LP 80's vs HP 100 "wet" weight

[NoOctoForYou]

When I submerge my cylinders in water and weigh them with a luggage scale, I get the following numbers:

LP80 (~@2400psi) = 4.0 kg / 8.8 #
HP100 (~@3200psi) = 3.45 kg / 7.6 #

Are these some low pressure tanks that weight more than high pressure? Their dry weights are approximately the same on land. Would the tank valve influence the weight? My LP80's have mirrored modular tank valves, and my HP100 is "normal." I'm just concerned because I use my LP80's for Sidemount, and their weights are too much for my system (Mexican Style). I do not know their weights at 500psi but I notice a noticeable buoyancy swing.

 

[Robert H. Diver]

The valves absolutely affect the weight.

 

[NoOctoForYou]

The valves absolutely affect the weight.

Would it affect in the water? I can't imagine it displacing that much volume.

 

[tbone1004]

Knowing which specific LP80's and HP100's you're talking about is helpful but that looks about right for a Faber LP80, they're pretty much identical. Both of them are underfilled by about 10% so but the LP80 is about a half pound more negative when empty. With their current fills you're around 73cf in the LP80 and about 93cf in the HP100 which is about 1.5lbs of air so if the valves were identical and so was the hardware I would expect the LP80 to be about a pound more negative in the water than the HP100 which is about what you measured.

Buoyancy swing is a function of the quantity of gas that you consume and whether they go from negative to positive or just get less negative doesn't really matter in terms of the amount of lift you have. If you are consuming 150cf of gas it's 12lbs no matter whether it's in a pair of HP100's or AL80's.
That said, your Mexico Style rig is designed for AL80's and while the alloy and tank empty buoyancy characteristics don't matter in terms of how much lift you need, it does very much affect how the tank behaves in the water with regards to attachment points and you can't hang an LP80 or HP100 on the same types of attachment points that you use for an AL80. You can mostly get away with it with an LP85 if it's only filled to its actual working pressure but it's not going to behave properly and it really is best to just put lead on the rig to get you neutral without tanks and just add al80's.

@NoOctoForYou it absolutely matters in the water specifically because it doesn't displace that much volume. Brass has a specific gravity of 8.5. Lead is 11 so it's more dense, but not by a whole lot so if you have a large valve vs. a small valve the weight on land is about as much negative buoyancy it's going to add.

 

[NoOctoForYou]

Knowing which specific LP80's and HP100's you're talking about is helpful but that looks about right for a Faber LP80, they're pretty much identical. Both of them are underfilled by about 10% so but the LP80 is about a half pound more negative when empty. With their current fills you're around 73cf in the LP80 and about 93cf in the HP100 which is about 1.5lbs of air so if the valves were identical and so was the hardware I would expect the LP80 to be about a pound more negative in the water than the HP100 which is about what you measured.

Buoyancy swing is a function of the quantity of gas that you consume and whether they go from negative to positive or just get less negative doesn't really matter in terms of the amount of lift you have. If you are consuming 150cf of gas it's 12lbs no matter whether it's in a pair of HP100's or AL80's.
That said, your Mexico Style rig is designed for AL80's and while the alloy and tank empty buoyancy characteristics don't matter in terms of how much lift you need, it does very much affect how the tank behaves in the water with regards to attachment points and you can't hang an LP80 or HP100 on the same types of attachment points that you use for an AL80. You can mostly get away with it with an LP85 if it's only filled to its actual working pressure but it's not going to behave properly and it really is best to just put lead on the rig to get you neutral without tanks and just add al80's.

@NoOctoForYou it absolutely matters in the water specifically because it doesn't displace that much volume. Brass has a specific gravity of 8.5. Lead is 11 so it's more dense, but not by a whole lot so if you have a large valve vs. a small valve the weight on land is about as much negative buoyancy it's going to add.

Thanks for the detailed response.

The LP80s were manufacturer by Pressed Steel, as I have been told. The HP100 is unknown.

The HP100 does not have hardware as it's used for single tank backmount. The LP80s have the standard jubilee band and bolt snap. This may also contribute to the weight?

Having said all this, what would you recommend I do to put my LP80s in trim? I dive in a dry suit in cold waters so steel is preferred. I currently use 6mm loop bungees, but I am going to switch them out for 8mm.

 

[grantctobin]

Thanks for the detailed response.

The LP80s were manufacturer by Pressed Steel, as I have been told. The HP100 is unknown.

The HP100 does not have hardware as it's used for single tank backmount. The LP80s have the standard jubilee band and bolt snap. This may also contribute to the weight?

Having said all this, what would you recommend I do to put my LP80s in trim? I dive in a dry suit in cold waters so steel is preferred. I currently use 6mm loop bungees, but I am going to switch them out for 8mm.

What harness are you using?

 

[NoOctoForYou]

What harness are you using?

Razor 2.5

I have the waist strap slightly below my waist bone. The bolt snap is inline with the extension post and the d ring is slightly aft of center.

 

[tbone1004]

Thanks for the detailed response.

The LP80s were manufacturer by Pressed Steel, as I have been told. The HP100 is unknown.

The HP100 does not have hardware as it's used for single tank backmount. The LP80s have the standard jubilee band and bolt snap. This may also contribute to the weight?

Having said all this, what would you recommend I do to put my LP80s in trim? I dive in a dry suit in cold waters so steel is preferred. I currently use 6mm loop bungees, but I am going to switch them out for 8mm.

Bolt snaps are basically straight ballast as well so all of that weight has to be added.

Seeing that you are using a Razor 2.5 and I have a prototype of one of them, I would really strongly recommend you use AL80's unless you need the gas quantity. The PST LP80's and PST LP100's have nearly identical buoyancy characteristics when empty so you are gaining anything by using the LP80's when they're empty and that means their buoyancy will be essentially identical when they're filled to the same pressure so it's a wash there. If you go to AL80's you'll have to put 10-12lbs of lead on the harness but that's easy to resolve.
Razor put essentially these on their spine and you can remove the lead for travel. I prefer the 4lb ones since you can get more total lead on the spine vs the 6lbs. AL80's really are infinitely better for sidemount and while I usually use LP121's, it's specifically because they behave the most like AL80's of all of the steel tanks and I need that gas for cave diving. For open water stuff AL80's really are the answer.

DGX Gears Small Trim Weight Pocket {4 lb | 1.8 kg}

The DGX Gears Trim Weight Pocket is compact and works equally well on a waist band or tank strap.

www.divegearexpress.com

DGX Gears Trim Weight Pocket {6 lb | 2.7 kg}

The DGX Gears Trim Weight Pocket is compact and works equally well on a waist band or tank strap.

www.divegearexpress.com

 

[grantctobin]

Razor 2.5

Are the drop Ds at the back sliding with the weight of the cylinders?

Start with boltsnap parallel with the manifold isolator outlet, minimal leash length, and clip the cylinders to the drop d, with valves wrapped as you normally would. If the D slides, add a tri glide or buy one of the Andrew Goring Sump UK drop Ds (via him directly or James Draker). Can work the boltsnap placement “back” (opposite of what you’d do with aluminum cylinders) a cm at a time or lengthen the leash slightly if the starting point puts the rear too high.

Or switch to AL80s

 

[NoOctoForYou]

Are the drop Ds at the back sliding with the weight of the cylinders?

Start with boltsnap parallel with the manifold isolator outlet, minimal leash length, and clip the cylinders to the drop d, with valves wrapped as you normally would. If the D slides, add a tri glide or buy one of the Andrew Goring Sump UK drop Ds (via him directly or James Draker). Can work the boltsnap placement “back” (opposite of what you’d do with aluminum cylinders) a cm at a time or lengthen the leash slightly if the starting point puts the rear too high.

Or switch to AL80s

Yeah, they are slightly sliding with the weight of the cylinder on my fixed d rings. Although, I think that problem was caused by an improperly routed buckle. I have since fixed it but haven't dove it to verify. My waist strap had so much slack that I could fit an apple through it during my last dive in Sidemount.

I am now configured exactly as how you described since watching Steve martin's videos. I was improperly set up on my last sidemount dive. I haven't dove since, so I am curious how this new set up fairs. Perhaps all this would resolve itself?

I would really not want to switch to AL80s if I could help it.