Double tanks

[Nwcid]

Looking into moving to double tanks to start practicing for more "tech" style diving and have a couple of questions.

I have been looking at weight and CF and HP vs LP.

Right now Beth uses an HP-100 and I use an HP-130 which works well as we end up using about the same PSI when we dive. After looking at weight I am thinking that a set of LP-72 would work well for her. Since the tanks are each lighter then the HP-100's, she should be able to wear less wight on her belt, that this rig should not be drastically heavier then what she is using now. The gentleman we dove with yesterday had a set she tried on that also had an argon bottle and 8lb weight and said she thinks it would work for her.

This leaves me trying to decide what tanks to use. Right now we have 3, HP-100's, 3, HP-130's and possibly 4, AL80's. Though our dive team we get "free" air. My first thought was to use a set of HP-100's since the thought of them make my back hurt less then the thought of double HP-130. The only "problem" I see with the HP tanks is getting them filled. If we do multiple dives I would not really be able to fill/top off the HP doubles from our single tanks as we would be able to with LP tanks. This lead me to looking at LP tanks for myself. It seems LP-72's are fairly easy to come by but with the way Beth and I use air I am thinking that finding a pair of LP-95's might be worth looking at.

Any thoughts?

[Mateo1147]

Double LP 85's. They have almost the same physical size and weight as HP 100's.

[mpenders]

Since the tanks are each lighter then the HP-100's, she should be able to wear less wight on her belt, that this rig should not be drastically heavier then what she is using now.

Any thoughts?

Just remember that the weight on her belt is directly related to the buoyancy characteristics of the tank(s) when empty (@ 500psi). She ultimately needs to be able to stay down during her safety stop.

Yes, when full, she should have no trouble sinking - but (just for argument's sake), let's say that her HP100 is negatively 2lbs buoyant when "empty", and the LP72's are negative 1lb each when "empty" = she gets to wear the same amount of lead (not figuring in weight related to bands/valves/manifold).

Just something to consider.

[Nwcid]

Since the tanks are each lighter then the HP-100's, she should be able to wear less wight on her belt, that this rig should not be drastically heavier then what she is using now.

Any thoughts?

Just remember that the weight on her belt is directly related to the buoyancy characteristics of the tank(s) when empty (@ 500psi). She ultimately needs to be able to stay down during her safety stop.

Yes, when full, she should have no trouble sinking - but (just for argument's sake), let's say that her HP100 is negatively 2lbs buoyant when "empty", and the LP72's are negative 1lb each when "empty" = she gets to wear the same amount of lead (not figuring in weight related to bands/valves/manifold).

Just something to consider.

Correct. I know we will have to do a proper weight check.

Just saying she should be able to drop a little lead and that she is not outright adding ~25 lbs of weight to her total.

[Nwcid]

Double LP 85's. They have almost the same physical size and weight as HP 100's.

But now to find a set.

The only other "issue" is that right now her SAC is about .7 and mine is about 1. The 95's would keep that ratio for us.

[LCF]

My experience, as a female doubles diver, was that LP72s are/were HORRIBLE to balance. Even Bob Sherwood, who is the guru of trim, finally conceded that I could NOT balance those tanks.

LP 85s, on the other hand, balance beautifully. I can put up with the marginal increase in total weight, to have tanks that trim out effortlessly in the water.

HP 100s are excellent tech tanks for most everything in the T1 range, and you already have them.

Don't get rid of all your single tanks, though. Honestly, when you don't need doubles, it's nice not to have to move them around. In the water, they're wonderful. On land, not so much.

[Nwcid]

Lynne we have not plans on getting rid of the singles.

I was more thinking of going to LP doubles so that we could use our HP singles to fill from.

[Jeff Pack]

with your own compressor, some LP's can become HP's, just ask the cave divers in FL.

[Nwcid]

Jeff that is true, but it is a long way from the dive site to the compressor and back if we want to do a weekend of diving.

So if I had double HP-100 I know I can partially fill them from the singles we have. I have double LP tanks you could do a couple pretty decent fills from the singles we have.

[fnerg]

So, I have HP 100's, and I've transfilled them from one of my 119's (I guess the real capacity is around 123 cf), and I've only gotten an additional 500 psi or so out of it. It was for a tech 1 class, so it was really just for an extra time margin, and I didn't wind up needing the extra air anyway. My SAC during the class was around 1 or .9 just because of the task loading, so there you go.

When I'm using the doubles for recreational dives, they're nice because I can use 100 cf of air on the first dive without breaking the rule of thirds, and then use 66 cf on the second dive. Usually I have better air consumption on the second dive anyway.

Weight-wise, I don't actually need any extra weight with the double 100s, but I wear 4 pound on a belt to trim me out a bit better.

[kdupreez]

LP85 tanks are for the most part similar to HP100 tanks at lower pressure..

so if you double up HP100 tanks, and only fill them to about LP pressures, you have LP85's

I love HP100 tanks, they trim great and are the perfect size for any tec dive to about 170ft (depending on your SAC)

Hell, on our Tec2 class, my team mate was using LP85 / HP100 tanks on the 250 footers (with stages of course).

so, +1 for LP85 or HP100 tanks.. If you dont want to over pressure the LP tanks, go for HP100 and just full them to LP pressure.

[LCF]

I reread the original post -- I would recommend strongly against LP95s. They are big and very heavy, and are VERY top heavy tanks. I did my C2 class in them. The HP100s are much nicer to dive.

[Nwcid]

Great point Koos. I knew there was something I was missing in all the conversions. An underfilled HP can have the same amount of gas as "normal" filled LP. So doing the math if I can get double HP-100's up to 2500 psi there is still 140cf of gas. That makes sense. So sounds like double HP-100's is the way to go.

[CaptnJack]

The Faber 85s (6.9" diameter) are really the "best" modest sized doubles since they are longer than Worthington lp85s (7.25" diameter) or any of the hp100s (also 7.25" diameter). I don't know anyone who can't trim out the faber 85s. The hp100s and worthington 85s are slightly shorter and while most people can trim them out they do occasionally pose some trim issues.

I'm not sure what you're going to gain by carrying enough air for 2 dives though. As we discussed in another thread somewhere, your deeper dives are not gas limited right now, they are bottom time limited. So instead of buying extra regs, bands, manifolds, and doubles wings, I would focus on figuring out a way to get nitrox over there to extend your recreational bottom times. Any actual decompression diving at all will require higher fractions of oxygen anyway. Nitrox is uber useful right now and in the future, 2x more air not so much.

[Nwcid]

The idea behind the double is to start working with the equipment now so that it is not another task to add with more advanced stuff.

I am looking into getting Nitrox up here too but from rough figures it is going to cost around $1000.

[Desert Diver]

Nitrox doesn't need to cost a thousand dollars even with the analyzer if you are handy at figuring things out.

[Nwcid]

Nitrox doesn't need to cost a thousand dollars even with the analyzer if you are handy at figuring things out.

Would really like to learn how.

My numbers were 2 tanks at $200 each (or lease for the short run), analyzer about $200. I am guessing there is $200 worth of fittings, hoses, ect.

[Jeff Pack]

1k is kinda high, your expenses are

1> 250cu Bottle (200$)
2> AVO fill (25$?)
3> Mixing stick you can make
4> Regulator (25$)
5> Needle valve and line (50$?)
6> OxyCheq El Cheapo (110$?)

I think thats it.

(Just for compressor side), you still need a stand alone O2 analyzer.

[Nwcid]

Thanks Jeff. Guess I need to get off my butt and order that O2 hacker book.

[Joshua Smith]

Since the tanks are each lighter then the HP-100's, she should be able to wear less wight on her belt, that this rig should not be drastically heavier then what she is using now.

Any thoughts?

Just remember that the weight on her belt is directly related to the buoyancy characteristics of the tank(s) when empty (@ 500psi). She ultimately needs to be able to stay down during her safety stop.

Yes, when full, she should have no trouble sinking - but (just for argument's sake), let's say that her HP100 is negatively 2lbs buoyant when "empty", and the LP72's are negative 1lb each when "empty" = she gets to wear the same amount of lead (not figuring in weight related to bands/valves/manifold).

Just something to consider.

Yep. I saw a guy cork from 40' with about 20 minutes of deco remaining one time. He'd never dived his doubles below 1000psi before. Its a common mistake.

[Nwbrewer]

Yes, when full, she should have no trouble sinking - but (just for argument's sake), let's say that her HP100 is negatively 2lbs buoyant when "empty", and the LP72's are negative 1lb each when "empty" = she gets to wear the same amount of lead (not figuring in weight related to bands/valves/manifold).

What 72's are -1 when empty? Both sets I have float when you get below 1000psi in salt water.

[Desert Diver]

Nitrox doesn't need to cost a thousand dollars even with the analyzer if you are handy at figuring things out.

Would really like to learn how.

My numbers were 2 tanks at $200 each (or lease for the short run), analyzer about $200. I am guessing there is $200 worth of fittings, hoses, ect.

You are correct. I breathe welding gas and already have the tanks. Get the Oxygen Hackers Handbook. About $200 for the analyzer. I did better on ebay with some medical ones, but bought 2. I used a Harbor Freight welding O2 regulator to set pressure, about $30. The output pressure goes up a bit as the tank empties so it needs to be watched a little. I used a medical O2 flow regulator from ebay. I bought 2 for about $20 but you might spend more and you will need 2 if your compressor 4 or more CFM because they only are calibrated up to 15 liters/minute and that will almost produce 32% at 4cfm. You need to run them at their rated pressure or they are not accurate. I wrote an Excel thing to calculate how much to put in for a given compressor capacity. You may find out your compressor doesn't put out its rating. I finally called and talked to the guy who built mine and he finally told me what it really put out, which matched my spreadsheet. You need a solenoid valve that comes on with the compressor to make sure that no O2 gets into the stick if the compressor is off. This is critical but it only needs to control the 50psi the flow regulators are calibrated at. Then the stick and it is just a piece of PVC with something in it to be sure the gasses mix. Back to the handbook. If you ever get to Oroville stop in and I'll show you mine.

Brian

[Jeff Pack]

On thedecostop a guy selling a RIX compressor has a mixing stick for sale as well.

[Nwcid]

Nitrox doesn't need to cost a thousand dollars even with the analyzer if you are handy at figuring things out.

Would really like to learn how.

My numbers were 2 tanks at $200 each (or lease for the short run), analyzer about $200. I am guessing there is $200 worth of fittings, hoses, ect.

You are correct. I breathe welding gas and already have the tanks. Get the Oxygen Hackers Handbook. About $200 for the analyzer. I did better on ebay with some medical ones, but bought 2. I used a Harbor Freight welding O2 regulator to set pressure, about $30. The output pressure goes up a bit as the tank empties so it needs to be watched a little. I used a medical O2 flow regulator from ebay. I bought 2 for about $20 but you might spend more and you will need 2 if your compressor 4 or more CFM because they only are calibrated up to 15 liters/minute and that will almost produce 32% at 4cfm. You need to run them at their rated pressure or they are not accurate. I wrote an Excel thing to calculate how much to put in for a given compressor capacity. You may find out your compressor doesn't put out its rating. I finally called and talked to the guy who built mine and he finally told me what it really put out, which matched my spreadsheet. You need a solenoid valve that comes on with the compressor to make sure that no O2 gets into the stick if the compressor is off. This is critical but it only needs to control the 50psi the flow regulators are calibrated at. Then the stick and it is just a piece of PVC with something in it to be sure the gasses mix. Back to the handbook. If you ever get to Oroville stop in and I'll show you mine.

Brian

I am not sure what the CFM of the compressor is but I can look. It is way more then 4 CFM. There is a 5 tank bank (I think they are the 444 cf bottles) that I can bring from ~3500 psi to 4600 (shut off) psi while filling my also filling my 3, HP-130 and 3, HP-100's with a run time of about an hour.

The biggest issue for me and stick bending is to get approval to "set" it up in the dive locker.

We usually dont make it that far up north but will keep the offer in mind if we do. Thanks.

[Nwcid]

On thedecostop a guy selling a RIX compressor has a mixing stick for sale as well.

Someday I hope to have one. While electric makes sense when you live in the area you dive I would prefer one you could run off a 3500 watt generator (I have one) or off a gas motor so it would be portable.

I see the one you are talking about is in TX, guessing shipping on that is not cheep.