Tech expectations

[rob.mwpropane]

TBH, the reason I got my tech certs was to dive in comfort without having the NDL boogeyman chasing me. Getting to swim the whole wreck and really look at it is simply enjoyable.

The stuff I dive is mostly at 110'-120', with deep for me being 130'-160'. Love the bottom time.

I came here to write something and you already said it. Same same. Having to cut things short because of a clock sucks. So much freedom after NDL.

It can be done with an additional dive for "helitrox" which is (or was) up to 20% helium that isn't overly expensive to reduce the narcosis. OK, helium's much more expensive now.

Last time I checked helitrox was up to 35%. Almost a moot point with the cost though.

​

 

[Alekseolsen]

Just for the sake of "building a dive".
Diving with 28% and 50%, GF50/85
30 minutes at 40 meters gives a runtime of 62 minutes, using 20 liters RMV and you should breathe 3600 liters 28% and 850 liters 50% O2.

If you loose the deco gas, you are looking at 70 minutes runtime and 4600 liters consumption of the backgas.


So if you have a set of large doubles - you get quite a bit of bottomtime without hanging for crazy long

 

[Tribal]

So, if you you just want to dive deeper and with some "light" deco, there is no need to enter the technical diving world, you can do this staying within the recreational system (with the proper agency, of course).

Then again so many Cmas divers that do deco dives can’t hold their stops half descent. They hang vertically or they keep swimming/moving or they have to stay on a line…

Follow some intro to tech course, use a good instructor. The agency doesn’t matter aslong as the instructor cares about skills.

 

[Wibble]

Last time I checked helitrox was up to 35%. Almost a moot point with the cost though.

I think it changed since I did ANDP+Helitrox. From my 2016 cert card:

TDI TDI Helitrox Diver Certification Card
Certified to utilize Nitrox and Helium mixes with no greater than 20% He, and up to 100% Oxygen for decompression diving to a maximum depth of 45m/150ft only.

My deco gas of choice is 80% nitrox. Not as dangerous as 100% and easier to get a decent fill as it's 100% topped off with air (e.g. an ali80 of 80% would need 155bar of oxygen, then add 54bar of air). 80% gives you accelerated decompression which you can get on to at 9m/30ft and significantly shortens the dive without the challenges of running pure oxygen.

Example: Diving to 40m/130ft for 30 minutes on the bottom with 28% backgas, using GF50:80 with SAC/RMV of 20 litres per minute (which is higher, so more conservative planning):

• Using only 28% backgas:
  • runtime is 71 mins
  • using 3,237** 4,400 litres of 28%
• 28% backgas and 50% deco gas:
  • runtime is 58 mins
  • using 2,462 3,178 litres of 28% and 518 845 litres of 50%
• 28% backgas and 80% deco gas:
  • runtime is 52 mins
  • using 2,564 3314 litres of 28% and 302 499 litres of 80%

It's fun to play with MultiDeco/whatever! The above planned on MultiDeco. For some reason it was different to @Alekseolsen's result, so listed all three. (On courses and elsewhere, it's quite common to be comparing MultiDeco / other planner's settings to try to get the same results)


** edit: corrected wrong gas volumes (error as had left it on my personal default planning SAC of 15 bottom and 12 deco, not 20 & 20)

 

[Tracy]

I know that in some places where I dive there are good dive sites that the operators will not go to because they are not deep enough for the tech divers to get excited about but too deep for NDL divers to get decent bottom time. A certification that allows you to extend your time at sites like that would be ideal. As I wrote above, tech 40 does exactly that, but it is not promoted for that purpose so people do not realize it.

There is nothing that gives me more anxiety than running charters to 130' wrecks.
The tech divers blow it off and do stupid things, the recreational divers are the deepest they have ever been and panic.
I try to avoid them whenever possible.

 

[VikingDives]

I think it changed since I did ANDP+Helitrox. From my 2016 cert card:


My deco gas of choice is 80% nitrox. Not as dangerous as 100% and easier to get a decent fill as it's 100% topped off with air (e.g. an ali80 of 80% would need 155bar of oxygen, then add 54bar of air). 80% gives you accelerated decompression which you can get on to at 9m/30ft and significantly shortens the dive without the challenges of running pure oxygen.

Example: Diving to 40m/130ft for 30 minutes on the bottom with 28% backgas, using GF50:80 with SAC/RMV of 20 litres per minute (which is higher, so more conservative planning):

• Using only 28%backgas:
  • runtime is 71 mins
  • using 3,237 litres of 28%
• 28% backgas and 50%deco gas:
  • runtime is 58 mins
  • using 2,462 litres of 28% and 518 litres of 50%
• 28% backgas and 80%deco gas:
  • runtime is 52 mins
  • using 2,564 litres of 28% and 302 litres of 80%

It's fun to play with MultiDeco/whatever!

I don't understand why 80% isn't more popular. There's definitely an argument for safety in the ocean, and no disadvantage in run times.

I do realize that I'm summoning the ghost of GI 3 to show up and call me a stroke, but truly, I don't see the advantage of 100% over 80%, but maybe someone can explain it to me.

 

[rob.mwpropane]

I think it changed since I did ANDP+Helitrox. From my 2016 cert card:


My deco gas of choice is 80% nitrox. Not as dangerous as 100% and easier to get a decent fill as it's 100% topped off with air (e.g. an ali80 of 80% would need 155bar of oxygen, then add 54bar of air). 80% gives you accelerated decompression which you can get on to at 9m/30ft and significantly shortens the dive without the challenges of running pure oxygen.

Example: Diving to 40m/130ft for 30 minutes on the bottom with 28% backgas, using GF50:80 with SAC/RMV of 20 litres per minute (which is higher, so more conservative planning):

• Using only 28%backgas:
  • runtime is 71 mins
  • using 3,237 litres of 28%
• 28% backgas and 50%deco gas:
  • runtime is 58 mins
  • using 2,462 litres of 28% and 518 litres of 50%
• 28% backgas and 80%deco gas:
  • runtime is 52 mins
  • using 2,564 litres of 28% and 302 litres of 80%

It's fun to play with MultiDeco/whatever! The above planned on MultiDeco. For some reason it was different to @Alekseolsen's result, so listed all three. (On courses and elsewhere, it's quite common to be comparing MultiDeco / other planner's settings to try to get the same results)

I end up with either 70% or 80% depending what's left in the bottle... poor folks can't afford boosters yet, lol. I'm fine with either as I like the calmer 25' waters anyway.

There is nothing that gives me more anxiety than running charters to 130' wrecks.
The tech divers blow it off and do stupid things, the recreational divers are the deepest they have ever been and panic.
I try to avoid them whenever possible.

That's interesting. I think most people here dive within their limits. Last week there was 3 "newer" divers with single tank mixed in with doubles and rebreathers on a 130' wreck, everything went well, but they all seemed really humble about where they were at, asking questions and just being excited to be there. None of them pushed the envelope at all.

They were with a shop... I think he vets them a good bit before taking them out on a charter.

 

[Alekseolsen]

I think it changed since I did ANDP+Helitrox. From my 2016 cert card:


My deco gas of choice is 80% nitrox. Not as dangerous as 100% and easier to get a decent fill as it's 100% topped off with air (e.g. an ali80 of 80% would need 155bar of oxygen, then add 54bar of air). 80% gives you accelerated decompression which you can get on to at 9m/30ft and significantly shortens the dive without the challenges of running pure oxygen.

Example: Diving to 40m/130ft for 30 minutes on the bottom with 28% backgas, using GF50:80 with SAC/RMV of 20 litres per minute (which is higher, so more conservative planning):

• Using only 28%backgas:
  • runtime is 71 mins
  • using 3,237 litres of 28%
• 28% backgas and 50%deco gas:
  • runtime is 58 mins
  • using 2,462 litres of 28% and 518 litres of 50%
• 28% backgas and 80%deco gas:
  • runtime is 52 mins
  • using 2,564 litres of 28% and 302 litres of 80%

It's fun to play with MultiDeco/whatever! The above planned on MultiDeco. For some reason it was different to @Alekseolsen's result, so listed all three. (On courses and elsewhere, it's quite common to be comparing MultiDeco / other planner's settings to try to get the same results)

I likelybhave both some gas change time and possibly slow decent based on some shore-dives I do quite frequently, the gas numbers seems Odd though

Found it, my app "diveprome" calculates 4 min descend + 30 min at the bottom + ascend

5 ATM @20LPM × ~30 MIN = 3000 Liters before ascending.
If 4 minutes at 20 m avg --> 3×20x4 =240
5 atm ×20 lpm ×30 min --> 3240 --> litres before ascending

 

[Wibble]

I likelybhave both some gas change time and possibly slow decent based on some shore-dives I do quite frequently, the gas numbers seems Odd though

Found it, my app "diveprome" calculates 4 min descend + 30 min at the bottom + ascend

5 ATM @20LPM × ~30 MIN = 3000 Liters before ascending.
If 4 minutes at 20 m avg --> 3×20x4 =240
5 atm ×20 lpm ×30 min --> 3240 --> litres before ascending

Thanks for spotting that!

Fixed it — all volumes were wrong, had left the SAC on my planning defaults.

Excellent example of checking your figures.

 

[Bazzathemammoth]

Thanks for all the replies. I had a play with my peregrine deco planner last night after posting here and came out with fairly similar figures.

My rmv on my last two dives was 16l/m (active dive) and 14l/m (chill dive), so 20 L/m seems like a fairly conservative number to me. I’m not sure that is correct though. I’ve only ever used that much once when I’ve been tracking it and that was because of a very free flowing regulator.

I think I’ll get the rest of my rec certs (deep and rescue) locally, then travel again for the tec40 training. The tec40 instructor is the only one I know of that regularly does tech dives and really knows his stuff.

My favourite dive buddy that I’ve had is a cmas instructor and we have done a couple of cmas style deco dives. Unfortunately getting my cmas qualification isn’t an option and I’d like to dive “by the book” moving forward.