Nitrox-Badger by Glenn Lawyer a.k.a. Honeybadger March 27, 2006
1
Disclaimer
I wrote this document to help me study for my nitrox cert. Please think about that. This document was made by someone who has never taken a nitrox course (let alone passed one!), something to consider before making decisions based on information presented here. Let me put that another way. I’m reasonably smart, but woefully ignorant. I didn’t write about what I didn’t know about, and that’s the bit that could kill you.
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What is nitrox?
Nitrox is any mix of nitrogen and oxygen, but not other gases (except in trace amounts). Hyperoxic nitrox, also known as Enriched Air (EAN or EANx), is a nitrox blend containing more than 21% oxygen. When blending nitrox by enriching air with oxygen, the trace gasses are included in the percent nitrogen figure. The history of nitrox is the history of research into oxygen toxicity. Oxygen toxicity was first demonstrated by Paul Bert in 1878, who showed that high partial pressures of oxygen caused convulsions. In 1899 Lorrain Smith observed that long-term exposure to moderate partial pressures lead to pulmonary problems. Nitrox was introduced into recreational diving by NOAA’s Dr. Morgan Wells in the 1970’s. The two NOAA mixes are Nitrox 1 (32 O2 / 68 Ni) from 1978 and nitrox 2 (36 O2 / 64 Ni) from 1990.
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Physics and Physiology
Safe nitrox diving is based on gas physics and human physiology. The physics is needed to calculate the changes in partial pressure of nitrogen and oxygen
from which nitrox derives is benefits and risks. The relevant physical law is Daltonâ&#x20AC;&#x2122;s law of partial pressure. P P = F rac â&#x2C6;&#x2014; Abs which states that the partial pressure of the gas (PP) equals the fraction of the gas in the mix (Frac) times the absolute pressure of the gas (Abs). The benefit of reducing the percentage of nitrogen in the blend is that nitrogen absorption by the body is reduced, allowing for longer dives with shorter recovery intervals and a lower risk of DCI. Some divers also report that multiple dives are less tiring. The cost is that hyperoxic mixes require an understanding of oxygen toxicity, a factor which does not come into play for normal air at typical recreational depths (< 40m). The increased partial pressure of oxygen in nitrox means that nitrox is a shallow-water gas. EAN32 is not suitable for dives deeper than 33.8 meters salt water. Oxygen toxicity takes two forms: central nervous system (CNS) and pulmonary. CNS is also known as the Bert effect, after Paul Bert (1878). CNS is a fairly rapid response to increased partial pressure of oxygen. Symptoms include: dizziness, nausea, fatigue, anxiety, confusion, lack of co-ordination, and ultimately convulsions/unconsciousness. Most common early symptoms are twitching of the perioral muscles and small muscles of the hand. These early symptoms are not reliable indicators, as they may or may not occur, may or may not be noticed, and may proceed to convulsions before the diver has the chance to react. Ascending a few meters will reduce the P P O2 , and in general alleviate the symptoms. Aborting the dive at this point is a wise choice. Pulmonary Oxygen Toxicity is also known as the Smith effect, after J. Lorain Smith (1899). He observed that long term exposure to raised partial pressures of oxygen damage lung tissue. Symptoms generally do not manifest until after a dive, and resemble flu or pneumonia. Coughing, difficulty breathing, lack of co-ordination (caution: can also be a symptom of DCI or other maladies), sore throat/chest. Recovery is a matter of a few weeks. Experiments over the years show that both inter- and intra-subject response to oxygen toxicity is highly variable. This is a fact well worth remembering. Previous personal survival beyond the established guidelines may not be evidenced in repeated experimentation.
4 4.1
Practical Implications Equivalent Air Depth
Absorption of inert gas into tissue is controlled by the difference in partial pressure in the tissue and the ambient environment. EAN has lower partial pressure of nitrogen, reducing absorption rates. The Equivalent Air Depth (EAD) is the depth on air which would give the same nitrogen partial pressure as our actual depth on nitrox. !
EAD =
%N2 ∗ (depth + 10m) − 10m 0.79
where depth is in meters. For EAN 32, this reduces to EAD32 = (0.86 ∗ depth) − 1.4m and for EAN 36 EAD36 = (0.81 ∗ depth) − 1.9m Please see table 1 and the figures for values at different depths.
4.2
CNS
One needs to remember that the partial pressure of oxygen also changes with the new mix in order to find a safe depth limit. It is strongly recommended that P P O2 be kept below 1.4 at all times. The highest fraction of oxygen for your mix which keeps the P P O2 below 1.4, for a given maximum depth in meters is 1.4 f rac O2 = depth + 10
4.3
Oxygen clock
The oxygen clock is a way of measuring multi-day oxygen exposure. It is based on the oxygen tolerance unit (OTU), alternately referred to as the unit pulmonary toxic dose (UPTD). The OTU represents the effect of breathing pure O2 for 1 minute at 1 bar ATA. To calculate OTU’s for other situations, the following formula is used: OT U = minutes ∗
(P P O2 − 0.5)−0.83 0.5
Table 1: EAD table for EANx32 and EANx36, with P P O2 and max 24 hour exposure limits Actual EAD32 P O2 Max 24hr EAD36 P O2 5 2.9 0.48 – 2.2 0.54 10 7.2 0.64 9.5 6.2 0.72 10.3 0.90 15 11.5 0.80 7.5 20 15.8 0.96 5.0 14.3 1.08 18.4 1.26 25 20.1 1.12 4.5 30 24.4 1.28 3.5 24.4 1.44 35 28.7 1.44 3.0 26.5 1.62 30.5 1.80 40 33.0 1.60 2.5
where the exponent is based on best fit to experimental observations and 0.5 is the threshold below which no significant pulmonary toxicity has been observed. Most people can breath pure oxygen for 24 hours without symptoms. Thus the maximum accepted dose is 1440 OTU’s per day (1 min * 60 min/hr * 24 hr/day). That limit is fine if you are undergoing hyperbaric treatment at a supervised medical facility. NOAA recommends the following more conservative limits for diving: Day 1 2 3 4
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Dose/day Total 850 850 700 1400 620 1860 525 2100
Day 5 6 7
Dose/day Total 460 2300 420 2520 380 2660
Equipment considerations
Look, it’s air. Ordinary tanks/regulators are fine except for high concentrations of oxygen (P P O2 > 0.4, during or after mixing). Higher oxygen concentrations can lead to problems unless the equipment is oxygen compatible and oxygen clean. While you don’t need a dedicated nitrox cylinder, cylinders should be clearly labeled as to contents. This reduces the risk of mistakes when plan-
Table 2: NOAA Maximum Exposure Duration Limits P O2 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
Single 24-hr 12.0 12.0 9.5 9.5 7.5 7.5 6.0 6.0 5.0 5.0 4.0 4.5 3.5 4.0 3.0 3.5 2.5 3.0 2.0 3.0 0.8 2.5
ning dives and also make the emergency medical treatment people happy. The above statement is dependent on how the gas is mixed. Partial pressure mixing begins by pumping pure O2 into the cylinder. In such a case oxygen serviceable materials are needed.
5.1
Gas analyzer and logging
Because safe nitrox diving limits are based on P P O2 and P P N i levels, it is vital to know what is in the tank. When a tank is filled it is supposed to be analyzed and tagged by the filler. The diver should check the tank after filling and before diving, and log the readings. Mistakes and mix ups do happen, and gas meters may have drifted off calibration. There are two kinds of gas analyzers. A paramagnetic analyzer takes advantage of the fact that oxygen is attracted to a magnetic field. these are accurate, stable, relatively expensive, and somewhat delicate. They are great for research laboratories, but arenâ&#x20AC;&#x2122;t very portable. Portable units are generally electrochemical. An electrochemical cell breaks oxygen into ions and electrons and measures the current generated; this current is proportional to the partial pressure of oxygen to which the sensor is exposed. Electrochemical analyzers are relatively inexpensive, can be made portable
and rugged, and show little interference from other gases. However, they tend to be unstable and may need frequent calibration, especially as the cell begins to age. Cell life depends on manufacturer and use, and can be anywhere from 6 to 18 months. Calibrating an analyzer is done in two steps. It is zeroed with an inert gas (argon, nitrogen, helium) and then spanned with normal air (O2 = 20.95%). Both the calibrating gas and the sample mix should be passed through the analyzer at the same flow rate. A good flow rate is 1 liter/minute. Readings should be taken for one minute.
5.2
Mixing
Three ways to mix nitrox are common: Partial pressure blending. Oxygen is pumped into an empty tank, followed by nitrogen or air to achieve the proper blend. If the tank isnâ&#x20AC;&#x2122;t oxygen service ready, you are going to have problems. Continuous blending. High-pressure oxygen is injected into the air as it is sent to the compressor. You donâ&#x20AC;&#x2122;t need an oxygen clean tank (for the fill). Membrane separation system. Nitrogen is removed by pumping the mix through a permeable membrane. The oxygen penetrates faster than the nitrogen. Unlike oxygen mixing and blending setups where high oxygen concentrations require oxygen clean equipment, this system never has more than 40% concentration, reducing oxygen fire hazard concerns.
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Handling Accidents
Divers should always be on the lookout for signs of oxygen toxicity, both in themselves and in their buddy. The immediate response is to ascend a few meters to reduce the P P O2 levels. Aborting the dive allows you the chance to dive again; continuing a troubled dive may not. Treatment at the surface is similar to other dive emergency treatment. The victim should be put on oxygen, kept warm, encouraged to lay down, and monitored until medical personel can arrive. It is vital that emergency response teams be informed of the gas mix and exposure times. This is so proper hyperbaric treatment can be devised with consideration to oxygen toxicity exposure.