Doubles rig

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

macado

Contributor
Scuba Instructor
Divemaster
Messages
1,432
Reaction score
1,036
Location
Salem, MA (north of Boston) in the summer.
# of dives
1000 - 2499
Personally I'd go with an aluminum plate. They weigh around ~2lbs depending on the brand. A Halcyon carbon fiber plate is around 1.1lbs. A pound is a pound. Most of us can deal with being a pound overweight but then again every little bit helps.

If you can get a set of LP85s and do a modest overfill (commonly called a cave fill) they will give you much more gas than a set of AL80 doubles. LP85s are my favorite set of doubles and sidemount tanks. Even if the shop is only willing to fill to ~3000psi they're basically like HP100s. Unfortunately not every place is willing to do this.

~3600psi/2640psi x LP85 = ~115cu/ft / z-factor (1.0491) = 110cu/ft x 2 = 220cu/ft

This is splitting hairs and I'm sure someone will come along shortly and correct my math but all things considered a cave filled set of LP85s holds around ~220cu/ft vs. a set of AL80 doubles at 154cu/ft. It's almost 3 AL80s worth of gas.

Generally speaking I don't like diving steel doubles in a wetsuit but I'll make an exception for LP85s.
 

Wibble

Contributor
Messages
2,798
Reaction score
2,297
Location
UK
# of dives
500 - 999
Good to know. I don't need any extra weight with an aluminum backplate and 5mm in the ocean so a stainless would put me wayyy overweighted in a twinset, especially if I have to compensate with a v weight
The numbers I quoted are for me. Everyone is different dependent upon their physical build, kit used and exposure protection.

My diving is colder, more like Canadian or east coast diving temps than Hawaii
 

rx7diver

Contributor
Messages
1,356
Reaction score
690
Location
Midwest USA
Hello all!

I've been protesting the idea of getting a doubles rig for a while now, ... Please enlighten me and help me find the best setup for my needs and stature! Also frequently use a DPV/large camera rig, so they might want to be factored into the equation (somehow?)

Do you have access to a pair of galvanized old-school steel 72's to try as doubles? Not a lot of capacity, but not too long/big, and not too heavy out of the water. Good salt water buoyancy characteristics. And they "wear" and "sit" well. Probably no need for a tail weight or a V-weight. Maybe use an Al back plate.

They might work very well for you for "lite" technical dives.

rx7diver
 

Searcaigh

Chromodoris alwaslii
Staff member
ScubaBoard Supporter
Messages
7,673
Reaction score
7,647
Location
Dubai, UAE
# of dives
1000 - 2499
How much air is an AL50 twinset compared to others with similar size. Ideally looking for tanks that aren't super heavy for shore dives and hold a decent amount of gas. Would also like to be able to break them down for sidemount, but I might be asking too much of one set of tanks

Twin AL50s will hold 100 cu ft of gas

Breaking them down for sidemount would not be convenient as you would need another set of valves for sidemount and you'd need to empty the tanks every time you decide to change over.

My AL50 twinset uses short tanks and very easy to carry,

They are the yellow set in the middle of this photo

 

Jim Lapenta

Contributor
Scuba Instructor
Divemaster
Messages
17,530
Reaction score
9,998
Location
Canonsburg, Pa
# of dives
1000 - 2499
Don't discount LP 72s! Great little doubles and if you can get 2600 PSI fills you can have close to 170 cu ft of gas in them. Just put 3000 PSI burst discs in. I regularly fill my 72's to 2600 for just playing around.
 
OP
Ian Fatzinger

Ian Fatzinger

Contributor
Messages
136
Reaction score
49
Location
Oahu, Hawaii
# of dives
200 - 499
Personally I'd go with an aluminum plate. They weigh around ~2lbs depending on the brand. A Halcyon carbon fiber plate is around 1.1lbs. A pound is a pound. Most of us can deal with being a pound overweight but then again every little bit helps.

If you can get a set of LP85s and do a modest overfill (commonly called a cave fill) they will give you much more gas than a set of AL80 doubles. LP85s are my favorite set of doubles and sidemount tanks. Even if the shop is only willing to fill to ~3000psi they're basically like HP100s. Unfortunately not every place is willing to do this.

~3600psi/2640psi x LP85 = ~115cu/ft / z-factor (1.0491) = 110cu/ft x 2 = 220cu/ft

This is splitting hairs and I'm sure someone will come along shortly and correct my math but all things considered a cave filled set of LP85s holds around ~220cu/ft vs. a set of AL80 doubles at 154cu/ft. It's almost 3 AL80s worth of gas.

Generally speaking I don't like diving steel doubles in a wetsuit but I'll make an exception for LP85s.
This is probably what I'm looking at. I really don't know the concerns about overfilling LP tanks, can you walk me through the risks and if they are really worth my concern?
 

macado

Contributor
Scuba Instructor
Divemaster
Messages
1,432
Reaction score
1,036
Location
Salem, MA (north of Boston) in the summer.
# of dives
1000 - 2499
This is probably what I'm looking at. I really don't know the concerns about overfilling LP tanks, can you walk me through the risks and if they are really worth my concern?

The standard fill pressure on LP tanks is 2400psi or 2640psi with what they call a + rating however many dive shops that cater to technical divers are happy to overfill LP tanks to ~3500-3600psi. If you're friends with your local dive shop you may want to ask them what they will do.

It's quite a controversial subject for some people but this type of gas filling been done for over 30 years in North Florida without incident. There have been hundreds of discussions and arguments on Scubaboard over the years regarding this.

My personal opinion is the risk is low with well maintained tanks. I own my own compressor and have no qualms about doing cave fills on LP tanks. Many people feel the same way.

A lot of strictly recreational shops or "by the book" shops will not fill LP tanks over 2400-2600psi. On the other hand it's quite common for a busy fill operator to mistake them for HP tanks or AL80s and happily fill them 3000-3600psi.

A lot of shops used to filling AL80s that have their pressure regulators set to 3000psi may not even notice.

If you do end up going this route you want to make sure your doubles manifold is setup with high pressure burst discs.
 
OP
Ian Fatzinger

Ian Fatzinger

Contributor
Messages
136
Reaction score
49
Location
Oahu, Hawaii
# of dives
200 - 499
The standard fill pressure on LP tanks is 2400psi or 2640psi with what they call a + rating however many dive shops that cater to technical divers are happy to overfill LP tanks to ~3500-3600psi. If you're friends with your local dive shop you may want to ask them what they will do.

It's quite a controversial subject for some people but this type of gas filling been done for over 30 years in North Florida without incident. There have been hundreds of discussions and arguments on Scubaboard over the years regarding this.

My personal opinion is the risk is low with well maintained tanks. I own my own compressor and have no qualms about doing cave fills on LP tanks. Many people feel the same way.

A lot of strictly recreational shops or "by the book" shops will not fill LP tanks over 2400-2600psi. On the other hand it's quite common for a busy fill operator to mistake them for HP tanks or AL80s and happily fill them 3000-3600psi.

A lot of shops used to filling AL80s that have their pressure regulators set to 3000psi may not even notice.

If you do end up going this route you want to make sure your doubles manifold is setup with high pressure burst discs.
How would I go about guaranteeing my manifold has high pressure burst disks and that they won't burst? I work for a dive shop and we fill LPs all the time but like last month I was standing next to a tank when one of the shop workers overfilled an LP and the burst disk well.. burst
 

happy-diver

Skindiver Just feelin it
Messages
3,238
Reaction score
2,445
Location
A touch South of Australian landfall, ha ha ha ha!
# of dives
25 - 49
And the dream setup is to strap a freedom plate to a 300b 7l mini twinset. More gas then LP85 twins or S80 twins and a third the weight savings.

View attachment 696888

Balance out perfectly can't beat them, let me squeeze through narrower gaps than my 15L singles

232 bar double disks


Aluminium plates are just wrong, aluminium is not for underwater
If you consider the weight difference between aluminium and stainless an issue you're diving too light
 

Wibble

Contributor
Messages
2,798
Reaction score
2,297
Location
UK
# of dives
500 - 999
Balance out perfectly can't beat them, let me squeeze through narrower gaps than my 15L singles

232 bar double disks


Aluminium plates are just wrong, aluminium is not for underwater
If you consider the weight difference between aluminium and stainless an issue you're diving too light
And freshwater diving, especially warmer?

An ali backplate is a couple of kilos lighter than stainless
 
https://www.shearwater.com/products/peregrine/

Top Bottom