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Jordi Gadea Alfaro

Scuba Instructor International Diving Educators Association

From Glenn Lawyer a.k.a. Honeybadger March 27, 2006 publicationFEDAS Nitrox manual, IDEA Nitrox manual

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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 (National Oceanographic and atmospheric association) 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 Dalton’s law of partial pressure.

PP = Frac x Abs.Which states that the partial pressure of the gas (PP) equals the fraction

ofthe gas in the mix (Frac) times the absolute pressure of the gas (Abs).

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The benefit of reducing the percentage of nitrogen in the blend is thatnitrogen absorption by the body is reduced, allowing for longer dives withshorter 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 oxygentoxicity, a factor which does not come into play for normal air at typicalrecreational 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.or

EAN36 is not suitable for dives deeper than 28.8 meters salt water.In order to respect 1.4 Pp O2

so

PLAN THE DIVE & DIVE THE PLAN

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Oxygen toxicity TTakes two forms: central nervous system (CNS) and pulmonary.

CNS is also known as the Bert effect, after Paul Bert (1878). CNSis 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 PPO2, and in general alleviate the symptoms.

Aborting the dive at this point is a wise choice.

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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 notbe evidenced in repeated experimentation.

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Practical ImplicationsEquivalent 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.

where depth is in meters.

For EAN 32, this reduces to EAD32 = (0.86 depth) − 1.4mFor EAN 36 EAD36 = (0.81 depth) − 1.9m

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CNSOne 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 PPO2 be kept below 1.4 at all times.

The highest fraction of oxygen for your mix which keeps the PPO2 below 1.4, for a given maximum depth in meters is:

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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:

− 0.83 (PPO2 − 0.5) OTU = minutes x -------------------- 0.5

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DOSIFICATIÓN UNIT PER ATA (BAR)  P O2 / ATA OTU / MINUTE PO2 / ATA OTU / MINUTE

0.50 0.00 1.25 1.390.55 0.15 1.30 1.480.60 0.27 1.35 1.560.65 0.37 1.40 1.630.70 0.47 1.45 1.850.75 0.56 1.50 1.780.80 0.65 1.55 1.840.85 0.74 1.60 1.920.90 0.83 1.65 2.000.95 0.92 1.70 2.071.00 1.00 1.75 2.141.05 1.07 1.80 2.221.10 1.16 1.85 2.281.15 1.23 1.90 2.351.20 1.32 2.00 2.49

Maximum accepted dose is 1440 OTU’s per day (1 min * 60 min/hr 24 hr/day).

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CNS

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Table of O2 % CNS

Every 90 minutes in surface – 50%

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ExerciseCalculate the % de EMT in one EAN 35 / -30 m. /30’Calculate the % de EMT in one EAN 35 / -30 m. /30’With a second dive after 2:15 hours using EAN 40 / - 22 m. With a second dive after 2:15 hours using EAN 40 / - 22 m.

/50’/50’

20%EMT1 +135 min 10% EMT2EMT f 38%

30m 30min

22m 50min

At 30 m. Pp. O2 1,4 atm EMT 150’

% EMT1 20%

At 22 m. Pp. O2 1,3 atm EMT 180’

% EMT2 28%

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Equipment considerations

Look, it’s air.

Ordinary tanks/regulators are fine except for high concentrations of oxygen (PPO2 > 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 planning 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.

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Gas analyzer and logging

Because safe Nitrox diving limits are based on PPO2 and PPNi levels, itis 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 aren’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.

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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 theanalyzer at the same flow rate.

A good flow rate is 1 liter/minute.

Readings should be taken for one minute.

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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 isn’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 don’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 AccidentsDivers should always be on the lookout for signs of oxygen toxicity, both inthemselves and in their buddy.

The immediate response is to ascend a few meters to reduce the PPO2 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 personnel can arrive.

It is vital that emergency response teams be informed of the gas mix andexposure times.

This is so proper hyperbaric treatment can be devised with consideration to oxygen toxicity exposure.

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LABELING

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O2 ANALIZER

1.1. Turn onTurn on2.2. Calibrate on the air (%OCalibrate on the air (%O22=20.9 ±1)=20.9 ±1)3.3. Connect to the tapConnect to the tap4.4. Introduce the sensor into the tubeIntroduce the sensor into the tube5.5. Open the tap (slow)Open the tap (slow)6.6. Messure the%OMessure the%O22

7.7. Close the tapClose the tap

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The messurement betwen the operator and the user will not The messurement betwen the operator and the user will not exceded more than 1% exceded more than 1%

Mixex over 40% needs equipement in O2 service!!!

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NITROX COMPUTERS

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DANGER

Oxygen manipulation should be only done by qualified personnel

High risk of spontaneous explosion if any contact with hydrocarbures

O2 is the highest comburent

FLAMABLE !!!!

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