Beep beep going into decom :-( or :-)

[blackice]

Thanks for the advice, from you wise divers, I am lucky that nothing happened from the limit push.

I know that I want to go deeper or longer and I know that diving is always a learning sport, each dive you learn something.

I'd say that we can close this thread as I think I have all the advice I need on this one.
Cheers,
blackice

 

[DA Aquamaster]

blackice once bubbled...

I was just wondering if others had had this happen before, as I had never had it happen in 70+ dives, but on that dive 5 people went beyond the limits.

To be fair it can and does happen, I can relate to your situation and I have been in similar situations myself. But, to be honest it comes down to poor planning and/or a lack of discipline which can still get you hurt even if the poor planning or disipline is on the part of the buddies.

We have high altitudes, short bottom times and what most people would consider poor visibility (30' is a REALLY good day) where I primarily dive. Consequently, I have been on dives where the dive buddies just ignore the freindly "time to go up sign" and the much less freindly "get your butt up now" signs and manage to lose track of time and end up in deco mode.

If they are less experienced and ill equipped, I end up feeling obligated to stay with them as I at least have a redundant air supply and can increase the odds that they can complete the deco requirements. (Of course I also make it a point to never let them on my boat again.) Dive buddies like that can get you into trouble and are actually one of the more compelling arguments for solo diving.

If you are not in a position of being the boat owner and/or the diver who could be viewed as in charge and legally liable for THEIR stupid behavior (America is a great country but here are way too many lawyers) I would suggest you consider giving them the ascent sign and then doing your ascent at the appropriate and planned time and leave them to roll the dice on deco on their own.

I am not opposed to planned deco diving and as indicated above it is a natural step, but it's a serious step requiring specialized equipment and training as well as a lot of thought as to your specific requirements and philosophy. To do it anywhere short of that is a lot like sky diving with out a reserve parachute - sooner or later you are going to have an equipment failure and you are going to bounce.

I don't think anyone on the site is trying to cut you down so much as to express an appropriate level of concern. As a counselor my approach tends to be to help people understand the complexities involved and let them make the obvious conclusion on their own that they may be in way over their head. Some people (including a lot of counselors) are just a lot more directive in what they say. it really is to your credit that you brought the question forward. Asking questions is how people learn and I would not want to discourage that in any way.

 

[Dr Deco]

Dear Readers:

You have good advice from many of the responders. No, you will probably not get DCS if you extend the limit by a little. However, as is true with any limit, it indicates where the territory is becoming a hazard. Diving can have serious consequences, such as paralysis.

You never want to get into the habit of thinking, I avoid DCS once going over the limt, maybe I can do it again. Maybe those earlier times you were well hydrated, light on the physical activity, and so on. The next time, when those conditions are not met, you get hit.

Planning for the decompression is not really that difficult.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[Peo]

DA Aquamaster once bubbled...
For deco diving I would plan on a minimum of a pony bottle with enough air to ascend and complete any required deco for the dive. Depending on your air consumption this would be 19 to 30 cu ft MINIMUM.

[...]

I dive independent doubles to ensure enough air to complete the dive with a healthy reserve even with a total air loss from one tank. In deco diving it does not take long to realize that there is nothing a pony bottle can do that a set of properly configured doubles can not do a lot better and safer.

Out of curiosity, what kind of deco diving are you doing in this setup? What is a typical profile? How do you do your gas planning?

 

[DA Aquamaster]

Most of my dives are to depths of 120 to 150 ft. Bottom times to the start of the acscent run 10-15 minutes.

The worst case plan would be a square profile to 150 ft for 15 minutes. I take my SAC rate of .53 cfm at a normal swim rate times 1.5 for planning purposes.

With these numbers about 67 cf of air is required to the start of the ascent and leaves roughly 75 cf for ascent and reserve. I plan on 30 cu ft for ascent and deco based on the same 1.5 times my SAC rate which leaves 45 cf in reserve.

This is just a bit less than 1/3rd for a reserve and I regard 150' for 15 as the absoulte max for a set of steel 72's even though my actual air consumption is less. But for planning purposes I should have 750 psi left in each tank at the end of the dive which is still a comfortable margin. In reality it is closer to 1200 psi in each tank at the end of the dive as my profiles are not that square and my actual air consumption is less.

As for air management during the dive I use 800 psi out of the left tank (1/3) then switch to the right tank. This normally occurs halfway through the 15 min bottom time and if there is a problem with the right tank, I still have more than enough air to abort and surface on the left tank. Actually the 1600 psi remaining in the left tank is more than enough to do the ascent and deco required at the end of the 15 minutes at 150 ft. This provides an out for me if things go bad with the right tank and/or reg at the worst possible time (ie the end of the 15 minute bottom time with 20 minutes of deco required). I then use 800 psi out of the right tank which normally brings me to the end of my 15 minute bottom time and means I begin my ascent with 1600 psi (2/3rds) remaining in each tank. And regardless of the plan and actual bottom time, I go up when I have used 800 psi from each tank whether the 15 minutes is reached or not. (rule of thirds)

An ascent to 30' with a 2 minute stop there, a 5 min stop at 20' and a 13 min stop at 10' normally requires another 800-900 psi of air which I spread between both tanks and the end result is normally about 1000-1200 psi (1/2) in each tank at the end of the dive. (I add a minute or two at the deeper stops and 2-3 minutes at the 10 ft stop for a saftey margin so the air remaining at the end of the dive is actually around 1000 psi per tank and I normally switch back to the left tank when the right tank reaches 1000 psi.

The conservative estimate for air consumption in the dive plan calls for a bit more than 2/3rds of the air but the reality is that I normally only use about half the air available.

The big critisim of independent doubles is the need to switch regulators, but in practice it requires one switch half way through the dive on the bottom and another during your deco when you are bored and blowing bubble rings anyway.

The most likely failure I would encounter would be due to the cold water I dive in (low 40's at depth) and the potential for a first stage freeze up and resultant free flow. With an isolator manifold, I would need to shut off the valve to the offending reg or, less than ideally, shut the isolator valve between tanks to save the other half of the air.

With independent doubles, I don't have to worry about this not being possible (for whatever reason) although it is still a good idea to shut the valve on the offending tank and save the air, as the reg will thaw in just a couple minutes after the airflow is stopped making it again avilable if needed.

In one respect, for my dive profiles, it can be argued that my independent doubles are actually a bit safer as if I get a frozen and free flowing first stage I can actually screw it up and/or do nothing and still have adequate air to finish the dive in the other tank even if the contents of one entire tank are lost. With an isolator manifold, I would have to do something or lose all of the air in both tanks.

This configuration also allows me to use the second tank as a very large pony on shallow no deco dives that can be completed on a single tank with an acceptable no deco reserve in that tank. This has the advantage of 100% redundancy and also of allowing me to dive the same configuration all the time deep or shallow without wasting air in half used doubles. After a shallow dive I then only have to switch out the left tank and can leave the lift bag on the still full right tank where it is bungeed with surgical tubing.

I moved to doubles when the dives planned exceeded the air available in a 30 cu ft pony (if you need a stop deeper than 10', in my opinion, you really ought to be diving with doubles as even a 30 cu ft pony is cutting it a little close in some cases).

Initially I used a pair of regs on a conventional set of hard banded doubles and a sherwood manifold until I figured out there was no real advantage to it for the dives I was doing. It was also a pain to remove the doubles and install an STA for the shallow dives, a pain to transport my two sets of doubles for fills and it also meant diving two very different configurations for deep and shallow dives.

 

[JeffG]

blackice once bubbled...
Thanks for the advice, from you wise divers, I am lucky that nothing happened from the limit push.

I know that I want to go deeper or longer and I know that diving is always a learning sport, each dive you learn something.

I'd say that we can close this thread as I think I have all the advice I need on this one.
Cheers,
blackice

All the advice you need?: You don't need advice..you are already doing deco dives before you are trained for it. Hurray for you...Transcend your training....

Why did you bring the subject up, and now want to close it?

Did you do something bad?

 

[ianr33]

DA Aquamaster once bubbled...

Initially I used a pair of regs on a conventional set of hard banded doubles and a sherwood manifold until I figured out there was no real advantage to it for the dives I was doing. It was also a pain to remove the doubles and install an STA for the shallow dives, a pain to transport my two sets of doubles for fills and it also meant diving two very different configurations for deep and shallow dives.

How do you attach the doubles to your BP (I assume you have a BP as you mention a STA??)

Do you consider this setup better than using a stage rigged 80 as a (large) pony. Stage rigging would seem to be less trouble to set up?

My reason for asking is that I am thinking of using a similar rig myself,although it would be in MUCH warmer water!!!!!

 

[DA Aquamaster]

ianr33 once bubbled...


How do you attach the doubles to your BP (I assume you have a BP as you mention a STA??)

I bought a set of Aqua Explorer's travel bands. Actually they are called "Ultimate Velcro Double Bands". They use center pieces of stainless steel that fit between the tanks and attach to the BP with normal 3/8 SS bolts. Standard cam bands and straps are then used in a slightly modified fashion to hold the tanks to the pair of center pieces. The system is actually a lot more low profile than it sounds and is very well designed. The "velcro" in the name is a bit of a misnomer as the only purpose of the velcro is to keep the ends of the cam straps from dangling. The strap ends also tuck nicely between wing and backplate and are not likely to snag anything.

I made one small change to replace the plastic keepers on the nylon straps with stainless steel keepers as I had one break. I considered doing the same with the plastic cam buckles themselves but have not had any problems with them.

When you are doing a single tank dive and only carrying the right tank for redundancy, you end up with one full tank and one tank with 500 psi by the end of the dive. I though this could cause problems with my CG moving to the right, but in practice the effect has not been noticeable.

Changing tanks is a snap with the travel bands as each tank can be changed individually with the remaining tank and backplate just sitting there. (I kept the boots on my tanks, which some folks regard as bad form.) So changing tanks is a very quick and lift free process.

It also allows me to keep all of my tanks available for other divers in the family rather than tying them up in sets of manifolded doubles that only I would use and that would take up additional space aboard the boat.

The independent doubles and travel bands idea also work in rental situations when traveling to places where taking your own tanks is not practical. In a pinch you could even use two tanks of different sizes.

I tried staged rigged 80's and 72's. And in my opinion both are a little bulky for general diving. And using only one stage bottle of that size (particularly a steel 72) can cause center of gravity issues and requires some weight shifting to balance it out. I am not a big fan of using anything larger in diameter than an AL 40 for a stage bottle unless it is absolutely neccesary (and I don't dive that deep or long.)

Also for any dive deeper or longer than 150' for 15 min, I like to use the D-rings for 4o cu ft stage or deco bottles in addition to the back gas. Using independent doubles instead of a single and a stage bottle keeps the configuration changes to a minimum and more evoutionary in nature when I need to carry additional tanks.

It isn't a configuration that will warm the hearts of the hard core DIR types, but it is very flexible, has it's uses (and limitations) and works very well for the type of diving I do. I think there is a lot of value in putting careful thought and consideration into the specific needs, demands and requirements of the diving an individual does and to configure accordingly rather than to just adopt and adhere strictly to a single and potentially uncompromising philosophy. I pretty much adhere to Gary Gentile's philosophy that:

"All too often, minimums become maximums and standards become limitations...The only standard that I accept is the non-standard."

 

[ianr33]

Thank you for your reply. There is LOTS of interesting stuff to think about there. I am hoping to start extending my depths/times before too long. Traditional doubles would of course work fine but are not really an option,you can't fly with them and not too many places rent them. For the sort of diving I will most likely be doing (warm water,good vis,no overhead ) I dont really see the need for manifolds and DIN valves,sure they would be nice,but far from essential. I will probably start off with a stage rigged 80 ,see how that goes and maybe get the travel bands at a later date.

And yes,I am getting the training,Advanced Nitrox and Deco in September

Thanks Again ,
Ian

 

[Peo]

DA Aquamaster once bubbled...
As for air management during the dive I use 800 psi out of the left tank (1/3) then switch to the right tank. This normally occurs halfway through the 15 min bottom time and if there is a problem with the right tank, I still have more than enough air to abort and surface on the left tank. Actually the 1600 psi remaining in the left tank is more than enough to do the ascent and deco required at the end of the 15 minutes at 150 ft. This provides an out for me if things go bad with the right tank and/or reg at the worst possible time (ie the end of the 15 minute bottom time with 20 minutes of deco required). I then use 800 psi out of the right tank which normally brings me to the end of my 15 minute bottom time and means I begin my ascent with 1600 psi (2/3rds) remaining in each tank. And regardless of the plan and actual bottom time, I go up when I have used 800 psi from each tank whether the 15 minutes is reached or not. (rule of thirds)

An ascent to 30' with a 2 minute stop there, a 5 min stop at 20' and a 13 min stop at 10' normally requires another 800-900 psi of air which I spread between both tanks and the end result is normally about 1000-1200 psi (1/2) in each tank at the end of the dive. (I add a minute or two at the deeper stops and 2-3 minutes at the 10 ft stop for a saftey margin so the air remaining at the end of the dive is actually around 1000 psi per tank and I normally switch back to the left tank when the right tank reaches 1000 psi.

The big critisim of independent doubles is the need to switch regulators, but in practice it requires one switch half way through the dive on the bottom and another during your deco when you are bored and blowing bubble rings anyway.

I don't think that switching regulators is THAT much of an issue. It DOES add to the task loading of the dive, but, personally, I have some other doubts about diving independent doubles.


The biggest issue with diving doubles as I see it is gas management.

Since you at any time during the time must account for loosing half your gas supply (one regulator failure => loss of (sometimes more than) half your gas supply). That means you'll have to carry more gas, i.e. larger tanks, for a given dive. Put another way, for a given dive, you will have shorter bottom times since you loose half your gas in case of a problem, and thus will need to cut the dive shorter to be able to get out.

Start doing some gas planning using the rule of thirds for independent doubles and for a set of doubles with a manifold, and you'll start to see the difference...

You need an extra SPG.

If you believe in long hose (which I do), you're hose routing becomes anything but trivial. You probably want long hoses on both your regulators, our you need to be very much on-top when your buddy goes OOA to be able to donate the right regulator.

If you start thinking about independent doubles, make sure you think all the consequences of that through before your decision, just as you should with any change in your scuba configuration.

When you get into any kind of overhead diving, you need to be able to resolve all problems then and there.