Is Rule of Thirds incompatible with decompression diving?

[jstotz]

I want to pose a physics problem where a tech diver attempts to obey rule of thirds.

(Reminder, physics problems take place in physics land, where there is no friction or air resistance, no changes in SAC, no current, no changes in swim speed, etc. )

So, the diver plans a square profile with staged decompression using a 3000psi Twinset of aluminum 80's. He immediately descends to 135ft and upon reaching depth, swims into a long horizontal cave for 20 minutes before he hits 2000psi. He then turns his dive around because he was told to always obey rule of thirds to be safe.

So he retraces his steps at exactly the same swim speed until reaching the ascent/descent line and starting decompression. But due to the decompression burden, he needs to use more gas for the total return trip than he used on the initial descent and going out.

He therefore surfaces with far less than 1,000 psi.

How is it possible to obey rule of thirds in decompression dives?

If it isn't possible, then what application does Rule of Thirds have in technical diving?

 

[PEDiver]

Your interpretation of the rule of thirds in this scenario is very flawed. Additionally, the rule of thirds is the minimum safety factor you can utilize - many divers might use 1/4 or 1/6 depending on the dive profile.

You need to account for decompression in your gas planning. Also, if you’re executing a decompression dive, you will probably have a decompression mix (I.e. 50%, pure O2) which is included in your overall gas plan for the dive.

My advice would be to actually take a technical diving course from a reputable instructor where you will learn this.

 

[SpeedyJ]

In technical diving, the rule of thirds is generally used where a direct ascent to the surface is not possible. Diving in a wreck, a shipping channel, areas with high surface current, etc. After calculating usable gas and minimum gas for the dive, a diver would apply the rule of thirds to their usable gas, ensuring that they can make it back to the ascent point with more gas than the minimum gas required to make a safe ascent.

I would agree with @PEDiver. If you're interested in beginning technical training, there are several good Michigan based instructors, and a really bad one who has been banned from nearly every boat on the lakes. The Great Lakes hold some amazing wrecks, but diving to the depths required to get to some of the gems requires proper training.

 

[rjack321]

I want to pose a physics problem where a tech diver attempts to obey rule of thirds.

(Reminder, physics problems take place in physics land, where there is no friction or air resistance, no changes in SAC, no current, no changes in swim speed, etc. )

So, the diver plans a square profile with staged decompression using a 3000psi Twinset of aluminum 80's. He immediately descends to 135ft and upon reaching depth, swims into a long horizontal cave for 20 minutes before he hits 2000psi. He then turns his dive around because he was told to always obey rule of thirds to be safe.

So he retraces his steps at exactly the same swim speed until reaching the ascent/descent line and starting decompression. But due to the decompression burden, he needs to use more gas for the total return trip than he used on the initial descent and going out.

He therefore surfaces with far less than 1,000 psi.

How is it possible to obey rule of thirds in decompression dives?

If it isn't possible, then what application does Rule of Thirds have in technical diving?

You're starting with 154cf.
Remove the gas necessary to complete the ascent, either to the surface or to your deco bottle.
The remainder is what you can use for the "working portion" and you can decide how much gas use for the outbound leg, how much for the return leg, and how much to reserve.

Double AL80s at 135ft don't leave much gas to work with and you're unlikely to be doing 20min penetrations.

 

[Rilelen]

Rule of thirds is for backgas penetration (let’s ignore stages for now). You need to calculate your deco gas separately.

Any serious deco dive is going to include a separate tank and gas for deco (at least a 100% O2 tank for your 20’ stop, possibly another tank for 50% for deeper stops). You should plan your deco so that you can still complete your deco if for some reason your deco bottles went missing or were unusable. That’s where the 1/3 to enter, 1/3 to exit, 1/3 for emergencies comes in.

Plus, remember, most of your deco obligation is going to be at very shallow depths (~20-30’) where gas goes a long ways.

Concrete example from yesterday: we did a one stage dive at Ginnie on HP100s, with 100% O2. Dropped the O2 on the gold line when entering. Dived a third of the stage, dropped it. Dived a third of our backgas, turned the dive. Exited the dive using just a sixth of our backgas because high flow cave, stage still 2/3 full (we could have switched to the stage instead). Picked up our AL40s of O2.

Thirty minutes of deco, including about twenty-some minutes at 20’. We had more than enough oxygen in each of our AL40s to deco out three times over, plus our additional remaining 150cf each of 32% in our backgas and stages (which, with our SAC rate, would last for 4-5 hours of deco at twenty feet).

 

[pisauron]

Rule of thirds is strange.

So I have 2 tanks each 12l with 200 bar.
I need to leave 30bar.
So I have 170*2*12 = 4080 liters.
I can use a third so that's 1360 liters.

A single 12l tank has more gas - 2400.

Must be missing something.

 

[rongoodman]

You're starting out with 4800 liters, not 4080. Not sure where "I need to leave 30 bar" is coming from.

 

[Alekseolsen]

Rule of thirds is strange.

So I have 2 tanks each 12l with 200 bar.
I need to leave 30bar.
So I have 170*2*12 = 4080 liters.
I can use a third so that's 1360 liters.

A single 12l tank has more gas - 2400.

Must be missing something.

Why do you believe you are missing something?

Of course a third of a double 12 set is less than a single 12l.

Now even the list basic Rec dives there is out there does some variation if this "must be onboard diveboat with 50 bars" means leaving the bottom with say 70 bars, beeing a third of the bottle. Big difference when diving somewhere you need to get back to the start, and easy math say you will use same gass "in and out" leaving you with a third in, a third out, "and 70 bare to reach surface with 50 bars".