Right I have a question... I am wanting to make use of access that I have to a 300 bar compressor and so am looking at getting hold of a 300 bar set of twin 12s.

I have just heard that due to the nature of gas compression you really don't get that much more bang for your buck above 270 bar, that it isn't a linear thing ie you don't get a usable 7000+ litres of gas...

I'm a bit lost with this to be honest and had always assumed it was just a case of more pressure equals more usable gas in a lockstep progression.

So if that is the case I'll just keep using the doubles I have at 250-260 bar and not shell out more coin for diminished returns..

Opinions anyone?

Its true. What we are talking about is the differance between ideal gas laws and actual gas laws which tend to not match up above 220 bar. We could get into the safety of over filling 232 bar tanks to 260 bar. But the other thing you need to consider is the extra weight of the tanks and gas. Depending on your set up this may work in your favor.

Aye true but with the + hydro, very well looked after less than 2 yr old steels and a 300 bar manifold I think the risks are low.. and as long as only filled to 250, legal..

Well learnt something new today! weight wouldn't be an issue but as you concur on the diminishing returns then it's all beside the point as I no longer see a point in getting 300 bar tanks.

what sort of pressures can the carbon fibre cylinders take?

They're 300 bar...

But alas they're not rated for use with scuba whatever that means... probably just the valve would need to be changed to something in brass..

There are actually about 80 or so that I know of that are 6 or maybe 7 litre scott CF cylinders all new and all just sitting there due to a cock up in ordering.. Might have to see if they'll tender them, would have to find out how much lead would be needed with em of course say about 30 kgs!!

Not sure how your get 250 bar legal.

The tanks are rated at 232 bar. As I understand it ( and I may be wrong) there is a 10% allowance for temperature. So you may fill up to 255 bar with the tanks being hot. But when they cool they should be at 232 bar @ 15degC.

Now you could say OK I will fill them cold to 255 bar. Problem then is storing/transporting them and they heat up in the car. Pressure rises to 280 bar+.

Now the question is since you seem happy to over fill 232 bar tanks are you just as happy to over fill 300 bar tanks? If you are then you should get about 20% more bottom time which is a fair bit.

Back to weight. If you currently need weight to get neutral then going to 300 bar tanks shouldnt upset things to much. You may have to drop 3 or 4 kgs from your belt. But you dont want to end up over weighted. For me a switch to 300 bar tanks would mean I would be close to perfectly weighted with not extra lead.
Remember that even though these are steel tanks you do needto compensate for the mass of air lost during the dive.

The real question should be how much gas do you need for the dive.

Yeah eventually.. but as we are not talking about a specific dive here it's a little soon.....

Packie there's nothing in any notes I have that ties in the 10% to only being allowable for the initial "hot" fill.. I think you may have been told that by a dive shop...

Modern 232 bar steel tanks are designed to be able to withstand 2000 cycles of 350 bar.

Does anyone have the 10% thing in print? (Oh and in print that doesn't include the words "PADI" "resort" "5 star" "ask our friendly staff" etc..)

So where do you get your 10% from then? Temperature must be taken in to consideration or you end up getting more preesure on a hot day.

Example 232 bar at 15deg C =268 bar at 60deg C
250 bar @15 deg = 289 bar at 60 deg C.

I believe a car interior can easily get to 60c in summer.

Sure the tank can handle it. And there shouldnt be any problem with a 232 bar manifold as these are just as strong as a 300 bar manifold but the burst discs are another story. (easy fix though).

I know in the USA there is a + hydro that allows 10% extra on the fill but there is no such system operating in NZ that I know of.

The burst disc will go before the tank does...if you're worrying about the tank blowing up. One of the reason that dive bottles are filled to 232 is related to the valve, which has a lower rating than the tank. I remember the dive centre not filling my tank to 180 bar because I had a unrated valve on it.....hmmmph....I topped it up to 250bar... and didn't die!

OK so the t232 bar tanks can handle 350 bar. The manifold and burst discs are not an issue.

Why not just pump them up to 300 bar and forget about new tanks?

Cos that'd be more than 10%

You're pretty spoilt for choice really, Faber have it stated in their spec sheets among many others but still no reference to this only being to compensate for cooling post fill..

I guess as with anything it's up to you to ensure you store and transport in a safe manner. Yet ironically there is no law regarding tanks in the passenger comparment with you (battering rams) and you can happily legally get away with a 60kg LPG tank on the passenger seat of a 70 deg car, the ullage is supposed to stop them going fizz but... Go figure.