Why are AL80 tanks often refered to as 12L tanks (rather than 10L?)

[RainPilot]

I dive to a 50bar reserve so I'de only gain that, how do you get a 100bar gain?

On a 40m dive, my rock bottom gas on single backmount is 100 bar, so after 2 dives I would have 200 bar remaining in my 2 tanks. In sidemount, I will end the day with 50 bar in each tank so 100 total. This is measured at time of commencing ascent, ie 100 bar extra bottom time over the 2 dives.

If my buddy is in sidemount as well, then the gas planning gets even more friendly, assuming single failure planning (which I generally don't out of personal choice)

 

[Mike1967]

On a 40m dive, my rock bottom gas on single backmount is 100 bar, so after 2 dives I would have 200 bar remaining in my 2 tanks. In sidemount, I will end the day with 50 bar in each tank so 100 total. This is measured at time of commencing ascent, ie 100 bar extra bottom time over the 2 dives.

If my buddy is in sidemount as well, then the gas planning gets even more friendly, assuming single failure planning (which I generally don't out of personal choice)

Ah I get it. When I say 50bar reserve, that's back on the boat, so the bottom time would be more. But yeah, definitely some advantages to diving independent twins.
Cheers for the info

 

[almostDIR]

back to the original topic, I am currently reading the Mark Powell book (Technical diving -An Introduction) and he clearly refers 80cf alu tanks as "12L" multiple times.
It has to have something to do with the divers used to dive 12L steel stages in the past and transferred much later to the aluminium 80cf stages so that the "12 litres" sticked even when the real bottle size they currently use is just that 11.1 L.

Otherwise they would just call them 11L tanks. No one would round the number up to 12 when it is really only very slightly over 11... unless there has been a very common bottle size in the past for same use which was really 12L

 

[darksider]

I recently discovered that in Egypt they fill both tanks AL80 and AL100 to around 200 bars.

Based on the specification AL80 is ~11.1L @ 207 bar.
If we assume that the pressure is 200 bar, we get
10.72L @ 200 bar = 2144L of air.

AL100 is actually 13.2L @ 228 bar.
If we assume that the pressure is 200 bar, we get
11.57L @ 200 bar = 2314L of air.

This way the difference is only 170L which gives you extra ~15 bars compared to the AL80 tank.

Assuming your SAC is 20L/min, with an average depth of 15m it gives you additional ~3.5 minutes of diving.

So the benefits of using so-called "15s" are relatively small. Especially since they are extra paid and you have to carry 4kg more.

The funny thing is that they call them "15s", they are really 13.2L and because they are filled to 200 bars they are only 11.5L .

If my assumptions are wrong, feel free to correct me !

Reference:

 

[L13]

I recently discovered that in Egypt they fill both tanks AL80 and AL100 to around 200 bars.

Based on the specification AL80 is ~11.1L @ 207 bar.
If we assume that the pressure is 200 bar, we get
10.72L @ 200 bar = 2144L of air.

AL100 is actually 13.2L @ 228 bar.
If we assume that the pressure is 200 bar, we get
11.57L @ 200 bar = 2314L of air.

This way the difference is only 170L which gives you extra ~15 bars compared to the AL80 tank.

Assuming your SAC is 20L/min, with an average depth of 15m it gives you additional ~3.5 minutes of diving.

So the benefits of using so-called "15s" are relatively small. Especially since they are extra paid and you have to carry 4kg more.

The funny thing is that they call them "15s", they are really 13.2L and because they are filled to 200 bars they are only 11.5L .

If my assumptions are wrong, feel free to correct me !

Reference:
View attachment 783581

The 11.1L is 11.1L regardless of fill pressure. The In most of the world, the tank size is it's interior volume which does not change (significantly) with fill pressure. Same for the 13.2L, it is always 13.2L even when empty. and the 15L is 15L always.

In the USA and a few other places, tanks are rated by the gas contained at rated pressure, ie. cubic feet. To further confuse things, they are often rounded to the next "nice" number. Thus, a 11.1L tank is 77.4 cu.ft @3000 psi, but is called an "80 cu.ft". It would need to be filled to 3100 psi to actually contain 80 cu.ft of air, but would still have an interior volume of 11.1L.

 

[inquis]

To further confuse things, they are often rounded to the next "nice" number.

The ideal capacity of the AL80 is 80.1 cf, possibly giving a bit more validity to the AL80 moniker. Gas compressibility reduces the real capacity to 77.4 cuft.

 

[BlueTrin]

A staff in Egypt told me my AL80 tank was 12

I recently discovered that in Egypt they fill both tanks AL80 and AL100 to around 200 bars.

Based on the specification AL80 is ~11.1L @ 207 bar.
If we assume that the pressure is 200 bar, we get
10.72L @ 200 bar = 2144L of air.

AL100 is actually 13.2L @ 228 bar.
If we assume that the pressure is 200 bar, we get
11.57L @ 200 bar = 2314L of air.

This way the difference is only 170L which gives you extra ~15 bars compared to the AL80 tank.

Assuming your SAC is 20L/min, with an average depth of 15m it gives you additional ~3.5 minutes of diving.

So the benefits of using so-called "15s" are relatively small. Especially since they are extra paid and you have to carry 4kg more.

The funny thing is that they call them "15s", they are really 13.2L and because they are filled to 200 bars they are only 11.5L .

If my assumptions are wrong, feel free to correct me !

Reference:
View attachment 783581

I am in Egypt and the local dive centre is saying that the cylinders were 12l: I assume that they must be AL80 so 11.1l?

Where is it marked at the top that it is an al80?

Edit: since it was a luxfer I should see something like below where 80 is the designation for an AL80

LUXFER 10A05 SO80

 

[darksider]

A staff in Egypt told me my AL80 tank was 12

I am in Egypt and the local dive centre is saying that the cylinders were 12l: I assume that they must be AL80 so 11.1l?

Where is it marked at the top that it is an al80?

Edit: since it was a luxfer I should see something like below where 80 is the designation for an AL80

LUXFER 10A05 SO80

Yes, exactly.

 

[tursiops]

So I'm not the only one hearing 11.1L tanks called 12L - I wonder where this comes from... I guess it's because 10 / 12 /15L steel tanks are common, so aluminium 11.1L tanks somehow got upgraded to 12L?

You seem to be a bit obsessed with this.

 

[tursiops]

Based on the specification AL80 is ~11.1L @ 207 bar.
If we

Not really. The pressure is irrelevant. The AL80 has an internal volume in liters that does not depend on pressure.

The problem is the assumption that a tank will be filled to 200 bars. That does not quite fill an AL80 to its rated working pressure and comes nowhere close to filling an AL100.