Deco Dive Planning

This page is a supplementary page for decompression divers enrolled in a decompression course or certified decompression divers looking to refresh. You can not complete a decompression dive with a reasonable margin of risk management with this guide alone. It’s a supplement.

For dive planning we need to start with:

Surface air consumption rate (SAC rate): cuft/min@1ATA
Depth of dive site.
Any logistical restrictions.
Cylinder sizes*

*We can either plan the dive to match cylinders or find cylinders to match the dive. If we’re constrained by cylinder selection that becomes a starting variable.

We need to end the planning session with:

Runtime – length of the dive
Bottom time – length we’re at depth
Max time to surface (TTS) – length of ascent
Ascent pressure – Pressure of gas expected at the end of our planned bottom time
Run schedule (if using one tech computer+bottom timer instead of 2 tech computers)

Dynamic Diving

Dynamic dive planning uses Max TTS+ascent pressure. Dynamic deco diving is the idea of using an average depth and adjusting the bottom time during the dive – shortening or lengthening – to stay within the two critical variables. This style of planning is not on the fly, it requires preparation. It is more fluid, realistic to modern diving, and typically maximizes our dive while still staying inside our determined limits. This style of diving is only reasonable when 2 computers are used, it isn’t appropriate when using a computer+bottom timer.

Bottom Time

Bottom time is our planned length at depth that results in our expected decompression length. Bottom time is the least absolute aspect of our plan – it is important but it isn’t absolute. If we descend on the wreck and it’s slightly deeper than planned, or we get blown off the reef into a deeper reef – we cannot treat the bottom time as an absolute because it will result in an unplanned amount of deco. Assuming a square bottom dive is not realistic. TTS shows us our ascent and our SPG shows us our pressure. Those two metrics are the most vital for the successful completion of the dive. We need to know and respect the bottom time as it often dictates other logistical concerns, but we need to understand when it’s time to ascend based on gas reserves or TTS.

How do we get our bottom time? Well, we guess. Pick a time that we want to be on the bottom and input that into the planning software. Then see if it’s realistic and tweak it until it’s a reasonable plan. Eventually, you’ll have the ability to know what a reasonable bottom time is for a given depth. The bottom time we choose dictates all other portions of the dive profile. This is our starting point for any dive plan for a given site. I am quick to remind people that it’s not the only metric because I’ve seen the problems with hyperfocusing on bottom time. Specifically, I’ve seen many divers run out of gas or almost run out of gas because they had a bulletproof dive plan but the only metric they had in their head for the ascent was bottom time. “25min on the bottom, that’s the plan – plan the dive and dive the plan”. That’s a bad plan. Sometimes they had the wrong depth for the dive, sometimes they were just working too hard, other times they were just lying to themselves about an unrealistic SAC rate.

Ascent Pressure

We have our dive plan and need these data points to ensure a few things during the dive. We need our ascent pressure because our gas planning is contingent on having a specific amount of gas for the decompression portion of the dive (lost gas plan). If we start working too hard at depth and breathe more gas than expected we need to ascend early – our ascent pressure is that metric. If we use too much we have crept into our reserves and/or rock-bottom gas volume. Our dive plan isn’t based on a TOTAL gas volume – it’s based on a gas volume being used at specific phases of the dive.

As an example, we choose to go to 100ft for 20mins.

Variables:

Depth: 100ft
Surface Air Consumption Rate: 0.8cuft/min @ 1 ATA
Time: 20min
Cylinders: Aluminum 80s, 3000psi working pressure

First, convert depth to pressure:

100ft/33ft/atm =3atm+1atm = 4ata

Then determine the gas used per minute at 1 ATA:

0.8 cuft/min *20min = 16cuft

Then factor in the pressure to get the cubic footage used:

16cuft*4ata = 64cuft

Determine the cylinder baseline and multiple by 2 for doubles:

(80cuft/3000psi = 0.026cuft/psi)* 2=0.053cuft/psi

Convert cubic footage to psi

64cuft/0.053cuft/psi = 1207psi

If our cylinders are filled to 3000psi our plan says we will leave the bottom with a little less than 1800psi. Assuming we’ve planned the dive fully, that means we can complete the required deco and the contingencies with that volume. If we are 15mins into the dive and we have 1800psi that means we have to go up. We will not have enough gas to be within our plan parameters if we continue. If we have 2200psi at 20mins then awesome! We did better than expected.

Total Time to Surface (TTS)

TTS shows us the length of the ascent. This value is accent times and deco stops combined. We have a planned decompression ascent that factors gas volumes and contingency gasses. If our ascent is lengthened by mandatory decompression we might not have appropriate gas reserves. The best way to know we are within our planned TTS is to watch that value on the computer. We should not rely solely on our bottom time to ensure we’re within our decompression schedule. Shearwater computers also have “+5 TTS” features which allow us to visualize the change in TTS if we stayed at that depth for another 5 minutes.

Emergency Planning

When we’re planning our dives we need to focus on being prepared to handle reasonable “what ifs”. We can never guarantee a dive will go without hiccups – it’s usually quite the opposite. We also can’t spend 10 hours planning a simple dive or be so paralyzed by fear we don’t enjoy the dive. We need to focus on reasonable preparedness to solve problems. For example, we should get our regulators and cylinders serviced regularly and do a bubble check at the start of each dive. That gives us confidence that they will work. We should also be experts at valve shutdown drills as deco divers. None of that guarantees they will work which is why we have a dual regulator system. If one fails we can end the dive and will see it as an inconvenience rather than a life-or-death situation. If we are terrible at turning our valves off, don’t get our gear serviced, and don’t do buddy bubble checks then we’re planning for failure and riding on luck and our impromptu problem-solving. That’s silly, we should rely on the proven processes to deal with that emergency.

When it comes to emergency planning for deco dives there are plenty of “what ifs” – even what if we get back to the car and someone stole it!

From an emergency standpoint, our standard planning needs to consider:

We have enough gas to do the dive
We have contingency gas for emergencies
We can complete all required decompression on backgas (lost gas plan)

From an environmental standpoint, some examples are:

What do we do if we get separated from each other?
What do we do if we cannot make it back to the anchor line?
If we’re drift diving, what if an SMB fails?
What if we miss the wreck?

Plus 5 plannings

Two of the most common planning scenarios are +5ft and +5mins.

+5mins – What if we get entangled or distracted and spend 5 minutes longer at the bottom?

+5ft – What if we are forced 5ft deeper than our plan during the dive?

These are separate events. I encourage you to run these events as separate plans from the primary plan. What you’ll find is that time matters more than depth. While I encourage you to do these plans a few times to visualize and conceptualize deco penalties neither are necessary for standard dive planning in my opinion.

Lost gas

Lost gas planning is to ensure even if we lose an entire cylinder of decompression gas we have enough backgas to complete the required decompression. Simply disable the deco gas in the dive planning software and see if your backgas volume can accommodate this loss. This is primarily relevant to using 1 decompression gas. If we have 2 we don’t need to plan to lose both, only one. When we talk about “losing” a deco gas we’re usually assuming the regulator failed, not that we physically lose it. In real life we could probably feather the valve, use our buddy’s once they clear deco, or even fix the reg underwater. Regardless of our problem-solving, if we are using a single deco gas we should see it as an accelerant and a bonus, not a guarantee.

Contingency gas

We cannot surface with empty cylinders. We need to set aside a volume of gas that is for emergency use. This is typically referred to as rock-bottom gas. We need enough reserve gas for a gas-sharing ascent from the bottom to the first deco switch. This means we can share gas with our dive buddy in an emergency at the bottom, begin the ascent, and get to deco depth. That is the emergency scenario we are planning for. Rock bottom gas is set aside as untouchable during dive planning.

For deco gases, we need 1.5x the required gas for the dive. This buffer is not arbitrary, it allows a buddy deco gas-sharing procedure in the event of lost gas that minimally impacts runtime.

Thirds

Tech divers often default to thirds (⅔ useable ⅓ reserve) for deco diving. This is maybe appropriate in practice but it’s totally inappropriate in process. Thirds is for cave diving – enter, exit, reserve. Thirds is only appropriate in certain caves and is often not conservative enough for emergency procedures in caves. In deco diving thirds is irrelevant because we have no entry/exit of assumed equidistance. While thirds may end up being a proper amount of gas it’s a guess at best and an inappropriate assumption for deco diving.

Rock Bottom Planning

To get rock bottom gas there is a detailed long way and an abbreviated short way. The abbreviated short way is the real-life best method, but we should start with the long way because it gives us the best visualization of what we’re doing.

Rock bottom is contingency gas that fits a specific emergency scenario:

Your buddy runs completely out of gas at depth.
You share gas with them and start to ascend.
You need enough gas to get to your next air source. (Maybe that’s the surface, maybe that’s a deco gas depth.)

In the below chart, you can see the variables and the planned emergency – an ascent from 120ft to the oxygen deco gas at 20ft. This example results in only a few hundred PSI required, which seems low – because it is. We can buffer on top of this gas reserve to have extra confidence we have enough gas to complete the dive. If the rock bottom is less than 1000psi I typically round up to 1000psi.

While the chart is a good visualization of the emergency a simpler approach is to average the ATA of the ascent or plan the entire gas reserve at the deepest ATA. Either of these methods are appropriate. Regardless of the methods this gas reserve should not be touched during the dive. It is set aside from the gas planning in an emergency.

Example Dive Plans

These are examples, don’t use them for real dives. There will be slight variances for different apps. I don’t endorse these as plans, they’re made up examples for the sake of examples.

Surface Air Consumption (SAC) Rate: 0.8cuft/min @ 1 ATA
Gradient Factor 30/75

Plan 1:

Reef dive with a bottom at 100ft.

Mains: AL80s 3000psi, 32%
Deco: AL40 3000psi, 50%
Max Depth: 100ft
Bottom Time: 25mins
Gas reserve: 1000psi

Let’s start by putting this plan into our app. We can see it will result in a 31 minute dive, so a light amount of deco. We will only use a small amount of the 50% – our AL40 is a great choice. Our backgas used is ~1500psi, leaving us with 1500psi for reserves. This fulfills our requirements. Since this is a single deco gas dive we need to ensure we can complete our deco without the 50%. If we disable the 50% we’re still above our 1000psi limit.

How do we know when to go up?

Dive Time: 25mins
Minimum Ascent Pressure: 1500psi
Maximum TTS: 6

We need to ensure we start the ascent with no more than a 6min TTS and no less than 1500psi in our doubles. This is a conservative but reasonable dive plan. It might be appropriate to rerun the plan with a little more bottom time.

Plan 2:

Wreck dive with the sand at 130ft and the top at 90ft. Most of the deck is at 115ft. We can approach this plan one of two days – plan the entire dive at the deepest portion or utilize an average depth. The max depth plan seems self explanatory so I’ll focus on average depth.

Mains: AL80 3000psi, 28%
Deco: AL40 3000psi, 50%
Average Depth: 115ft
Bottom Time: 25mins
Gas reserve: 1000psi

Let’s start by putting this plan into our app. We see it will result in a 37 minute dive. We will use ~800psi of deco gas, well within our 1.5x contingency. Our backgas used is ~1600psi, leaving us with 1400psi for reserves. This fulfills our requirements. Since this is a single deco gas dive we need to ensure we can complete our deco without the 50%. If we disable the 50% we’re still above our 1000psi limit.

How do we know when to go up?

Dive Time: 25mins
Minimum ascent pressure: 1400psi
Maximum TTS: 12

Because we opted to use an average depth, our reason for starting the ascent is contingent on how the dive progresses. If we pop down and see the sand for a minute but spend the rest of the dive at ~110ft then we’ll probably be close to this 25min plan. If we spend a lot of time above 110ft then we’ll probably have plenty of gas and little/no deco when we hit the 25min mark. However, if we spend a lot of the dive – even half of it – at the sand level we will not be within the average depth of the dive plan. That’s why it’s crucial to watch our TTS and SPG to ensure we’re at a reasonable deco obligation and gas usage. Maybe we plan for this average depth and the current is so strong we have no choice but to get near the sand on the lee side – or maybe there’s a shark or stingray we’re following. Either way, we can flex our plan to that depth by watching TTS and our SPG and ascending according to that plan.