Weighting Impact of AL to Steel

[kurisuji]

Hi All — Question on weight adjustments when switching tank types:

When I switch from an AL80 to a HP100, I’m able to drop 6 lbs based on weight checks at end of dives.

However looking at the buoyancy characteristics of the two tanks, there is about an 11 lb swing in buoyancy at empty: +4 for AL80 vs -7 for HP100. If I never did the buoyancy checks, my initial assumption would have been that I could drop about 11 lbs, however the reality is that I can only drop around 6.

How do I explain this discrepancy? Im sure there’s a reason, but it’s not coming to me. If it was just a small difference it would not hurt my head, but with a nearly 2x difference I must be missing conceptually.

Thanks for any insights!

 

[Manatee Diver]

You are looking at the wrong number. You want enough weight to be neutral at the end of the dive, ie when the tank is empty. As such when calculating starting weight when changing tank types, you need to compare the empty weights.

+4 is about the empty weight of an AL80, but -7 is about the full weight of most HP100s (Faber HP100s). The empty weight of my HP100s is -1 lbs. So removing 5-6lbs compared to your AL80 weighting is correct.

 

[RyanT]

I'm not sure what data you're looking at, but a typical steel 100 is around -0.5 lb empty. Spun steel tanks, like the Ashai brand are around -6 lb empty. Some older steel tanks also have some different buoyancy characteristics from the modern steel Fibers. As you discovered, this is the value of doing weight checks.

 

[kurisuji]

@Manatee Diver @RyanT : Thanks for the replies! Yeah I need to poke around a little more ... the chart I'm using from "Huron Scuba Adventures" ( shows a Faber 100 at -14.76 lbs full and -7.26 lbs empty ..... hmmm ....

 

[kurisuji]

Wow ... just looked at the XSScuba website ... yeah the numbers I had are way off!

 

[kurisuji]

Okay mental crisis averted. Using the correct numbers from XS Scuba's website for AL80 (+3.4 lbs @ 500 psi) and HP100 (-0.59 lbs @ empty + 1.16 lb for the remaining weight of air at 500 psi = -1.75 lb total) ... gives a difference of 5.15 lbs ... which is pretty much dead on to my tested change of 6 lbs.

Thanks for your quick replies and for flagging the crazy number I had

 

[Bob DBF]

@Manatee Diver @RyanT : Thanks for the replies! Yeah I need to poke around a little more ... the chart I'm using from "Huron Scuba Adventures" ( shows a Faber 100 at -14.76 lbs full and -7.26 lbs empty ..... hmmm ....

That Faber sounds like an MP (mid pressure ) 100. The MP series is a 3AA tank with a service pressure of 3180# and with a + it's 3498#, that's what makes them heavy.

I like the MP series because I dive wet in cold water.

 

[rjack321]

@Manatee Diver @RyanT : Thanks for the replies! Yeah I need to poke around a little more ... the chart I'm using from "Huron Scuba Adventures" ( shows a Faber 100 at -14.76 lbs full and -7.26 lbs empty ..... hmmm ....

There are 2 types of Faber "hp" steels
3AA 3180psi+ and 3442psi exemption. The former are much heavier

 

[scubadada]

Hi @kurisuji

I lose 4 lbs when I switch from an AL80 to a HP100 (3442 psi)

 

[Scuba-74]

6lb is the right number. I dove NC this July with HP100 and Coz last week with AL80 in very similar configuration, I added 8lb in Coz. I could have gone with +6lb, but wanted to be slightly negative to adjust for currents,