Quick Deco Question

[bamafan]

The Petrel shows TTS based upon you doing the stops it has calculated. If you stop deeper the tts doesn't change immediately. It will still show a stop of a given time but because you are at a deeper depth it might take 5 minutes to clear a 3 minute stop. You can set the petrel so that is shows your gf% percentage while you are in deco as you go up and down this percentage will change.

 

[Subcooled]

I am not an experienced decompression diver, but I used this learning process:

I played with dive planning software to see how TTS is affected by stops at various depths. The software just needs to run the same model as the dive computer. I usually end up with rules like 5 min of extra bottom time at X m results in 5 min of extra deco at 6m, and one minute at 9m, or something like that. Making the rules a little bit conservative makes the rules simple (and then I verify the plan). If I go deeper, the numbers change a bit. It is also fun to look at how various deco gasses (or not) affect the decompression profile. Unfortunately the deco software I have (V-Planner and Suunto) only show a profile resulting from one model.

A Suunto Vyper (some flavor of RGBM), a Petrel (Buhlman) and V-Planner (VPM) can result in different deco profiles for light deco dives. In a mixed team this can be annoying. A suitable ascent profile might keep all computers and tables happy.

 

[ajduplessis]

@Declan Long
You are clearly asking questions you have little understanding of; and here lies a lot of potential danger for YOU!

Question if I may? Why are you "behind" in what the SW has calculated in terms of run? If you want to be more conservative (M Value) rather use more conservative setting.

From what I have seen in your posts you clearly have very limited practical experience in deco; or an understanding of the theory and how to apply this to your diving.

Many members here have raised the same question yet you still feel as being "attacked"

Find a mentor that will help you answer these questions. Your instructor should be an obvious choice? We are all strangers here .....

 

[DevonDiver]

When we have deco to complete when you have a stop at say nine metres and you are at eighteen and you ascend slowly, does this stop clear before you get there because due to the slow ascent you have off gassed enough so you can ascend shallower without reaching the m value?

The stop would clear eventually - obviously, this depends on the length of the stop and deco obligation... and, the algorithm/software used.

The real question is: Why would you NOT ascend and complete the stops as planned/directed?

Also when you have a stop in an sw petrel at 6 m why if you stay at nine does it still clear at the same rate as if i was at 6 asin the count down still counts down? explain please.

Firstly, there's no real reason why this should ever happen.

The time (depending on algorithm/software and dive parameters) wouldn't be identical. Ascending through stops brings a calculated super-saturation to permit predicted off-gassing. Staying deeper wouldn't provide that level of super-saturation (and gas pressure differential between tissue-cardiovascular-lungs) to provide the anticipated off-gassing. Hence, deco could be longer.

... i am a certed deco diver...

The fact that this is not apparent to any of the contributors to this thread should cause you some considerable concern.

There is an expectation that a diver 'qualified' in decompression diving would have a more robust knowledgebase than that demonstrated by the questions you ask.

 

[HIGHwing]

Why not give straight answers? ... The answer to your second question is it doesn't. Or at least it shouldn't, but there might not be a big difference. If you stay at 9m you will still be offgassing (based on the whole dive profile), so your 6m stop will eventually clear. It shouldn't be at the same rate that it would have cleared at 6m, but it might not be a big enough difference to notice. I'm not positive how the Petrel handles this, but with some computers (like the VR3) the deco time shows the same while the clock counts slower. In other words it might still show 3 minutes of deco, but it could take 5 minutes to count down those 3 minutes.

The Petrel handles this the same way. But... And this is where knowing what you're doing, or reading the materials/manuals is very important. IF you have gases programmed and active in your computer the deco calculation always assumes you're going to switch to those gases and the TTS and Time @ Stop results presented to the diver are as IF you're going to immediately switch to those gases. Several times now I've witnessed Petrel divers program and make active a bunch of gases they do not have on the dive. Absent the academic knowledge, I guarantee you it will confuse you and worse you could easily run out of gas executing computer based deco dives and programming gases for which you do not have access. The problem is the divers want to program gases they see in pretty technical dive photos. These divers do not have the academic knowledge and fail to realize the potential consequences.

It's really easy to get lulled into the trap. You get a new fancy computer and setup 21/00, 32/00, 36/00, 50/00, and 99/100 because those seem like cool gases. You drop down to 130 ft in your recreational AL80 setup on air and see 5 minutes of deco and climbing, time to leave becuase 5 minutes is kinda just like a long safety stop anyway. Next thing you know our intrepid diver is at the 20ft stop with a TTS of 5 and the 21/00 gas is a faint yellow color for whatever reason. Time and gas is burning, but the deco just won't clear. Eventually the gas is nearly exhausted and it's time to abort for the surface and skip all the deco. You get on the boat and conclude the computer said five minutes and I hung there for 10 actual minutes so I should be all set. No problems, right? PS - Don't forget to click "Confirm" on your computer to clear the Decompression Violation Error.

Decompression planning is often all about gas planning. If you don't understand the rules of the road for decompression diving, it's going to be increasingly difficult to have an adequate amount of gas for the dives in the long run. Eventually you're going to run out of something...

 

[Dr. Lecter]

Several times now I've witnessed Petrel divers program and make active a bunch of gases they do not have on the dive. Absent the academic knowledge, I guarantee you it will confuse you and worse you could easily run out of gas executing computer based deco dives and programming gases for which you do not have access.

Which is probably why the SW manuals beat readers over their ing heads with the importance of NEVER turning on a gas they don't have with them.

It's really easy to get lulled into the trap. You get a new fancy computer and setup 21/00, 32/00, 36/00, 50/00, and 99/100 because those seem like cool gases. You drop down to 130 ft in your recreational AL80 setup on air and see 5 minutes of deco and climbing, time to leave becuase 5 minutes is kinda just like a long safety stop anyway. Next thing you know our intrepid diver is at the 20ft stop with a TTS of 5 and the 21/00 gas is a faint yellow color for whatever reason. Time and gas a burning, but the deco has not decreased...until gas is nearly exhausted and it's time to abort for the surface and skip all the deco. You get on the boat and conclude the computer said five minutes and I hung there for 10 actual minutes so I should be all set. No problems, right?...

I disagree that it's really easy. I would say you have to really work hard to be that dumb. This is not a complicated question of deco theory or even something a self-taught deco diver could miss because they don't know what they don't know...this is just a question of RTFM for the computer you're taking with you.

Also, you won't be setting 99/100 or even 99/02 on a SW

 

[HIGHwing]

I disagree that it's really easy. I would say you have to really work to be that dumb.

Though we usually agree as a rule, I'm going to disagree with you here. Divers who buy the more advanced computers seem to love to setup gases for some reason. It's not a matter of intelligence; the divers do not realize there is an actual consequence attached to leaving a gas active that is not available. I've seen it done many times, and I believe it will continue to occur.

Bottom line is many divers fail to understand how the computer works and have no interest in further discovery because their open water instructor told them to blindly follow the computer - the computer knows all. The challenge we appear to have is people want to be "ready" for future dives and they program fancy gases without realizing the computer will consider the gases in the decompression profile.

 

[Dr. Lecter]

If they read the well-written manual for the fancy SW they just bought, they'd realize there is an actual consequence to leaving a gas active that is not available (and a bunch of other helpful things). Not reading the manual before using a dive computer, especially if flying the computer is the only form of deco planning you're doing, is unquestionably dumb.

And I'm fairly sure that even the laziest OW instructor/manual at least says to understand how the computer works before you follow it blindly.

 

[boulderjohn]

Hi Evryone

sorry but i am a certed deco diver. please can someone clairfy my understanding. i am not iterested in theese kind of comments. please answer.

Like many others, I am puzzled about the reason for some of the questions you have been asking. You say you are a certified deco diver, and your profile says you were certified by TDI. Do you have plans to continue to a higher certification level? TDI teaches a lot of classes in its decompression sequence. Can you tell at what level you are certified? Are you no longer in contact with your instructor(s)?

 

[Diver0001]

Decompression planning is often all about gas planning.

Hmmm... I see that differently. To my way of thinking decompression planning is just an ascent strategy. The gas plan is just a sanity check to make sure that what you intend to do can be done safely. The gas plan is never the main thing. The main thing is the ascent strategy.

At least the way I see it.

R..