Modifying my BMCLs for the Hammerhead Extreme CCR

I've started to make a few adjustments to my new Hammerhead CCR this week, in general the unit works a dream; but one minor concern I had is that the unit doesn't breath well as you move to a head down position due to air becoming trapped in the bottom of the lungs and not being able to reach the shoulder mounted T-pieces.

I don't really plan on being in a head down position too often, however wreck or cave diving can often require this and I didn't like the idea of restricted or unbreathable operation when more than likely being in a restriction.

Hose wraps on BMCL T-pieces

The modification suggested by Randy from SubGravity was the addition of the standard scuba hose wraps into the the lungs.  I actually didn't dive the unit without the wraps inside the lungs so can't comment on any improvement but I felt the sharp edges of the wraps could increase the risk of puncturing the lungs, while the performance with the wraps still made breathing slightly more difficult when horizontal and noticeably difficult when orientated head down.

I wanted something softer inside the lungs that gave a greater opening area to keep an airway between the t-pieces and the bottom of the lungs, initially I was thinking of a 1 inch diameter chemical braided hose (like a large diameter hose pipe) but was a little worried about the effect on minimum loop volume - a long length of large diameter hose would create quite a bit of dead space which I didn't want to carry in the lungs.  The hose also needed to be soft enough to not interfere with the lungs or risk damage, but firm enough to avoid collapse when under a negative pressure in the lung.

Standard firm garden hose with holes

I weighed up a few different options and had intended to try two or three different options, namely a standard garden hose, a slightly larger hose and the larger 3/4 inch hose.  My final choice was just to go with the garden hose as the hardware store nearby only had that in stock unless I bought the full 20m hose!  I had two choices, soft or firmer and went for the firmer hose type.

I cut the hoses to reach from the t-piece to around 10cm/4" from the bottom of the lungs - this could mean I couldn't quite work them at absolute minimum loop volume but was willing to sacrificial that to avoid the end been sealed or increased risk of water in the hoses.

To stop water entailment from the bottom of the lungs into the breathing loop, and to allow gas movement from anywhere in the counter lung, I drilled several large holes in the hoses.  I had originally intended to drill more larger holes but found they made the hose shape more flexible and was concerned the hoses could close up and reduce the breathing area when under a negative pressure in the lung.

Hose connection on BMCL T-piece

The hose was then tied using a cave line with two half hitches to the counter lung t-pieces, keeping the hose and as close as possible to the t-piece connection.

Performance wise I definitely noticed a significant improvement in the work of breathing when fully horizontal, head down was still noticeable harder to breath but it did prevent the complete close-up of the lungs - although the breathing resistance was still very high.

I've since added a second hose to each counter lung to double the hose opening are, the second hose was added in the same way as the firs.  Adding the second hose seems to line up well with the t-piece water trap openings.

I've not yet had a chance to dive the unit with the second hose so can't compare performance, however I'm hoping there's a further improvement - logic would suggest breathing should be easier again given the increased hose sectional area in each lung.  Sadly I don't think I'll get a chace to dive the unit again for almost 2 weeks due to other commitments but as soon as I do I'll give an update on how it works!

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.

Recreational Gas Management and Planning

One thing we try to emphasize in our courses is the importance of proper gas planning at all levels.  In technical diving this normally means allowing for enough gas for a team to finish the dive with one team member suffering a complete catastrophic loss of gas.

We normally apply the same logic to recreational no decompression limits diving and focus that the amount of gas reserve we need to maintain is dependent upon the depth rather than air consumption rates.  The gas planning currently used by most divers for gas planning (low on air at either 50 bar/700 psi or 70 bar/1000 psi) is not really an appropriate method for most dives while use of the ‘rule of thirds’ can lead to many other problems.

Most divers have a Surface Air Consumption rate (or SAC rate) of around 15-25 litres per minute, this is a regular breathing rate – in case of an out of gas emergency or panicked diver this is normally around the 30 l/min mark.  Given this bit of basic information it’s possible to work out the amount of gas we’d need to get from a specified depth to the surface.

Sadly we don’t normally work in litres when underwater so it’s good to use a unit we’re all familiar with; pressure.  A typical recreational aluminum scuba tank, the S80 or 12l tank, has an internal volume of around 10.5 to 11 litres, based on 30 l/min consumption rate this would correspond to a pressure of around 2.85 bar of gas per minute at the surface (30 l/min / 10.5 l = 2.856 bar/min).

We now need to allow for the fact that in a worst case scenario there would be two of breathing from the same tank – normally a team member who is out of gas.  In this case the breathing rate doubles from 2.85 bar/min to 5.7 bar/min.  With this information we can work out the gas required to ascent from any depth by using the pressure at the average depth during the ascent (i.e. half the depth) and the 5.7 bar/min consumption rate.

The final steps we need to consider is we can’t suddenly ascend as soon as our team member is out of gas; typically we are looking at up to 30 seconds to donate gas and a minute to stabilise the situation before starting the ascent – i.e. another 1.5 minutes at the bottom before ascending.  It’s also a good idea to factor in a safety stop at 5m, it’s not required for most dives but allowing at least 1 minute at 5m at the end of the dive is highly recommended – the full 3 minute stop is better again, however in our emergency planning we’ll just allow for a 1 minute stop for now (it’s easy to extend in a real situation if we still have gas available!).

With all the information above we can now work out some gas volumes for dives, lets say we want to dive to 30m and need to know or reserve – we can calculate it as follows:

Maximum depth = 30m

Pressure at maximum depth = 4 bar

Average depth for Ascent = 15m (30m / 2)

Pressure at average depth = 2.5 bar

Ascent time = 3 minutes (allowing for 10m/min ascent rate)

Safety stop = 1 minute at 5m

Gas used:

At bottom = 34 bar (1.5 min x 4 bar x 5.7 bar/min)

In ascent =  43 bar (3 min x 2.5 bar x 5.7 bar/min)

At safety stop = 9 bar (1 min x 1.5 bar x 5.7 bar/min)

TOTAL = 86 Bar

The first thing most people notice is this is more than the standard 50 bar or 70 bar many divers follow – what does this mean?  Well, in short it means you really didn’t have enough gas to safely ascent to the surface if your buddy ran out of gas, sadly this is something most divers are doing regularly without realizing it.

We can also try the number for a shallower dive, say 12m:

Maximum depth = 12m

Pressure at maximum depth = 2.2 bar

Average depth for Ascent = 6m (12m / 2)

Pressure at average depth = 1.6 bar

Ascent time = 1.2 minutes (allowing for 10m/min ascent rate)

Safety stop = 1 minute at 5m

Gas used:

At bottom = 19 bar (1.5 min x 2.2 bar x 5.7 bar/min)

In ascent =  11 bar (1.2 min x 1.6 bar x 5.7 bar/min)

At safety stop = 9 bar (1 min x 1.5 bar x 5.7 bar/min)

TOTAL = 38 Bar

This time we need less reserve than the normal 50 bar most divers work too.

This seems a little complex to calculate for every dive or change in depth, however if we look at this for different depths we see a pattern start to emerge in the ratio between depth and the required pressure reserves:

Depth	10m	15m	20m	25m	30m	35m	40m
Average Depth	5m	8m	10m	13m	15m	18m	20m
Ascent time (10m/min)	1.0 min	1.5 min	2.0 min	2.5 min	3.0 min	3.5 min	4.0 min
Donate Gas (1.5 minutes)	17 bar	21 bar	26 bar	30 bar	34 bar	38 bar	43 bar
Ascent Gas Use	9 bar	15 bar	23 bar	32 bar	43 bar	55 bar	68 bar
Safety Stop (1 minute)	9 bar	9 bar	9 bar	9 bar	9 bar	9 bar	9 bar
Total Gas Required	34 bar	45 bar	57 bar	71 bar	86 bar	102 bar	120 bar
Depth to Pressure Ratio	1:3.42	1:2.99	1:2.85	1:2.82	1:2.85	1:2.91	1:2.99

You can see that based on the above information the depth to pressure ratio for non-stop recreational diving limits is generally within a 1:3 pressure to depth ratio when using metric units, i.e. if you multiply your depth in metres by 3 that is the minimum gas reserve you should maintain in bar.

This is where our depth times 3 gas reserve that we use for recreational non-stop in courses and guided dives is derived from, in other words if we're at 30m any divers at 90 bar of pressure (30 x 3) should signal 'low on gas' to the guide, if we are at 15m the low on gas pressure would be 45 bar.  If we have divers using PSI PSI we use 50 times the depth in metres or 500 psi per 10m of depth.

This is the basis of gas planning we use for all our recreational courses, you’ll find that when comparing the reserves above to the standard recreational table non-stop limits a lot of dives in the 20-30m range would result in gas reserve becoming the limiting factor for the dive duration, not the non-stop limit time.  However you’ll also see as you ascent towards the end of the dive the gas reserve required also reduces, allowing the dive to be continued while still maintaining an adequate safety reserve of gas to finish the dive.

This is the basis of our suggestion that any dives to 30m or deeper should be done in either twin tanks/sidemount configuration or down with an additional slung tank to provide the team an adequate gas safety reserve.

Please note this is based on recreational non-stop diving only and doesn’t apply to overhead environments or decompression diving.  We’d also suggest staying above these minimum reserves rather than working directly to them, particularly for the shallower depths and when using rented or unserviced equipment.