I have both HP 100's and Steel 72's set up as doubles.
They both have the same internal volume and consequently the same tank factor (ie: how many cu ft they hold per 100 psi., which we'll over later)
Trim wise I find little difference between the HP 100's and the steel 72's but the qualifyer is that steel 72 vary a bit between manufacturer and some are heavier and trim differently than others. Heights and shoulder profiles also vary so they can be a challenge to match.
Volume wise, a steel 72 overfilled about 400 psi to 2670 psi holds as much as an AL 80 at 3000 psi. More importantly, an AL 80 filled to 2800 psi (a common hot filled AL 80 pressure) holds 71.8 cu ft. - almost identical to the 71.2 that a steel 72 holds at the plus rated pressure of 2475 psi. In short a plus filled steel 72 holds the same gas as a slightly short filled AL 80 and there is just not that much difference between them.
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For technical diving gas matching becomes important.
To determine a tank factor, take the volume of the tank divided by the pressure and then multiply it by 100. For an AL 80 that holds 77 cu ft at 3000 psi: 77/3000= .02567x100= 2.57 (rounded to a tank factor of 2.6)
For a steel 72 this works out to 2.88 cu ft per 100 psi and a rounded tank factor of 2.9. If you double the tanks you also double the tank factors so a set of double AL 80's would have a tank factor of 5.2 while a set of steel 72's would have a tank factor of 5.8. The tank factor just makes the mental math easier when gas matching.
For overhead diving it is common to plan to turn the dive on thirds (in some cases you'd turn more conservatively on 1/4's or 1/6ths) and to do this with different sized tanks in the buddy team you need to consider the smallest set of tanks in the group.
If you had 2400 psi in the steel 72, 1/3 is 800 psi. If you take 8 times 5.8 you get 46 cu ft.
In the larger double Al 80's, you take the same 46 cu ft and divide it by the tank factor of 5.2 to get 885 psi. You'd add some conservatism by rounding that down to 800 psi to get your turn pressure. However, mentally it is easier to figure it backwards by trying 8 or 9, etc times 5.2. For example: 9x5=45 plus 9x.2 = 1.8, 46.8 total (a bit more than 46) and 8 times 5.2, 8x5=40 plus 8x.2=1.6, 41.6 total (a bit less than 46). In practice 9 is ball park close enough as the missing 15 psi is going to get lost in the noise of SPG accuracy anyway.
So...the Al 80's start at 3000 psi and the diver will turn at 2100 psi (or 2200 psi if you round down to 800) giving up only 100 psi compared to an actual third in the AL 80. Which makes the point that the difference between an Al 80 and a Steel 72 is pretty minimal even when doubled -as long as the Steel 72 is not short filled.
The difference gets bigger with larger differences in tank volumes.
For example, an HP 100 holds 100 cu ft at 3442 psi compared to the steel 72's 71.2 cu ft at 2475 psi. Both have the same tank factor of 5.8 when doubled but the starting pressures differ greatly. If the Steel 72s are filled to 2400, you'd have the same 46 cu ft third which would be about 800 psi in the HP 100's (roughly 6x8=48 - easy to do in your head floating on the surface prior to the dive). So if your HP 100's are filled to 3600 psi (a common fill pressure for HP 100's) you'd be giving up 400 psi compared to your normal 1/3rd turn pressure (turn at 2800 psi rather than 2400 psi).
If the steel 72's were filled to only 2200 psi you'd round down to 2100 (first 100 psi increment evenly divisible by 3) and figure a 700 psi third. 7 times the 5.8 tank factor is 40.6 cu ft, and again 40 divided by the HP 100 tank factor of 5.8 in your head is about 700 psi (actually 700 exactly if you work out the math) so the diver with the larger double 100 cu ft tanks filled to 3600 psi would turn at 2900 psi rather than 2400 psi, giving up 500 psi and almost 30 cu ft. compared to the normal 2400 psi turn pressure. A third diver with double AL 80's filled to 3000 psi would use the same 40.6 cu ft third and divide it by his tank factor of 5.2 to get a turn pressure 700 psi below the fill pressure (since 8x5.2 is 1.6 cu ft too big of a third). In this case the double AL 80 diver will be giving up 300 psi and about 15 cu ft compared to the normal 1/3rd turn pressure. In the extreme, if a 4th diver had LP 104's (tank factor of 8.0 when doubled) cave filled to 3600 psi (288 cu ft of gas), he would still turn on the same 40.6 cu ft "third" and would turn at 3100 psi rather than 2400 psi and would be giving up 700 psi and 56 cu ft. compared to his normal third.
The idea is that if everyone figures thirds based on the smallest tank, everyone will turn on the same volume of gas and, if needed, could get out of the overhead breathing the reserve third in those smallest tanks. The bad news is that the guys with the larger tanks don't see any advantage to having them. In the real world, diving the same sized tanks with buddies with similar SAC rates allows everyone to take full advantage of larger tank volumes - but it is important to note this poentially has the effect of actually lowering the overall reserve of gas in a multi member team.
So in effect, it is often peer pressure that drives the selection of a larger tank, since divers with smaller tanks tend to be less popular with the divers hauling around larger tanks, particularly if the goal is longer dives or penetrations.
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Steel 72's really suck as stage bottles - they just do not ride as well as an AL 80.
I've honestly got NO clue what I've got in this 72...It's old, original hydro is '73, and there aren't any markings that give away who made it (at least not to me, but I don't know what most markings mean anyway..)
