Bottom stage buoyancy equivalencies

Joe Hesketh · January 5th, 2007, 11:37 AM

Right…geek out time again.

I was musing about multiple stage bottles and the positive stream lining benefits of helium in pushing an outside heavier bottle up out of the way, etc. It occurred to me that for training/practice purposes you could only replicate this effect with a nitrox stage if it were emptier.

Obvious so far, but then I thought, just how empty? And what difference does varying helium content make? And how do you compare air pressure in fresh water to a helium-based dive in the sea? So I ran some numbers. Probably far too many numbers as it turned out, but what I came up with was a series of look-up charts so you can find an air pressure in a Luxfer 80 that gives theoretical equivalent buoyancy characteristics to a stage of any of the four deeper standard GUE trimix gases. There are also four sets of numbers that cover conversions between salinities (fresh-fresh, fresh-salt, salt-fresh and salt-salt).

So, if you’re having a plop in your local fresh water quarry and want (without helium) to accurately synthesise the buoyancy a full 15/55 stage would have in salt water, now you can. There are also figures for air and EAN32 but it makes very little difference.

The tables also give the buoyancy figure for each pair of equivalent pressures (negative numbers in red parentheses indicate negative buoyancy) - note this is minus pillar valve or regulator but for conversion purposes this makes no odds as the extra mass is the same in each case.

Some of the results are interesting, particularly the trimix (fresh) to air (salt) where, for a given buoyancy increase, the air pressure starts out lower but crosses over and ends up higher than the trimix pressure.

Attached is the pdf (usual caveats apply), which hopefully someone may find either interesting or useful or both. If anyone wants the spreadsheet with the workings too, PM me.

Joe

[EDIT - now includes figures for 50/25 vs. EAN50]
[EDIT - high pressure version for Hoffi - warning kids: don't fill AL80s to 250 bar!!]

**neilh** · January 5th, 2007, 11:42 AM

Fascinating stuff Joe - you have far too much free time on your hands

Anders Knudsen · January 5th, 2007, 01:17 PM

Thanks a lot – just what I was looking for – amassing how much it helps to add just a little helium.
Is the buoyancy in kg is without valve and regulator?
If it seriously becomes 2 kg positive when empty carrying three would make you 6kg lighter than diving without stages would be very difficult to use the same amount of weight without being over/under weighted.

Joe Hesketh · January 5th, 2007, 02:02 PM

Quote: (Originally Posted by Anders Knudsen)

Thanks a lot – just what I was looking for – amassing how much it helps to add just a little helium.
Is the buoyancy in kg is without valve and regulator?
If it seriously becomes 2 kg positive when empty carrying three would make you 6kg lighter than diving without stages would be very difficult to use the same amount of weight without being over/under weighted.

Yes, it's just a theoretical bouyancy based on bare tanks (no reg or valve, etc). However, that doesn't affect the equivalency conversion.

I've updated the file to include more accurate density figures (makes naff all difference though) plus another set of figures with EAN32 equivalents.

*David Martin* · January 5th, 2007, 03:15 PM

Interesting.

An extension that would interest me would be Tx 50/25 versus Nx50 in both an AL80 and also the old style 7s, since it's often said that adding helium to the 21m bottle is a good thing from a buoyancy/bottle-trim perspective.

David

Joe Hesketh · January 5th, 2007, 03:58 PM

Quote: (Originally Posted by David Martin)

Interesting.

An extension that would interest me would be Tx 50/25 versus Nx50 in both an AL80 and also the old style 7s, since it's often said that adding helium to the 21m bottle is a good thing from a buoyancy/bottle-trim perspective.

David

OK - file now updated.

The numbers seem bear out the practice - a full 50/25 equals a nitrox only stage with 160 bar in, which is about the point an AL80 becomes noticeably un-negative (though still not neutral as such). I've included figures for a 7L but it makes no difference to the pressure comparisons (though obviously the buoyancy figures are different).

Joe

lamont · January 6th, 2007, 12:08 AM

Isn't this as easy as just taking off the helium %age from the service pressure.

So, over in the states, 1950 psi of EAN32 ~= 3000 psi of 21/35:

(1.00 - .35) * 3000 = 1950

You can go the extra mile to treat N2 and O2 differently and correct for the weight of helium, but doesn't that work closely enough?

Joe Hesketh · January 10th, 2007, 04:50 PM

Quote: (Originally Posted by lamont)

Isn't this as easy as just taking off the helium %age from the service pressure.

So, over in the states, 1950 psi of EAN32 ~= 3000 psi of 21/35:

(1.00 - .35) * 3000 = 1950

You can go the extra mile to treat N2 and O2 differently and correct for the weight of helium, but doesn't that work closely enough?

Hi

Sorry for the delay - forgot to reply to this. I guess your method would be approximate enough, especially if you’re dealing with same salinity conversions. Trying to guess between these and/or when using higher helium percentages then the errors get larger. For instance with same-salinity and converting from 200 bar of 21/35 the equivalent air pressure difference is 11 bar when using your shortcut, rising to 20 bar for 10/70. Where you’re crossing salinities it doesn’t really work at all and you get some huge differences.

Joe

Mats Edenfeldt · January 10th, 2007, 09:19 PM

Great job. It would be interesting if it was possible to get the figures for the actual bottle with valve and regulator and rigging. For the 7 liter it looks about right, i think if i am not mistaken that my 7 liters starts to lift at about 130 bar when i use a 50%, but i rarely do just because it’s not comfortable with the 7 lit hanging under you the whole dive. I don’t remember when it starts to lift with a 50/25 but it is way earlier.

Is it that hard to get the exact figure? Just ad the weight of the typical rigging, what could that be…1-1,2 kg?

Joe Hesketh · January 11th, 2007, 11:29 AM

Quote: (Originally Posted by edenfeldt)

Is it that hard to get the exact figure? Just ad the weight of the typical rigging, what could that be…1-1,2 kg?

Yes, that would be possible, and the buoyancy figures would of course change by that amount. The point of the comparison though wasn’t really to give the buoyancy figures, it was to compare the equivalent pressures between different gases, and there the weight of the rigging is irrelevant. It doesn’t matter whether the stage has no extra weight or lots, the pressure differential will be the same in each case.

January 5th, 2007, 01:17 PM	#3 (permalink)
Anders Knudsen(Offline) Gadevang Underwater Explorers Join Date: Dec 2005 Location: Denmark Posts: 179	Thanks a lot – just what I was looking for – amassing how much it helps to add just a little helium. Is the buoyancy in kg is without valve and regulator? If it seriously becomes 2 kg positive when empty carrying three would make you 6kg lighter than diving without stages would be very difficult to use the same amount of weight without being over/under weighted. __________________ If someone standing on dry land tells you they're safe diving deep on air, give them a wide berth. If they're that stupid at 1 ATA, you know they won't get any smarter at depth

January 10th, 2007, 09:19 PM	#9 (permalink)
Mats Edenfeldt(Offline) Cave is fun :) Join Date: Dec 2005 Location: Sweden - Stockholm Posts: 157	Great job. It would be interesting if it was possible to get the figures for the actual bottle with valve and regulator and rigging. For the 7 liter it looks about right, i think if i am not mistaken that my 7 liters starts to lift at about 130 bar when i use a 50%, but i rarely do just because it’s not comfortable with the 7 lit hanging under you the whole dive. I don’t remember when it starts to lift with a 50/25 but it is way earlier. Is it that hard to get the exact figure? Just ad the weight of the typical rigging, what could that be…1-1,2 kg? __________________ GUE: DIR-Fundamentals, Cave 1, Cave 2 NAUI: Trimix 1, Technical diver, Instructor, DM, Wreck penetration, Oxygen service technician IANTD: Advanced Nitrox, Nitrox Various: Apeks tech, Poseidon tech "Doing it right really means to do it properly. No skimping and no compromises. It sounds easy enough but it is not - it requires balls of steel" Peter Steinhoff

January 5th, 2007, 11:42 AM	#2 (permalink)
neilh(Offline) Haemoglobin on the bus... Join Date: Nov 2005 Location: Barnsley, UK Posts: 1,900	Fascinating stuff Joe - you have far too much free time on your hands

January 5th, 2007, 02:02 PM	#4 (permalink)
Joe Hesketh(Offline) New Member Join Date: Nov 2005 Location: London, UK Posts: 657	Quote: (Originally Posted by Anders Knudsen) Thanks a lot – just what I was looking for – amassing how much it helps to add just a little helium. Is the buoyancy in kg is without valve and regulator? If it seriously becomes 2 kg positive when empty carrying three would make you 6kg lighter than diving without stages would be very difficult to use the same amount of weight without being over/under weighted. Yes, it's just a theoretical bouyancy based on bare tanks (no reg or valve, etc). However, that doesn't affect the equivalency conversion. I've updated the file to include more accurate density figures (makes naff all difference though) plus another set of figures with EAN32 equivalents.

January 5th, 2007, 03:15 PM	#5 (permalink)
David Martin(Offline) Instructor candidate Join Date: Jan 2006 Location: London, E11 Posts: 419	Interesting. An extension that would interest me would be Tx 50/25 versus Nx50 in both an AL80 and also the old style 7s, since it's often said that adding helium to the 21m bottle is a good thing from a buoyancy/bottle-trim perspective. David

January 5th, 2007, 03:58 PM	#6 (permalink)
Joe Hesketh(Offline) New Member Join Date: Nov 2005 Location: London, UK Posts: 657	Quote: (Originally Posted by David Martin) Interesting. An extension that would interest me would be Tx 50/25 versus Nx50 in both an AL80 and also the old style 7s, since it's often said that adding helium to the 21m bottle is a good thing from a buoyancy/bottle-trim perspective. David OK - file now updated. The numbers seem bear out the practice - a full 50/25 equals a nitrox only stage with 160 bar in, which is about the point an AL80 becomes noticeably un-negative (though still not neutral as such). I've included figures for a 7L but it makes no difference to the pressure comparisons (though obviously the buoyancy figures are different). Joe

January 6th, 2007, 12:08 AM	#7 (permalink)
lamont(Offline) APLP Join Date: Nov 2005 Location: Seattle, WA Posts: 181	Isn't this as easy as just taking off the helium %age from the service pressure. So, over in the states, 1950 psi of EAN32 ~= 3000 psi of 21/35: (1.00 - .35) * 3000 = 1950 You can go the extra mile to treat N2 and O2 differently and correct for the weight of helium, but doesn't that work closely enough?

January 10th, 2007, 04:50 PM	#8 (permalink)
Joe Hesketh(Offline) New Member Join Date: Nov 2005 Location: London, UK Posts: 657	Quote: (Originally Posted by lamont) Isn't this as easy as just taking off the helium %age from the service pressure. So, over in the states, 1950 psi of EAN32 ~= 3000 psi of 21/35: (1.00 - .35) * 3000 = 1950 You can go the extra mile to treat N2 and O2 differently and correct for the weight of helium, but doesn't that work closely enough? Hi Sorry for the delay - forgot to reply to this. I guess your method would be approximate enough, especially if you’re dealing with same salinity conversions. Trying to guess between these and/or when using higher helium percentages then the errors get larger. For instance with same-salinity and converting from 200 bar of 21/35 the equivalent air pressure difference is 11 bar when using your shortcut, rising to 20 bar for 10/70. Where you’re crossing salinities it doesn’t really work at all and you get some huge differences. Joe

January 11th, 2007, 11:29 AM	#10 (permalink)
Joe Hesketh(Offline) New Member Join Date: Nov 2005 Location: London, UK Posts: 657	Quote: (Originally Posted by edenfeldt) Is it that hard to get the exact figure? Just ad the weight of the typical rigging, what could that be…1-1,2 kg? Yes, that would be possible, and the buoyancy figures would of course change by that amount. The point of the comparison though wasn’t really to give the buoyancy figures, it was to compare the equivalent pressures between different gases, and there the weight of the rigging is irrelevant. It doesn’t matter whether the stage has no extra weight or lots, the pressure differential will be the same in each case.