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Save 22Lbs. by switching from LP85 to FX71 or FX80? Please Check My Math & Understanding

Discussion in 'Sidemount Diving' started by 73diver, May 11, 2017.

  1. 73diver

    73diver Solo Diver

    # of Dives: 200 - 499
    Location: Milwaukee, Wisconsin
    107
    56
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    I dive sidemount off a boat. My preference is to kit up and walk off the back of the boat. With two Worthington LP85's and ballast, I'm supporting about 80#. I noticed that there is an FX71DVB cylinder made by Faber and marked BSE71 rated at 3442PSI. The catalog lists the FX71 as 25# while the LP85 is listed as 37#. The buoyancy empty and full compares equally. Of course there is less air in the 71's. However, it would appear that I could take 22# off my back with a tank change. It is approximately the same switching to FX80 (i.e. 20#). This seems too good to be true, I'm wondering if I have missed something. How about the fore-aft trim? I did dive a pair of M71DVB's (3300PSI marked BSM71) which are positively buoyant halfway through the dive. Thanks
     
  2. runsongas

    runsongas Great White

    # of Dives: 500 - 999
    Location: California - Bay Area
    4,173
    1,868
    113
    LP50 with an overfill is a better idea.

    The short HP tanks are fat and negative the whole dive. they don't ride well in sidemount as they are too short to reach rear drop rings or rails for most.

    if the 65cf with an overfilled lp50 isn't enough, look into lp72s. you will shave almost 16 pounds off with them although they top out around 87cf with an overfill at 3k.
     
    northernone likes this.
  3. DA Aquamaster

    DA Aquamaster Directional Toast ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: NC
    11,518
    1,705
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    I'll second the suggestion for steel 72s. They are light weight - less than an AL 80, they are not overly negative, going neutral to slightly positive at around 500 psi, the 6.9" diameter is low profile, and as noted above they have good capacity with a moderate over fill. They hold about 80 cu ft at 2800 psi and 86 cu ft at 3000 psi. I don't advise pushing them past 3000 psi, and I stay with 2800 psi, unless I actually need the extra 12 cu ft of gas from a pair of steel 72s. Trim varies a bit with brand of steel 72s, as some will be neutral when near empty while others will be positive and tail light like an AL 80. Most of the difference usually boils down to whether they are galvanized or not, as the cold dipped galvanizing added a couple pounds. It pays to get matching tanks.

    I also like the recent production Cyl-Tec aluminum 72s for dives where 140 cu ft is adequate. They are also light weight, the 6.9" diameter is again quite streamlined and they go neutral at about 500 psi.
     
  4. gfaith

    gfaith Solo Diver

    # of Dives: 100 - 199
    Location: Desert SE of NM
    444
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  5. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,927
    1,264
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    An LP85 should be 31 n ot 37# empty. 37# IS A GOOD WEIGHT FOR A FULL TANK. I cant imagine a tank that is 25# FULL.
     
  6. runsongas

    runsongas Great White

    # of Dives: 500 - 999
    Location: California - Bay Area
    4,173
    1,868
    113
    worthington lp85 is 36.8 empty if you include the valve and boot for the 7.25 version. the 7 inch version is closer to the faber at 34 pounds with the valve and boot.
     
    KWS likes this.
  7. decompression

    decompression Instructor...seriously...

    # of Dives: 5,000 - ∞
    Location: Victoria, BC, Canada
    4,126
    1,649
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    A small point, when I teach SM I make sure that students know that tank weight isn't part of your individual weighting requirements just because they aren't permanently attached to you like BM.
    So of course this matters mainly in water, so if you get a capacity that works for you, the tanks are neutral (ish) when empty and are considerably lighter on the surface, sounds great.

    Locally, there is no such thing as cave fills, so you get what you get and you don't get upset. So, HP100's are the go to tanks and to a lesser extent, LP 85's.
     
  8. 73diver

    73diver Solo Diver

    # of Dives: 200 - 499
    Location: Milwaukee, Wisconsin
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    56
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    Thanks for the comments everyone. When I want "real punishment", I sidemount with a pair of Worthington HP120's. With those there is no swinging one tank out in front and I notice the length of the tanks when doing a frog kick.

    BTW for RunsOnGas, in 1973 I learned to dive in the Bay Area at Dave Olson's Olympic Scuba School, Walnut Creek. Open water dives were at Cannery Row.
     

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