When computers fail

[Se7en]

For rec diving, I believe a computer is a tool which extends the diving beyond what you could realistically calculate or plan for.

Most reef diving is continuous multilevel diving - my usually dive profile would be to drop down to somewhere between 30 and 40 metres, and work my way back up from there. For this - even if I was to calculate a multilevel profile with 1/2 dozen levels - or use the average depth - I CANNOT get the accuracy and granularity that my computer can. Therefore doing it manually, I significantly reduce my bottom time (some may say that this increases safety - but my fudge safety factor is keeping fit, not smoking, and using slow ascent rates - not padding an algorithm) over what I can get with my computer.

On a recent trip, I fell off the PADI tables on dive 2 - and stayed that way for the next 16 dives. However I dove quite conservative profiles for the whole trip and was no where near pushing the limits. Using the computer probably gave me an extra couple of hours of bottom time over the week over my best approximations with tables.
Great

As far as what to do if the computer craps out - I'd be very tempted to continue the dive (I'm talking rec dive here < 120 ft, no mandatory deco) based upon my buddies guages. I'd turn the dive with my buddy having enough air to get us both back to the surface with a couple of stops as a precaution - but I would probably not pull the pin on the spot if everything else is going OK.

I must add that of my last 70 or so dives, all bar 2 of them have been with one of two buddies. One of these has almost identical air consumption to me, the other uses about 5% more. So If I look at my buddies guage, it is as good as looking at my own.

Mike

 

[Lost Yooper]

Originally posted by Se7en
Before I start, a disclaimer: I'm not trying to argue against DIR. I believe it is the system which has had the most examination of any that people dive, and therefore represents the current high water mark of gear configuration. That doesn't mean however, that it is necessarily perfect...

Lost Yooper Mike

Doesn't this introduce a contradiction?

You (as an exponent of DIR) say that isolating a tank is practically unheard of - therefore two SPGs are NOT required for redundancy.

Yet DIR requires an isolation manifold - which sole purpose is to enable the isolation of a tank.

An isolator manifold also adds SIGNIFICANT complexity in terms of knowing which gas supply you are both breathing, and measuring the pressure of.

Surely on these grounds, the extra complexity of managing an isolation manifold, and the lack of requirements for one on the basis that tank O rings of burst valves practically never go, should indicate that the isolater is 'other stuff', that you don't need and shouldn't take?

Conversely, if you need the ability to isolate your tanks, why don't you need the ability to measure pressure in those tanks once isolated? After all, at this point you now have all the issues of managing independant doubles - with the added problem of not knowing how much gas you have in 1/2 of you system...


After all, even a free flowing first stage doesn't dump air that fast, and you have your buddy for backup...

Mike

The isolator valve doesn't pose a significant threat or risk to anything (I disagree with you there). Isolator valves don't fail all by themselves, so an isolator offers redundancy at virtually no risk. Maybe I don't understand what you mean by "complexity". They certainly aren't complex.

Assuming you had to isolate your tanks, the dive would be terminated, and you should be able to get up to your deco gas switches using your buddy and what's left in your tank you can breath off of.

Dealing with a free flowing first stage is easy to do and doesn't required isolating both tanks. Are we talking about two different scenarios here? I may be confused -- which doesn't take much ya know .

Mike

 

[Se7en]

Originally posted by Lost Yooper


The isolator valve doesn't pose a significant threat or risk to anything (I disagree with you there). Isolator valves don't fail all by themselves, so an isolator offers redundancy at virtually no risk. Maybe I don't understand what you mean by "complexity". They certainly aren't complex.

The 'complexity' I was referring too was the issue of getting fills with tank isolated, or diving with tanks isolated etc. So complexity of use, rather than complexity of construction. As mentioned earlier there IS some chance that someone stuffs with your gear at some point, and you don't pick it. (even a DM accidentally turning the isolator off as they grab your valve to 'assist' you into the water. I have had this happen with a single tank)

So it does pose a threat - probably a hypothetical rather than significant one, I'd agree.

What I was trying to do was compare the use of two SPG's with the use of isolating manifold.
Both of these are ONLY required in the event of a tank failure (ie O ring or burst disk).
If you need the isolator, then the second SPG becomes useful.

However DIR requires one and condems the other. Why?

The ramble about isolator use complexity was merely meant to compare to the complexity of a second SPG. Neither are particularly complex, but both pose some slight risk.

Dealing with a free flowing first stage is easy to do and doesn't required isolating both tanks. Are we talking about two different scenarios here? I may be confused -- which doesn't take muh ya know .

Mike

Nah - I expressed it badly and used the wrong example. What I meant to type was that a burst O ring doesn't loose air all that quickly - if diving thirds you'd have enough gas to make it home - with your buddy as redundancy (which we definately agree upon).

Mike

 

[Lost Yooper]

Yup.

Mike

 

[Uncle Pug]

Hi Mike,
The Iso is critical in the event of a tank o-ring or burst disk failure...
It is the only way you can save your gas in a least the one tank...

The extra SPG is not critical at all... it is superfluous....
When you isolate the dive is over....
It doesn't matter how much gas is in the remaining tank....
You are going to use whatever you need to to go home....
If you don't have enough...

1. you weren't diving thirds
2. an spg wouldn't get you more...
3. tough luck


On the topic of Isolators:

An easy way to make sure your Iso wasn't accidentally turned off before the fill or before the dive is to check your SPG every so often....

If it hasn't changed.... guess what!!!
Open it...

If it is changing faster that it should ..... guess what!!!
Head home....

Of course we all check our valves during the dive right?

 

[Se7en]

Aah.. I suffed up the reply syntax as well...

Mike

 

[Uncle Pug]

Oh shoot.... I see this thread was about computer failing and here I was posting stuff about Isolators and SPGs....

Sorry....

Ahhh...hmmm....

Yes, I agree.... computers can fail....
I have had it happen several times....
And when they do....
And you are relying on them....
Well, tough luck....

Does that qualify as an on topic post????

 

[Se7en]

Originally posted by Uncle Pug
Hi Mike,
The Iso is critical in the event of a tank o-ring or burst disk failure...
It is the only way you can save your gas in a least the one tank...

Hey Uncle Pug

I guess I was just thinking of what is more likely - dive shop monkey filling one tank only due to shut valve, or burst disk or O ring blowing underwater.
I've seen dive shops do all sorts of wonderful things to gear... like 280 bar in old steel 72's... but I've never seen a tank O ring go, and I've only seen burst disks go above water.

So it's more likely that you will experience a problem (such as diving with 40 bar in the rh tank and 200 bar in the left hand - would you notice your spg not mooving before you ran out of gas in the rh tank?) with a iso manifold, than without.

The consequences would likely be a bit different though... how fast is gas lost through an extruded O ring or burst disk?

Mike

 

[Uncle Pug]

Originally posted by Se7en


Hey Uncle Pug

I guess I was just thinking of what is more likely - dive shop monkey filling one tank only due to shut valve, or burst disk or O ring blowing underwater.
I've seen dive shops do all sorts of wonderful things to gear... like 280 bar in old steel 72's... but I've never seen a tank O ring go, and I've only seen burst disks go above water.

So it's more likely that you will experience a problem (such as diving with 40 bar in the rh tank and 200 bar in the left hand - would you notice your spg not mooving before you ran out of gas in the rh tank?) with a iso manifold, than without.

The consequences would likely be a bit different though... how fast is gas lost through an extruded O ring or burst disk?

Mike

Mike,
You let monkeys fill your tanks???

Seriously.... very seriously.... I have only once left my tank at an out-of-town shop to be filled while I went shopping at a nearby hardware store..... (never again)

The owner seemed knowledgeable and I should have taken that as a sign to be even more cautious....

I always check my valves before the dive.... my partner failed to notice that his iso had been shut (after the fill)....

He picked it up at the 5 minute check....

If you are diving doubles with an iso and letting monkeys fill your tanks then you are responsible to check them before you leave the shop! Simple check the iso... if it is open... good.... if it is closed and when you open it you hear air movement.... good... you caught it - now have them fill your tanks.

You do check your pressure and gas mixture before leaving the shop anyway.... right????

Now let me answer several other questions in you post:
With 40 bar in the rh tank and 200 bar in the lh tank and you don't pick up on the fact that your iso is closed because you are a sloppy diver.... all of a sudden you are ooa on the primary.... you put your backup in your mouth and breath while you check your gauge and open your isolator.... dive over - go home!

The consequences of an equipment failure such as burst o-ring or ruptured burst disk are taken care of by the iso....

the consequences of a double operator failure such as letting monkeys fill your tanks and then you not checking them is taken care of by turning the dive.... go home sloppy diver - go home!

Mike do you dive doubles???

BTW... monkeys don't get to touch my tanks much less fill them....
When I fill my tanks it is always through the right hand post with the left hand post on and the SPG monitoring the fill. Even if you don't have a fill station you can see to it that your monkey follows your protocols for filling...

By filling through the right post with the left post on it is impossible to get the senario you describe above.

Remember Mike,
Uncle loves you and only wants the best for you....

 

[100days-a-year]

Don't have an AI.If I did I'd have an SPG anyway.My logic being that over the years I've become accustomed to my turn pressures at certain depths.In the event of an AI failure the determining factors become depth,time ,tank pressure.Hey,isn't that what they tell you to watch in OW class.I would turn at 1000 psi(assuming I'm under 130' and AL80 tank SAC rate of ~.60cfm)after 2 hrs I'd be safe for another similar dive ;computer or no.This can be confirmed at any rec depth by a simple look at the tables or run it on the dive similator in your computer.A simple thinh to remember is the three pile system.Pile one stuff I must have to complete the dive plan.Pile 2 stuff that I'd be screwed without in an emergency.Pile three stuff I do't need to have even in an emergency.Take 1st 2 piles leave the rest in the car.