What is a deco bottle?

[alohagal]

Yes, people have been known to remove deco bottles from caves, causing the diver to come back and find he doesn't have his deco gas and must deco out on backgas (which is a much longer process, and hopefully he has the gas to do it). I always hope that the people who do this have no clue what the tank is for, and aren't just willfully taking gear and thinking, "So he gets bent, so what?"

Okay...what is backgas and why is it longer than a deco bottle?

 

[Rick Murchison]

Let's see if I can do this without getting "technical..."
"Back gas" is the gas you carry in the tank(s) on your back (or on your sides if you are side-mounting). It is safe to breathe at your maximum planned dive depth.
"Deco gas" usually has a high (above 49%) oxygen content. This is designed to increase the gradient between the partial pressure of dissolved inert gas in your body and the partial pressure of inert gas in the deco mix, so you'll offgas faster and can therefore come up sooner than on your bottom (back) gas. But due to its high oxygen content, deco gasses have a much shallower maximum safe depth for breathing.
So, having to deco on back gas takes longer because it takes longer to get rid of the inert gas in your body than if you're breathing a deco gas.
Rick

 

[DA Aquamaster]

As an example:

If I dive to a depth of 105 ft for 60 minutes, using 32% Nitrox for the back gas, I will have a required decompression of 3 minutes at 40 ft, 4 minutes at 30 ft and 34 minutes at 20 ft. and will use a total of 178 cu ft of back gas.

However if I use 100% O2 as a decompression gas, I will have a required deco obligation of 3 minutes at 40 ft , 4 minutes at 30 ft and only 13 minutes at 20 ft where I can use the 100% O2 rather than the back gas for decompression. (I have to wait until I get to 20' to switch as the 100% O2 is essentially toxic below 21'.) At the end of this dive, I will have only used 156 cu ft of back gas and 8 cu ft of O2.

So...it is faster and it uses less back gas.

But...a prudent cave diver will always assume that they may lose access to the decompression gas and will have a "lost gas" contingency plan. This is important in Ginnie in particular as due to it's location on the santa fe river and the facilities available, it gets a lot of swimmers and float tubers as well as a large number of OW divers.

It has happened where snorkelers have taken deco bottles left in the ear at the 20 ft stop as they are very visible there and within range of many people with a mask and fins. In the cavern itself, an OW diver may stumble up on a tank and reg and if we look at it charitably may just assume some one dropped it or lost it, lind of like a lost snorkel in a quarry. Or it could be that tanks and regs are expensive and diver sees it as an easy way to acquire a tank and reg.

That's why you almost never see deco bottles left near the log in the ear, and if you do, the odds are they are divers new to the area who have not been advised of the risk of doing that. Normally, a cave diver would leave a 100% O2 bottle at 20 ft to avoid the potential to switch to it deeper than 20 ft where oxygen toxicity could be a factor. Alternatively some cave divers are train to leave it 10 ft deeper to prepare for the gas switch during that 10 ft ascent. At Ginnie, they end up being left significantly deeper to keep them in an area where they are more likely to remain undisturbed.

Stage bottles are usually larger 80 cu ft tanks left deeper in the cave. Cave divers will breathe part of it going in, then leave it secured to the line so that they can breathe part of it going out. How much is a topic of some debate. Some divers will use 1/3 rd going in and 1/3rd going out with 1/3 left in reserve - the same as they do with the gas in their back gas tanks (a maximum of 1/3rd going in, 1/3rd going out, and a minimum of 1/3rd in reserve. Other cave divers using a stage will use a bit less than half going in and a bit less than half going out leaving only about 200 psi in reserve in the tank - choosing instead carrying all of the total reserve gas needed for the dive in the back gas tanks. In the end it usually ends up being the same size reserve, the difference is just the philosophy of where it should be carried as both have some pros and cons.

 

[DA Aquamaster]

As for cave diving. I cannot remember which book it was I read years ago...but it talked a lot about when people first starting exploring caves with scuba gear and Ginnie Springs was one of the main ones. Lots of lives lost trying to figure out how to go about doing it safely (back in the 60's?). Long before cave certs course, guide lines, penetration techniques, double tanks, staging and deco bottles.

Made for a bone chilling read!

Many of the advances in recreational diving have come from the cave commuity. The BC is one example.

I think one of the biggest advances in techology has been in the development of lights. Older halogen bulb primary lights were not horribly bright by modern HID and LED standards and the battery cannister were large. Primary lights had to be over volted a bit to get maximum brightness but that hurt reliability, and in any event battery life was very short even with the larger battery in the larger cannister as they drew a lot of current. Consequently, cave divers took a minimum of 3 lights on every dive with the smaller back up lights being under volted, quite dim, but a bit more reliable. Way back in the day, it was common for divers to take more than one primary light in addition to back up lights in anticipation of a light failure.

Today, a fairly small 10W HID offers more light than I ever got with my OMS phantom where I could choose between a comparatively dim 10 or 20 watts of halogen light, or 50 watts (about equal to a 10W HID now, but only an hour of burn time.) And today's LED back up lights offer exceptional reliability, lots of light and phenomenal burn times.

But a history of unreliable lighting has left it's mark on cave diving as lights out drills are a large part of the training. And it still makes sense as all the light in the world won't help in a total silt out where you still need to do it by feel.

The development of better gas management strategies as well as the isolator manifold has also had a significant impact on cave diving safety.

 

[tstormdiver]

When was down there a couple of weeks ago, My instructor & I left our deco bottles on the buoy chain in the ear. He teaches a lot of classes there at Ginnie. Nothing happened to them there (maybe just luck). However, a bunch of drunk boaters came up & were messing with them where we had placed them in the water next to the stairs, while prepping for the next dive. I was the fist one on scene, as I heard a regulator being purged & a voice call out, "you probably shouldn't be messing with that". I basically told the offender to put the bottle back in the water & leave it alone. The drunkard just looked at me blindly, barely even conscious. About that time my instructor came up & more sternly told the guy holding his bottle the same thing. The guy complied, walked up the stairs (we thought there might be an altercation coming), laid down on the bench & promptly passed out. His buddies then came, got their inebriated friend, put him in a boat & left the area.

 

[H2Andy]

yeah, what everybody said

backgas is the gas you carry on your back (either in a single or double cylinder setup) (if you're diving sidemount, then the tanks are slung under your arms, not on your back)

a deco bottle is a cylinder you carry clipped to your harness with its own regulator, and which you breathe from during the decompression part of your dive. normally it has a higher O2 content to speed up offgassing

the concept is that you have different gas mixes for the different parts of the dive, normally split into regular gas and deco gas. you can also have traveling gas to get you to depth, then your deep gas, then traveling gas to get you up to deco depth, then various deco mixes as you get shallower and shallower

 

[alohagal]

Okay...I know the difference now between the gases in a Deco bottle versus a backgas! Which makes complete sense.

Three more questions:
1. Are cavers usually using Nitrox for back gas?
2. Do you adjust computer after switching to 49% through 100% O2 Deco Bottle? Or relying on your brain to make these adjustments to account for proper Deco time?
3. What are the mechanics involved in switching? Just putting the Deco bottle REG in your mouth a turning on the valve? Are you hooking it up to your mount in anyway?

I know when I first saw the bottles in the video...it reminded me of discarded O2 Cannisters that litter the camps on Mt. Everest. I realize now they are not "discarded" in a cave situation.

Thanks again,
Mary

 

[alohagal]

Many of the advances in recreational diving have come from the cave commuity. The BC is one example.

I think one of the biggest advances in techology has been in the development of lights. Older halogen bulb primary lights were not horribly bright by modern HID and LED standards and the battery cannister were large. Primary lights had to be over volted a bit to get maximum brightness but that hurt reliability, and in any event battery life was very short even with the larger battery in the larger cannister as they drew a lot of current. Consequently, cave divers took a minimum of 3 lights on every dive with the smaller back up lights being under volted, quite dim, but a bit more reliable. Way back in the day, it was common for divers to take more than one primary light in addition to back up lights in anticipation of a light failure.

Today, a fairly small 10W HID offers more light than I ever got with my OMS phantom where I could choose between a comparatively dim 10 or 20 watts of halogen light, or 50 watts (about equal to a 10W HID now, but only an hour of burn time.) And today's LED back up lights offer exceptional reliability, lots of light and phenomenal burn times.

But a history of unreliable lighting has left it's mark on cave diving as lights out drills are a large part of the training. And it still makes sense as all the light in the world won't help in a total silt out where you still need to do it by feel.

The development of better gas management strategies as well as the isolator manifold has also had a significant impact on cave diving safety.

Doing cenotes in Mexico this summer was my first feel for the idea of "cave" diving. My Guide--who was stupendous--had a cannister type light on a long hose. It really was quite bright and I had never seen that type of set up before.
While in the depths the three of us covered our lights with our hands to see how dark it was. It was PITCH black. I know they call them caverns...but quite a few areas had no overhead light coming through.

I still am amazed at the video of Harry's crack and how tight the spaces are that cave divers manage to go through. With double tanks no less. I did not see any side mounts on that trip. Quite an interesting environment you cavers delve into. I want to "read" more about it.

Mary

 

[ucfdiver]

When was down there a couple of weeks ago, My instructor & I left our deco bottles on the buoy chain in the ear

Be real careful doing that, it's a good way to end up doing backgas deco

 

[Atomic_Diver]

My Deco bottle is a AL 40 O2 clean, with a Dive Rite Regulator, and SPG