THE TITANIC AND AVOIDING THE EVIL TWINS

Just seven days to go until the start of my Trimix cert course…  I just realised it will be the day after the 100^th anniversary of Titanic’s sinking.  I’ll use my wreck fascination to interpret this as a good omen!

My spare time is full of revision… gas laws, deco schedules, emergency procedures… oxygen toxicity, nitrogen narcosis, decompression illness…  And the abbreviations: EAD, HPNS, OTU…alphabet soup for divers.  It’s all come back, but it needed work.

I’m planning numerous dive scenarios, considering gas mixes, calculating oxygen exposures and allowing for contingencies.  I usually do all this stuff with a spreadsheet and software, but I find running through longhand brings it to the front of my mind.

I still enjoy ‘playing’ with dive plans, which is a good thing… it’s not unusual for planning of a technical dive to take longer than the dive! 

Warning - the theory that follows is a bit dry…

IDEAL GAS MIX, OXYGEN TOXICITY, EQUIVALENT AIR DEPTH

Avoiding deep-diving’s evil twins, oxygen toxicity and nitrogen narcosis, is the whole point of Trimix.

Helium replaces some nitrogen, reducing narcosis, and can replace some oxygen, avoiding toxicity.  For each depth, there is an ideal gas mix suited to the needs of the diver.  The ideal mix depends on the intended maximum partial pressure of oxygen and the degree of narcosis the diver is prepared to tolerate.

Humans die without oxygen.  The air we breathe is 21% oxygen (okay, 20.9%).  Our cells need it to burn fuel to provide energy, but too much oxygen is bad.  Oxygen is an aggressive oxidiser – rust on metal and the browning of a cut apple are oxidation reactions.  By a similar process, long-term exposure to oxygen at high pressure damages cells, the higher the pressure, the shorter the ‘safe’ exposure time.

A useful insight into oxygen toxicity comes from considering it as a drug and looking at the dosage.  For best effect, we use drugs at the correct strength, for the appropriate time; dosage is a function of concentration and duration.  A drug overdose is disastrous in the short-term and long-term abuse is debilitating.  Oxygen is no different.  We tolerate high pressures of oxygen for a short time, but must reduce the concentration (pressure) for extended use.  Very high pressures quickly cause seizures and unconsciousness, while long-term exposure to moderate concentrations results in lung damage, coughing, shortness of breath and loss of coordination.

A widely used method for calculating safe exposure to oxygen follows guidelines developed by the National Oceanic and Atmospheric Administration (NOAA), a US government-funded organisation.  Through research, they established single-dive exposure limits and cumulative 24-hour limits.

Oxygen Partial Pressure Time Limits (NOAA)
PO2	Single Dive	Percent per	24-Hour
(ATA)	Limit	Minute	Limit
1.60	45 mins	2.22%/min	150 mins
1.50	120 mins	0.83%/min	180 mins
1.40	150 mins	0.67%/min	180 mins
1.30	180 mins	0.56%/min	210 mins
1.20	210 mins	0.48%/min	240 mins
1.10	240 mins	0.42%/min	270 mins
1.00	300 mins	0.33%/min	300 mins
0.90	360 mins	0.28%/min	360 mins
0.80	450 mins	0.22%/min	450 mins
0.70	570 mins	0.18%/min	570 mins
0.60	720 mins	0.14%/min	720 mins

In a gas mix, each gas contributes to the total pressure.  The fraction of each gas is the same size as its fraction of the total pressure.  This is the partial pressure.  In a Trimix with 20% oxygen, the partial pressure of oxygen (PO2) is 20% of the total (eg at a depth of 40 metres, where the total pressure is 5.0 bar, PO2 = 20% x 5 = 1.0 bar).  From the NOAA table, the maximum exposure to a PO2 of 1.0 bar is 300 minutes.

For planning their gas mix, technical divers calculate their oxygen exposure for each level of the dive and aim to keep the total exposure below about 80% of the limits.  Although NOAA’s table provides data for oxygen pressure up to 1.6 bar, a PO2 of 1.4 bar is the accepted maximum for most diving (PO2_max).  Planning for the fraction of oxygen in the ideal gas mix uses this limit.  The diver converts the planned depth to pressure (P_amb), and then divides the PO2_max by that number to get the oxygen fraction (FO2).

FO2 = P_amb/ PO2_max

Example:

At 60 metres, P_amb= 7 bar, so FO2 = 1.4 / 7 = 20%

The next step in the planning process is calculating the fraction of nitrogen in the Trimix (FN2).  The more nitrogen, the worse the narcosis, so the diver must decide what level of narcosis to accept.  A reference for narcosis is the equivalent air depth (EAD).  EAD is the depth where a diver breathing air would experience a similar degree of narcosis.  Divers frequently plan for an EAD in the range of thirty to forty metres.  On a difficult dive, such as a wreck penetration, a diver might plan a shallower EAD to take advantage of the reduced narcosis.

Mathematically, EAD is the depth where the partial pressure of nitrogen in air is the same as the partial pressure of nitrogen in the Trimix the diver is breathing.  Calculation of the FN2 is a two-step process.  First, the diver finds the PN2 at the planned EAD, and then divides by P_amb for the planned depth.

PN2_EAD= 0.79(EAD/10 + 1)

FN2 = PN2_EAD/ P_amb

Example:

For EAD = 30 metres, PN2_EAD = 0.79(30/10 +1) = 3.16 bar

At 60 metres, P_amb= 7 bar, so FN2 = PN2_EAD / P_amb = 3.16 / 7 = 45.1%

Working out the fraction of helium (FHe) is the final step.  Simply, add FO2 and FN2; whatever is missing, is helium.

FHe = 1 – FO2 –FN2

Example

FO2 = 20%, FN2 = 45%

FHe = 1 - FO2 - FN2 = 1 - 0.20 - 0.45 = 0.35 = 35%

So, the ideal gas mix for a dive to 60 metres with a maximum PO2 of 1.4 bar and an EAD of 30 metres is TMX 20/35.  By convention, FO2 is written first and then FHe.  FN2 is the remainder.

If you made it this far, well done.