2 Examples of Why You Should Always Anaylse Your Gas

[Fiona]

Originally Posted by Mal Bridgeman

Although the rationale is put forward that it stops me breathing the wrong gas at depth, I think it's also to avoid breathing expensive trimix on a 6m quarry dive

HTH
Mal

Arh yes multiple twinsets - who needs more than one

I can appreciate that it would be very different with trimix fills but some of us only use 32%

and yes it is 03.53 and some of us are daft enough to be going diving

[Digger]

Originally Posted by Clare Gledhill

Indeed. But lets have another sanity check shall we? The day's date on the label. What can this guard against? Simple really - old labels.

Imagine that you do a dive, lets say out of Weymouth harbour. You jump off the boat, straight in to Old Harbour Dive Center where you get a fill in a hurry for the next days diving. You analyse your gas but don't pull off your old label - you forget in the rush to pay the bill before the shop shuts for the night.

The next day you wake up and the weather has turned. You trudge home and shove the set in the shed in a bad mood. Two weeks later, you get a call to go diving - go out and check the shed.... you have a stickered up, full set. Woohoo - we're going diving.....

The addition of a label which is dated the day of the dive gets round this. Is it unlikely? Of course, there are lots of things we would have to forget to get into this position in the first place - take the sticker off before diving, take it off when being taken in for refilling, forget to write out a new lable when analysing etc.... but I do not under estimate my potential to be stupid.

The day I think I am infallible is the day I should give up diving

So here you miss my important step - analyse the gas before you leave the shop.

I also have a step where I remove any contents labels when it goes in for a fill. This also prevents bottle monkeys assuming anything and cocking up their calculations.

So if I miss both these steps, then get home and forget completely I got the tanks filled, carry on in that fashion until the next time I go diving, have the confidence that the incorrect label is right (despite my go and get it analysed if it's been a while) then I could end up diving the wrong gas.

Now there's infallible, and there's covering your arse in more directions than required. That said, if you wanted to analyse the gas before you put it in the car or got on the boat, I wouldn't see that as a bad thing, I just personally wouldn't need to. I'd know what was in there.

On top of that I very very rarely top anything off with anything any more. It's usually either a straight O2 fill, a straight air fill (diluent) or I've filled the bailout with something different, which doesn't really happen often. If ever now. I've come up with a strategy with the 4 bailout tanks I have which should cover me no matter what dive I am doing. So no need for changing what I use in which tank.

Digs.

[A. Berk]

Originally Posted by GLOC

Yep, I would have also thought doing a proper reg check (ie breathing the regs to make sure the valves are turned on properly) on the surface would have showed up this issue because his Diver 1 voice would have gone rather high

Also, if he'd taken 3 - 4 breaths the sound of his head bouncing off the deck would have been a fair give-away

[Scubee]

Originally Posted by nigelH

That's interesting...
I haven't seen it that way round....

apparently, i have it the wrong way round... he asked for 80%, it was analysed as 50%....about 3 times. rolling it around mixed it up.

[Steve S]

I analyze at the shop, and always put a new label on with the exact mix on. I always remove the old label before taking it in, and as my bottles are ''different'' there isn't much danger of a mix up with any others.

I would not except a fill from anywhere without checking it myself.

Safe diving,
Steve

[David Pye]

Originally Posted by Scubee

apparently, i have it the wrong way round... he asked for 80%, it was analysed as 50%....about 3 times. rolling it around mixed it up.

I might misunderstand Nigel, but I think what he's trying to say is:

rolling it around makes no difference as there's nothing in there to make rolling things actually mix the contents.

He's saying that because the debris tube/valve is full of air as that's what was put in last, if you leave it hissing for a bit, then it'll give you a true analysis. Surely that could explain the situation you mention, and that the rolling around just 'appeared' to do it?

[Scubee]

Originally Posted by David Pye

I might misunderstand Nigel, but I think what he's trying to say is:

rolling it around makes no difference as there's nothing in there to make rolling things actually mix the contents.

He's saying that because the debris tube/valve is full of air as that's what was put in last, if you leave it hissing for a bit, then it'll give you a true analysis. Surely that could explain the situation you mention, and that the rolling around just 'appeared' to do it?

Nope. i would agree with Nigel to a point, however, my (limited) knowledge of physics is that gas is a gas because the molecules are further apart than if it were a liquid or solid. This allows the molecules to move. jiggling the cylinder around gives the molecules a bit more momentum, allowing them to mix up a bit.

Logically, gas is squished into a cylinder so there isn't a great deal of room for movement. However, the amount of gas that's in the debris tube & valve is minimal, and, quite honestly, I doubt it makes that much of a difference as it will be the first out when the valve is opened before the analyser is introduced.

[Nick_lincoln]

Originally Posted by Scubee

...hopefully contributing when they need a new sensor!

This has to be one of my pet hates. If a diver can afford Nitrox training, can afford to pay for the gas, then s/he should be able to afford something to analyse the gas with.

.

Try working for the environment agency! No cash......... and I dont believe anyone is in a position to comment on anyone elses financial situation. As I said it isn't ideal but its on the list to buy. I dont see a problem with using a buddy's analyser in the interim period - especially if it is used to analyse the gas a second time (after the blender does it) as I always do. The sensor will get a bit of a bashing granted, but if it starts giving erratic readings (i.e. the cell is knackered) then it wont be used and I would get the gas re-analysed by the blender or another person who is diving with us. If neither is available - no dive.

[Scubee]

Originally Posted by Nick_lincoln

Try working for the environment agency! No cash......... and I dont believe anyone is in a position to comment on anyone elses financial situation.

Not a personal dig at all, and there was no comment about anybody's financial situation. If your buddy is happy with the situation, then fair enough. I have heard people say they will get one, but it isn't needed at the moment coz they can always use the one belonging to X. Who knows what financial hardship X may have gone through to protect him/herself from oxtox?

[Smiling Dragon]

Originally Posted by Scubee

Nope. i would agree with Nigel to a point, however, my (limited) knowledge of physics is that gas is a gas because the molecules are further apart than if it were a liquid or solid. This allows the molecules to move. jiggling the cylinder around gives the molecules a bit more momentum, allowing them to mix up a bit.

Logically, gas is squished into a cylinder so there isn't a great deal of room for movement. However, the amount of gas that's in the debris tube & valve is minimal, and, quite honestly, I doubt it makes that much of a difference as it will be the first out when the valve is opened before the analyser is introduced.

That sounds a bit off I think. The gas molecules are going to have a large amount of energy getting released as heat but there's still enough to keep it a gas rather than having it condense to a liquid. It's enough to mix itself just fine if you give it a minute.

Any gas trapped in a narrow confine in the valve mechanism would be far slower to mix as mixing molecules have a smaller area to pass through.

Shaking the cylinder would only add a fraction of energy to the molecules (if any). If the two gasses were of a different density you might be able to do that, or if there was something else of a different density (like a lump of metal or water) to act as a stirrer, but I'm pretty sure Nitrogen and Oxygen are the same density (or close enough that the natural activity of the molecules prevents them from seperating).

You could heat it up I suppose - that would get things moving inside - maybe not a killer plan though :/

Not a physics major so don't stake any lives on this or anything