3 PO2, for which the allowable time is 180 minutes, an additional one-third, 33 percent or 0.33 is added, giving an oxygen clock now of 83 percent or a limit fraction of 0.83. When the total reaches 100 percent or
1.0, the diver is considered to have reached the allowable limit, and further exposure to elevated oxygen is at increased risk. Diving beyond the limit is not recommended. Although there has been no specific laboratory validation of this technique of interpolating the exposure times, it appears to work in practice. The NOAA oxygen exposure limits have been shown to be reasonable limits through extensive use.
