Deco on the Fly and Gas Planning

Deco on the Fly and Gas Planning

Following on from the short article we made on recreational gas planning it seemed a god idea to follow up with some guidelines on gas planning at the next level – entry level technical diving.

For most technical divers, dives down to 50m are often done on the fly.  We still normally start with a plan, but often something comes up or we can often end up staying longer than the plan or going a little deeper.

Modern technical computers make deco on the fly viable

Carrying out deco on the fly is often seen as a risky approach to technical diving, although this isn’t always the case with the correct planning and knowledge.  Most technical divers have spent $1,000 or more on a technical dive computer (often more than one!) so already have the decompression information at hand for a revised profile – the risky part is ensuring we have enough gas to complete the dive, and that's what we’ll cover here.

Basically what we are going to do is work out how long a deco bottle will last, we can then use this to estimate how much decompression (or time to surface on most computers) we can accumulate without running the risk of being out of gas during the deco.

The basis of these calculations are similar to the recreational article from a few weeks back (this link takes you there - http://www.maniladiveacademy.blogspot.com/2014/07/one-thing-we-try-to-emphasize-in-our.html).  We’ll assume a conservative breathing rate of 20 litres per minute at the surface to estimate deco bottle durations.

EAN50 Gas Planning

EAN50 is generally the most versatile deco gas

The most common decompression gas is the trusty 12l EAN50 bottle, this is normally breathed at 21m and shallower and is often combined with O2 for longer or multiple dives.  Like most decompression profiles the stops are generally longer as the stops become shallower, almost resembling a Fibonacci series pattern (i.e. 1-1-2-3-5-8-13-21 etc…) in many cases.

If we use a conservative average depth of 15m for EAN50 (it would normally be shallower) this would give an adjusted breathing rate of 2.5ata x 20l/min = 50 litres per minute.  A typical 12l tank at 200 bar will hold 2,200+ bar of gas – 2,200 litres / 50 l/min gives 44 minutes of gas – we’ll say 40 minutes to be conservative.

We can now use the 40 minute bottle as a benchmark – basically for every 10 bar we have 2 minutes of decompression gas in a 12l tank, or 1 minute per 10 bar in a 6l tank.  Using this logic a tank with 130 bar would have 26 minutes of decompression gas, or a 6 litre tank at 160 bar would have 16 minutes of decompression gas.

Factoring in Oxygen to Gas Planning

When using a second decompression gas as oxygen we can also factor this in.  In most case, the decompression time between 21m to the surface is split evenly between EAN50 depths and O2 depths; that is we’ll spend 50% of the time at 21m to 9m and the other 50% between 6m and the surface.  This has one obvious advantage, it effectively doubles the amount EAN50 deco gas we need, IF (and this is an important if) the O2 gas volumes are also planned!

Assuming the average O2 depth is 5m or 1.5 ata, our adjusted breathing rate would be 1.5 ata x 20l/min = 30 litres per minute.  Using a 12l bottle at 200 bar as the benchmark we can say this bottle has 2,200 litres / 30 l/min = 73 min, say 70 minutes of deco gas use.

Using the same numbers as above this corresponds to every 10 bar in a 12l cylinder representing 3.5 minutes of deco gas, we like to make sure we have some O2 left over for any post dive emergencies or just for in water contingencies so lets use 3 minutes per 10 bar as our benchmark, of 1.5 in a 6l bottle.  So, a 12l at 80 bar would be 24 minutes of O2 deco gas while a 6l bottle at 120 bar would be 18 minutes of O2 deco gas.

Back Gas Planning and Reserves

Maintaining sufficient back gas is critical

The key concept in this scenario is that in a worst-case(gas sharing at minimum reserve) situation we are prepared to skip any deco below 21m and ascend from depth directly to the gas switch depth.  Unless the dive is excessively long, or part of a multiple long dive series, it is unlikely we’d have any significant decompression obligation below 21m for dives up to 50m in depth.  This principle also requires EAN50 to be available at 21m, dives using only O2 as a decompression gas aren’t suitable for this method of gas planning and are not suggested as a result – make a plan and stick to it for these dives!

If we assume our ascent is at a maximum rate of 10m/min and we allow an additional 1 minute at the bottom to signal the team and start the ascent, plus 2 minutes at 21m to change gas as a team we get the following gas consumptions:

	30m Depth	35m Depth	40m Depth	45m Depth	50m Depth
Max Depth Pressure	4.0 ATA	4.5 ATA	5.0 ATA	5.5 ATA	6.0 ATA
Average Ascent Depth	25.5m	28.0m	30.5m	33.0m	35.5m
Average Ascent Pressure	3.6 ATA	3.8 ATA	4.1 ATA	4.3 ATA	4.6 ATA
Ascent Time	1.0 min	1.5 min	2.0 min	2.5 min	3.0 min
Gas Use at Bottom	80 litres	90 litres	100 litres	110 litres	120 litres
Ascent Gas Use	71 litres	114 litres	162 litres	215 litres	273 litres
Switch Gas Use	124 litres	124 litres	124 litres	124 litres	124 litres
Total Gas Use	275 litres	328 litres	386 litres	449 litres	517 litres
Assuming 11.3l twin tanks	12.2 Bar	14.5 Bar	17.1 Bar	19.9 Bar	22.9 Bar

You’ll notice the gas pressure required in twin tanks is just under half the depth in metres of the pressures above; if we allow for two divers both breathing at an increased rate of 30 l/min our gas consumption rates increase by a factor of 3 (20 l/min to 60 l/min) – so the half the depth becomes 0.5 x 3 = 1.5 times the depth in m.

Our benchmark then becomes 1.5 times the depth in metres assuming we are going to switch to EAN50 at 21m, we then complete the deco on EAN50 alone or a combination of EAN50 and O2.

Putting it all together

So, how do we use this?  Lets say we’ll do a dive to 50m and will be using EAN50 (in a 12l tank at 130 bar) and O2 (in a 6l tank at 110 bar) – how do we plan our gasses to suit the deco?

Bring a little too much gas for the dive is not a bad thing

The first factor is the back gas, we need 1.5 times the depth as a reserve to bring two of us up to the 21m gas switch – 50m x 1.5 = 75 Bar in this case.

Now the deco, we have 130 bar of EAN50 at 2 minutes per 10 bar, so 26 minutes of EAN50 gas.  The O2 tank is a smaller 6l bottle at 110 bar, at 1.5 minutes per bar that's just over 16 minutes of O2 deco.

So, if we keep 75 Bar of back gas at 50m we can run up a total deco time of 26 minutes if we use EAN50 alone, or 32 minutes if we use both EAN50 and O2 (16 minutes on EAN50 and 16 minutes on O2).

Things to consider

These numbers are conservative but give some flexibility in deco on the fly as having a little too much gas for a dive is rarely a problem; these figures give ball park estimates and should be used in place of proper gas planning.  We’d normally suggest factoring in maximum deco times of gas into the equipment check at the start of the dive; for example “I am carrying two decompression gasses, EAN50 and O2, my EAN50 is in a 12l bottle with 160 bar of pressure giving me a maximum of 32 minutes of decompression gas, my O2 is also in a 12l bottle at 90 bar of pressure giving me 27 minutes of decompression gas.  Using both gases I have enough decompression gas to complete 54 minutes of total decompression”.

A key consideration if using a computer to provide total deco time of time to surface (TTS) values is what this TTS time is based on, many computers use all gasses in memory to estimate this time – it’s important the gasses are switched on or off correctly when using this type of gas planning – for example if I am diving with 40 minutes of EAN50 and 10 minutes of O2 I need to consider I may not have enough O2 to complete the required deco if my deco is longer than 20 minutes – it’s best to switch off the O2 in the computer and use the TTS based on EAN50 alone – the O2 can always be switched back on the ascent portion of the dive or after the switch to EAN50 occurs.

Ratio decompression is also another method of providing decompression information on the fly, however we don’t suggest this should be used as a primary method of decompression as the availability of more accurate technical computers at a relatively low price and excessive times normally given for ratio decompression for dives in the 40-50m range make this technique less than ideal.  Ratio decompression does however make an excellent back-up tool.

Key Rules

There are basically three numbers to remember for this method of gas planning, as follows:

                - EAN50 gives 2 minutes of gas per 10 bar in a 12l bottle

                - O2 gives 3 minutes of gas per 10 bar in a 12l bottle

                - Back Gas reserves in bar should be 1.5 times the depth in metres when using 12l twins

A predive review of all gases carried is recommended

Try using these rules at the start of each dive as a double check on how much deco gas is available to each diver, it’s a good practice to get into regardless of whether deco on the fly will be performed or not.