TDI Valve Drill sequence?

[dave archer]

Graham,

I do get the difference, and the fact you guys don't need the gas cos your buddy has it for you, but the bit I can't get over is....

2 seconds is such a short amount of time that you could isolate while your buddy tells you which side, so the isolating has costs you no time or next to no time, so why not do it?

If it took 15 or even 10 seconds to isolate I'd buy it completely, but 2 seconds just seems a very minimal price to pay to safe guard your gas, even if you don't absolutely need it...

Not criticising, far to novice at twins for that, just failing to understand...

Cheers,
Dave.

[ardhill]

Originally Posted by Brian of Aquanauts

But shutting an isolator doesn't actually preserve gas, the leak will still be constant, only closing the correct valve will preserve gas.

Brian, it depends on where the fault is.

You have 3 main fault zones:
1. Right post
2. Left post
3. Manifold

If the problem is in the manifold, then only by shutting it down can you save the gas in the cylinders as turning the posts off won't effect it.

There is also a psycological aspect of isolating first, but if you don't get stressed that it won't be an issue for you - it is for others though. But yes you will save gas if it is a manifold fault.
.

[Brian of Aquanauts]

Originally Posted by dave archer

Sorry, I'm new to diving twins, but nothing said so far explains to me why guaranteeing half my gas is not worth two seconds before starting to diagnose...

Because most loss of gas problems are 1st stage/reg related, the chance of it being a problem that can't be fixed by closing left or right post are low.... therefore closing the manifold in my opinion only wastes time.

Having the manifold only partially open could create more problems. When you are stressed what if you turn it the wrong way? How do you know 100% which way is the correct way to turn. I've done shit loads of dives and I hit my valves sweet every time in practice, I open and close cylinders every day when filling them and it's almost ingrained. But i've seen really experienced divers turn them the wrong way when they were being videod just out of nerves. So i don't know for sure that i'll turn them the right way when it's real. So manifold all the way open takes a bit more than 2 seconds to close. This also helps prevent it shutting accidentally.

Also if it is an unfixable problem and the manifold needs to be closed, you actually haven't made yourself any worse off as you still have the same amount of gas as if you had shut it first, just spread a tiny bit more between 2 cylinders. If you breath the broken side down you will have alot more than if you had isolated first and then breath good side only. However if you plan to isolate first and breath broken side, you still have the same amount of gas as if you didn't isolate first. If you've ever seen how much air pisses out of burst LP hose you'd understand why you'd want to shut that post off first.

So in my mind it makes sense to try to fix the problem first, as closing the isolator gives me no benefit and in fact in alot of situations will give me less gas too.

I haven't mentioned the GUE team reserve as not all non DIR divers would have that backup.

Best,

Brian

EDIT by unfixable i mean that closing the valve doesn't resolve the gas loss. If ever a valve is closed and that doesn't stop the bubbling my instant reaction is to isolate

[Scuttler]

I seem to remember a recent dive mag trying to ascertain what difference it made having the isolator fully open or two turns open...in terms of lost gas.

They deduced that the difference was not significant enough to advise people to dive with the isolator almost closed.

There have also been a number of valid points stating that someone could turn the valve the wrong way in a "panic" situation exacerbating the problem.

I have to say I was never really satisfied with the diving mags conclusions because it left so much technical detail out so I decided to investigate a little further.

Bear in mind this is on a hypothesis based on operating parameters. I haven't actually tried this!

Some of the leading second stages are capable of delivering a flow rate of 17l/s. The best first stages are capable of around 42l/s.

So.....assuming that sometimes it's the first stage and sometimes it's the second stage that fails we have an average of 29.5 l/s.

If we say the average diver has a SAC of roughly 22 l/s SAC then the difference between the two turn/1.5 sec isolation and the 10 turn/8 second isolation equates to just over 8 minutes.

That seems like a lot. However digging a bit further.....

The worst stage for this to occur would be on the ascent. If we're running rule of thirds then on a twin 12 rig we have just started the ascent with a maximum of 3540 L and a minimum of 2640 L [1]

So our rig pops and we have to isolate.

2 turns = 45 L lost
10 turns = 236 L lost

191L difference or less than 10%


OK.....all the two turn mob are yelling victory now BUT HANG ON.

This makes a lot of assumptions such as that the gas loss is uniform across both valves and that there is no restriction in flow as we shutdown. In reality however I suspect the the cylinder which has the problem on it will lose gas at a faster rate than the cylinder that is working (path of least resistance and all that) sooooooo....what can we conclude......need more scientific experiments on this.

[1] I'm assuming that we have 220bar and have started ascent on 1/3 or 1/2. No particular reason to choose that figure just it has to be less than 2/3 and I'm struggling with the maths tonight.

/Drunken post mode

[dave archer]

If I have isolater fully open (and I can see both sides of that one) then ok, isolating takes longer so "costs" you more.

And yep, if most failures can be solved and dive can continue then isolating hasn't gained me anything.

I get it

But if I'm solo, or diving a plan that says if post fails abort dive (e.g. me and/or buddy not happy about trying to fix underwater), then iso first makes more sense.

So it comes down to the fact isolator is not always dived 2 half turns open, and the As versus Bs approaches Mark listed.

I was just being thick

Cheers,
Dave.

[GLOC]

Originally Posted by dave archer

I do get the difference, and the fact you guys don't need the gas cos your buddy has it for you, but the bit I can't get over is....

Originally Posted by dave archer

I was just being thick

Cheers,
Dave.

Yep, as I am Gareth and not Graham

I am the good looking one, just see my Avatar for more details

[Scuttler]

Originally Posted by dave archer

or diving a plan that says if post fails abort dive (e.g. me and/or buddy not happy about trying to fix underwater), then iso first makes more sense.

Surely if a post fails EVERY dive is abort!!! You've lost your backup....all bets are off......unless you're in the bath!

[Mal Bridgeman]

Originally Posted by Scuttler

If we say the average diver has a SAC of roughly 22 l/s SAC then the difference between the two turn/1.5 sec isolation and the 10 turn/8 second isolation equates to just over 8 minutes.

/Drunken post mode

Have a look in the morning and rethink the 22l/s

[Scuttler]

Originally Posted by Mal Bridgeman

Have a look in the morning and rethink the 22l/s

I said I was a bit pished.....you know I mean 22L/m but the 1st and 2nd stage figures WERE l/s

I must spread the rep

[dave archer]

Originally Posted by Scuttler

Surely if a post fails EVERY dive is abort!!! You've lost your backup....all bets are off......unless you're in the bath!

Originally Posted by Mal Bridgeman

as I understand it the primary difference, other than sequence, is that GUE will attempt to fix a faulty 1st stage so the dive can continue, whereas the TDI method seemed aimed at training you to secure at least half your gas to perform your ascent.

If you reseat your first stage and all is fine, do you end the dive or go on...? Personally I end it, but maybe not everyone has that approach...