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Resident 'Jawling Man' and 'Graunching Specialist'
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Mark Chase may have already answered this question for me on another post but here goes anyway.....

I want to move up to manifolded twins at the end of the year and I'm planning on doing normoxic trimix dives with them. For dives up to 60m and bottom times not exceeding 25 -30m will twin twelves be enough gas to supply ample reserves for me and my buddy? Or should I be going for 15's? My concern is 15's are going to be too big and heavy for routine use.

Also what is the thinking on manifolds - are the barrel ones essential (I think scubapro make them) or are the MDE open face ones ok? Is there a cost difference?

Thanks for any advice.
 

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A short fat well off crap cave diver. Likes wrecks
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Whats your SAC?

I plan dives on 15 sac for bottom gas. on a 50m dive that will get me 55mins on the bottom out of 200bar with 50 bar left (250 fill) to get me to first gas switch stop depth.

Yes I can get 250 fills no problem

So 50min ish max at 50m and 40mins ish at 65 so long as I am on staged deco

Thats OK for me as the deco gets starts to get silly after that.

If your SAC is more like 20 upward I would go for 15's. If you are considering doing regular 60+ jaunts I would consider 15's.any way. If you are considering long bottom times at all I would go streight to 10's for deco stages and travel. If your using a lift bag or bags a 10 for travel / bag filling is essential.

15's are big and heavy but if you can handle them then why not go for it. You would have to breath very heavy to justifie them at max 50m depth IMHO.

Ample reserves for you and your buddy?? Have you done the Maths?

OK you are going for 66% usage with 33 in reserve. Thats third out third back and third in reserve.

66% of 250 bar is 150bar in a set of twin 12's

150 bar at 60m on a 20 sac is 25min bottom time

66% of twin 15s is 35mins

Have you any idea how much mony you will waste using this system? Tmix isnt cheep twin 15's are expensive to fill and you will be blowing off 33% of your expensive gas after every dive. A typical 60m fill will set you back £50 and your going to throw away £16.00 every dive.

Why not use all your back gas and carry a redundent supply or bail out travel mix that can be used on every dive? If you filled a pony with 16/40 that would get you out of most situations and would last you a whole season if nothing hit the fan.

Just an opinion

Hope this helps

Mark Chase

PS
MDE manifolds are considered standerd kit in my little circle of divers so you should be alright with them.
 

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Just a quick note,

I agree with Mark for the above, but, you need to be aware, helium has a habit of slowly disapearing, it is such a light gas that it just finds it's way out of the tin. There is little chance that you will sill have a 16/40 mix by the end of three mons.

Andrew
 

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A short fat well off crap cave diver. Likes wrecks
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Just curious

How does it stay in the J's then??


I had herd this but I didnt think it was that fast. Does any one have any data on gass loss using helium?

Mark Chase
 

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Ah, good point, the only thing I can think of is the seals in J's for Helium are different.

Hmmmmmmm,

I shall have to make a call or two tomorrow.

Andrew
 

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<font color='#0000FF'>Hi Spike

I'd get 12's for general use and plan for thirds. If the dive doesn't work on thirds you need 15's (or 18's!!)

Sod the cost, your life (and that of your buddies) is worth more than £16


And the helium escaping from a cylinder ? Old wives tale  
 Divings full of them.

Andy

PS IMHO
 

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A short fat well off crap cave diver. Likes wrecks
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[b said:
Quote[/b] (wetlettuce @ Sep. 02 2003,23:24)]Sod the cost, your life (and that of your buddies) is worth more than £16
Lets see, I have done 38 Trimix dives this year. £16 per dive thats £608 in wasted gas.

Or spend £100 on a 3 ltr pony or another 7ltr stage which will last for years and £15. on a fill. and be just as safe if not safer than diving rule of thirds on a wreck dive.

ATB

Mark Chase
 

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[b said:
Quote[/b] (Mark Chase @ Sep. 02 2003,23:33)]Or spend £100 on a 3 ltr pony or another 7ltr stage which will last for years and £15. on a fill. and be just as safe if not safer than diving rule of thirds on a wreck dive.
<font color='#0000FF'>Hi

Then thats Spikes choice isn't it. I know which one I'll be doing




Andy
 

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<font color='#0000FF'>
[b said:
Quote[/b] (wetlettuce @ Sep. 02 2003,23:24)]I'd get 12's for general use and plan for thirds. If the dive doesn't work on thirds you need 15's (or 18's!!)
Andy,

You only need to use 'thirds' if you are planning a wreck penetration. If you must get back to the shot line then you would use 'halves' but on most of our type of non-penetration/drift deco wreck dives the back gas is 'all usable' (all with a sufficient reserve on top for two divers to reach either the first deco gas or the surface, in the case of a minimum deco dive).

Regards,

Mark.  
 

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<font color='#0000FF'>
[b said:
Quote[/b] (Mark Chase @ Sep. 02 2003,19:43)]OK you are going for 66% usage with 33 in reserve. Thats third out third back and third in reserve.
You should also calculate a 'rock bottom' reserve in addition to the three thirds.

So for a 30m dive with 250Bar in your tanks you reserve 50Bar then split your 200Bar into three thirds.

You need to be able to get two divers out of the overhead from the furthest point of penetration (the 1st third turn pressure) then upto the deco gas/surface).

HTH

Mark.
 

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As for the barrel vs face manifolds.....well I can assure you you'll get a million different responses.

The barrel is considered safer or more sturdy but you will hear it said time after time that the MDE manifolds are perfectly adequate.....as they are.

The Scubapro barrel (apparently as I've never used them) are very stiff and a shut down takes a good deal of effort....

I think we had a thread on this some time ago (twinsets and manifolds)....I'll see if I can find it for you and post it up.

Cheers for now.

Kinetic
 

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That always makes me laugh - the same people who say "DIN are the best cos they've got captive O-rings" then slag off MDE manifolds - which use a DIN connection!

I breathlessly await the barrel O-ring DIN fitting from Halcyon, for people who think facing O-rings are a bad system
 

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Dom,

I wasn't slagging either off.....just pointing out some of the views and opinions already expressed here and elsewhere.

I'm still looking for that thread.....

Cheese for now.

Kinetic
 

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[b said:
Quote[/b] ]I wasn't slagging either off.....
I know. But other people do, as you mentioned in passing..
 

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This was from a post started some time back called "MDE" if you do a search you'll find it.

This post I copied here came from Wetlettuce and he gives some useful links here....if the links don't work just copy and paste.

Hope this helps a little

Kinetic




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Joined: Dec. 2002  Posted: May 15 2003,10:01  

--------------------------------------------------------------------------------
Hi

You will find that people will always recommend what they have themselves and encourage you to do it their way with no explanation other than 'any other way is bollox'  

There are generally two types of manifold : Facing O ring and Barrel oring.

A facing oring type ( http://www.omsdive.com/valves.html#M-SPEC ) uses DIN type fittings ie screws the centre piece into an oring to provide the seal. MDE provide this type. One thing to note is that the seal is made by the facing oring and may be less tolerant to twisting. However if properly setup this shouldn't occur with normal use which is why many people will say they have never had a problem. MDE are a leicester based company with an excellent rep and you can buy and see these manifolds at Divers Warehouse, SDS and most other dive shops

A barrel oring type such as Scubapro, halcyon and more recently OMS ( http://www.omsdive.com/news.html ) uses a thinner piece with two orings forming a seal. These are preferred by cave divers where overheads form a collision hazard. Because they have two orings they are supposed to be more resistant to movement. SDS sell scubapro. One thing I have been told is that if carried by the isolator these may bend and therefore leak. Solution: Don't let anyone carry it by the manifold

Sadly, we can now expect  although I have tried to be objective and I didn't mention kkk once.  

Edited by wetlettuce on May 15 2003,10:03
 

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Resident 'Jawling Man' and 'Graunching Specialist'
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Discussion Starter #18
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Thanks for all the replies and the advice. Very useful stuff!

My resting sac rate (ie gentle pootle along the Kyarra) is 13l/min. However for gas calculations I use my stressed/working rate of 17l/min.
The way I would work out my reserves would be as follows: 5 minutes to stop depth X sac X max atmospheric pressure X 2
gives 5 x 17 x 7 x 2 = 1190 free litrer/24 litre tank is 50 bar + 20 bar reserve means my "up pressure" would be 70 bar minimum ( this obviously is for open water only - no overhead environement). this leaves 160 bar for the dive which gives me 30 min at 60m.

Any flaws in this? Not DIR I know but workable?
 

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<font color='#0000FF'>Hello,

I use twin 12's which easily give me 30min at 60m with rule of thirds, then if you go deeper you would stay less due to deco penalty. What I also considered was that with twin 15's you have pleanty of gas but also plenty to carry on back also he cost of which you have to drain more with twin 15's.

Mark,

Do you fill trimix to 250bar? I have read that mixes become unstable over 200bar according to a theory that Daltons law of partial pressure does not remain linear.

Regards

Pierre
 

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Resident 'Jawling Man' and 'Graunching Specialist'
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Discussion Starter #20
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I've heard that too - trimix fills become a bit inaccurate at higher pressures. iIs this true? It wouldn't be a problem if you have a helium analyser - anyone out there using one?
 
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