AL100 to HP120... weight loss??

[BWDscott]

OK...

I'm not grasping this, for some reason.

I need someone to explain this to me V E R Y S L O W L Y!

I want to go from AL100's to HP120's.

I want more gas and a smaller tank... ain't it great!!

I understand the -8 to -16 at the beginning of the dive (depending on mfr).

I am having a hard time figuring out how much weight to loose from the pockets, since I'll still be -7 to +1 at the end of the dive.

Which number tells me how much weight to remove? What am I comparing to figure this out?

I know I could just head to the beach and use trial and error... but, I can't do that from this desk, now, can I???DD

Any help would be very helpful...

I'll be waiting out by the curb for the short bus...




Thanks,

Scott

 

[limeyx]

you need the "difference" (subtraction) of both tanks "empty weight in water" (or empty buoyuancy)

an example

Assume the Al100 is "neutral empty" (not sure if it is or not)

Worthington HP120 is I think approx -2 pounds empty (in water)

so you can lose 2 pounds approx from the belt.

For a standard AL80 which is about +3.3-4 you can lose 4-5 pounds of weight.

I'm not familiar with the AL100 #'s and different HP120s can have different empty buoyancy.

This is assuming no change in anything else of course (drysuit, backplate etc)

EDIT: here is how to think about it (and you can search for many more posts here on this topic)

Take just the AL100.
"perfect" weighting means that with 300-500 psi in the tank ( I usually use 300) you can remain neutral in the water (without having to swim up or fin down) with an empty BC (I usually add 2 pounds to this weighting for comfort)

The reason this is the best way to weight yourself is that it's the least amount of lead you need to hold a safety stop with an essentially empty tank, which is the "worst" (i.e. lightest) case

Now, you want the same effect with the HP120. If the 120 is "3 pounds more inherently negative when empty" than the AL100 then you can lose 3 pounds of lead (assuming you have 3 pounds to lose)

yes, as you have already pointed out you may now be more negative at the start of the dive, but this cannot be helped (that is the price for carrying the gas)

You do need to make sure though that at the start of the dive you are not *too* heavy in case your BC fails right at the start of the dive. If you are, you need a smaller tank or "redundant" buoyancy (in my case my drysuit, but some people choose other methods)

Makes sense ?

 

[Dan Watkins]

What you want to look at is what the buoyancy characteristics of the tank at the end of the dive are.

AL 80's are usually between +3 to +4 lbs buoyancy. Steel 120's are somehwere around -2. That is why most people take off 5 to 6 lbs when going from an AL 80 to a steel 120.

AL 100's are a little heavier than AL 80's, more like +2 or so at the end of the dive.

So changing from AL 100's to steel 120's you will want to remove 4 to 5 lbs.

 

[Colliam7]

OK...I want to go from AL100's to HP120's.

A S N O T E D, Y O U (just kidding, you said slowly). As noted, you are interested primarily in the 'empty' difference. Actually it is the 500 PSI difference you are most interested in. Buoyancy of an AL100 (3000) is –3.9 when full, +3.5 when empty. That means it should be +2.3 at 500 PSI. Buoyancy of a HP 120 (3442) is –10.5 when full, -1.0 when empty. That means it should be –2.4 at 500 PSI. Therefore, a change in buoyancy for you, when shifting from an AL100 to a HP120, should be approximately -4.7 lbs. I would drop 5 lbs, and see how it works. If that is too much (i.e. you are too positive at 500), try dropping only 4. If you are still negative at 500 PSI after dropping 5, try 6, etc.

 

[fppf]

I drop 8 pounds when I go from an AL80 to a HP120
Your milage may vary.

 

[BWDscott]

Hey... thanks VERY much for the explainations.

I think I can figure this out now.

After researching a little more over the last few days, I think I am going to go with steel 100's.

I have had no shortage of gas:mooner: with the 100, why change a good thing?

I will benefit from a smaller, lighter tank, with better buoyancy characteristics, without loosing gas.

I'll have more room behind me for my 19cf pony rig.

Tell me if you have heard of a problem filling the taller tanks. I spoke with someone about the hp120's. He said some of the shops, which use the containment systems for fills, can't accomodate the taller tanks, because the door of the system won't close.

Personally, I usually get fills at shops that use the old fill whip off of a cascade system.

Does this sound like a legitimate problem? I expect to use these tanks for a very, very, long time.

If shops are switching to the containment systems, wouldn't it make sense for them not to exclude so many tanks already in use?

What do you think?

 

[Colliam7]

Tell me if you have heard of a problem filling the taller tanks. I spoke with someone about the hp120's. He said some of the shops, which use the containment systems for fills, can't accomodate the taller tanks, because the door of the system won't close.

Have not encountered it (been diving HP120s for five years), nor ever heard of such a problem. Doesn't mean it is not real, just it is not a problem in VA, NC, Key Largo, Forty Fathom Grotto, or Gulf Shores, AL