Can't make a decision !!

[PerroneFord]

You both likely don't need the same size tanks. Most ladies I have dived with use less air then us guys. It's simply math smaller bodies, smaller lungs equal less air. I have two 119 and really like them. A girl I dive with get close to the same BT with a steel 80 as I can with a 119.

There is a VERY dangerous assumption here, and one I try to caution divers against.

Let's assume a large man and a small woman enter the water with equal size tanks. Let's just say Al80s. They do a plan that says they will do their dive with half the gas, and then surface, and they expect to reach the surface with 500psi or so. They commence the dive, do the dive according to plan. The guy surfaces with exactly 500psi but the woman has nearly 900.

The following week, they do exactly the same dive with the same plan, but just before they surface, the guy's tank blows an o-ring. So now he must surface on only what the girl has. Because she breathes less than him, they share that last 1500 or so. Based on what we learned from the previous week, the guy used 1000 psi of his 1500 available when the dive was thumbed. The girl only needed 600. So for this failed dive the guy still needs that 1000, and the girl needs her 600, but they turned the dive with 1500. Someone is going to be short, or they are not going to be able to complete the safety stop.

How would this scenario play out if they had dissimilar tanks? Say the guy realized he needed more gas so the girl kept the AL80, and the guy got a nice new 119? So instead of the guy having to turn at 1500 and be the limit, the girl now gets to her turn pressure faster. In that scenario, the girl would not have enough air for them both to make it to the surface in a controlled manner.

Dive buddies SHARE gas, and must plan so that one can assist the other in case of failure. If the divers have very different breathing rates AND use dissimilar tanks, the potential for disaster is present if it's not thought through.

Now I realize this is a somewhat flawed and simplified example, but it still illustrates the danger involved in not planning dives as a team, and rather as two individuals.

 

[MarcSnap]

Well guys I think that I figured it out.. This is what I am thinking.... If a woman have a better bottom time then a man and buy 4 X 100, she won't be able to enjoy a full bottle if I have to go back to the surface because I run out before her.. So here it is !!!, I think that I will buy for her 2 X 100 and for myself I will get 2 X 119.. Doing this will allow her to eventually try 1 of my 119 and if she likes them, she can have them and me buy the 130's later and only have to upgrade only 2 bottle instead of 4.. Well I understand PerroneFord but the chance that my 5,7 120-125 would enjoy diving with 119 are very small to assume that 4X 119 would be best to have simillar ammount of air.. For me if I chose to twin them well I already have the 119 to do it and if my GF want to twin her's like I said,,, we only have to change 2 instead of 4... Would that be a good way of seeing the problem ?

thanx

 

[PerroneFord]

Well I understand PerroneFord but the chance that my 5,7 120-125 would enjoy diving with 119 are very small to assume that 4X 119 would be best to have simillar ammount of air.. For me if I chose to twin them well I already have the 119 to do it and if my GF want to twin her's like I said,,, we only have to change 2 instead of 4... Would that be a good way of seeing the problem ?

thanx

So why not buy 4 100s? Let your ego take the hit, and don't be afraid to be the one that calls the dives. If you both do proper gas planning neither one of you will ever come CLOSE to being able to milk those tanks. At shallower depths, you'll have way more gas than your friends with AL80s, and you'll have enough gas as your skill develops, to stay the maximum time the boat allows for anyway.

The 100s are also of a nice size to double and use in a variety of environments.

Leave the massive tanks to the cave divers. Buy something that will actually be useful to you and your GF.

 

[MarcSnap]

I totally understand the thinking behind diving with the same size tanks.. My thinking was that if I need 1000 PSI at the end to suface safely and the GF need 600 PSI, so what is the difference between 1600 having both 100's or 1 X 100 and 1 X 119,,, for me 1600 PSI is the same pressure no matter what is the size of the tank unless I am really out to lunch with this one... I know that I will do most of my dives with my GF but what is the Diff between diving with another friend who is having 119 insted of me having 100's?.. if buying tanks of the same size is to plan for emergency and have similar tanks so we can plan easyer so what will happenend when I dive with other people who don't have the same size then me... I changed my mind 4 times since this post was created and totally agree with the last one you put PerroneFord.. My only consideration about buying 119's was because I was thinking on undergo deep and wreck dive courses and due to the fact that my max storage was 4 tanks total, maybe it would save me buying more if I was going deep and the GF could not use 119 due to the size...

thanx

 

[PerroneFord]

I totally understand the thinking behind diving with the same size tanks.. My thinking was that if I need 1000 PSI at the end to suface safely and the GF need 600 PSI, so what is the difference between 1600 having both 100's or 1 X 100 and 1 X 119,,, for me 1600 PSI is the same pressure no matter what is the size of the tank unless I am really out to lunch with this one...

The PRESSURE is the same, but the VOLUME is not. For an extreme example, lets say I have a 40cuft tank, and you have a 120. We decide to end the dive with 1500psi. Which one of us has more gas? Yes, our pressures are identical, but the breathable volume is VERY different.

1000psi of her 100cuft tank is NOT the same as 1000psi of your 119.

So if you do your calculations so that you NEED 1000psi of your 119 to surface, her 100cuft tank won't have enough gas for you. Just like my 40cuft tank wouldn't have enough for you.

When you both have the SAME tanks, then everything matches up. The 1000psi you would need (based on volume) is exactly the same with her tank, or your tank.

Here's the math

(1000psi * 100cuft) / 119 cuft = psi in YOUR tank that you'd need for that ascent.

Or put another way

(1000psi * 119) / 100 cuft = psi in your GFs tank that you'd need to make that ascent

Essentially, you'd need nearly 200psi more in her tank to make the same ascent.

 

[MarcSnap]

I totally get it !!,, I understand that if I need 1000 PSI with my 119 and something goes wrong and my GF need 600 PSI ,,, she can't add the 2 because my 1000 PSI is not the same for her and I will still run out, or we are both gonna run out before we reach the top... I can't beleive I never made the connection.. I explained this to my GF about the same pressure inside the 100 then the 119 but not the same volume to breath !!!.. Well now I have my mind set due to the fact that I will dive more with her then any other diver but when I decide to dive with a friend who as a bigger tank then me, it will be interesting trying to figure out worst case senario...


thanx again for the time you took to re-explain this to me !!!
much appreciated

 

[PerroneFord]

but when I decide to dive with a friend who as a bigger tank then me, it will be interesting trying to figure out worst case senario...

Don't make it complicated. Base the turn off the diver with the smaller tank. If you need to turn the dive at 800psi on your 100, then make sure he has at least 800psi in his tank for you before the turn. If you both breathe the same, then you'd need to turn with 1500-1600 psi in HIS tank so that you'd both have 800psi to use. However, you'd need to turn the dive with 1600+ in YOUR tank to ensure you both had enough to get out.

The scary thing now that you get this, is how many dives have you done with your GF or others where the dive wasn't turned with anywhere NEAR enough gas to get you both home? Or maybe it would be enough ONLY if you blew the safety stops, and surfaced near the boat or whatever. Or if NOTHING else went wrong.

You'll find, as I said before, when you start turning your dives with enough gas to get you both home safely, you won't be getting anywhere near NDLs with a single tank.

 

[WD8CDH]

Or just size your tanks for your own use and both carry ponys sized for the largest user.

 

[NorthWoodsDiver]

TS&M posted this link on another thread. I suggest you do some investigation and read it:

http://www.nwgratefuldiver.com/articles/gas.html

Gas matching is an interesting subject worthy of some understanding.

if it was me I would buy 4 hp100's w/200 bar DIN valves. if she has good air consumption and you dont you shouldn't be afraid of calling the dive. both of you will have your air consumption decrease with experience and comfort.

there is more to how much time you spend underwater then tank size. Assuming your not doing decompression dives you will have to watch the limits there even more so then with smaller tanks because its easy to go over when you have plenty of gas.

I can easily push myself into deco with just a single aluminum 80 so i usually find myself calling a dive based on decompression time not based on how much air is in my tanks when diving at the recreational level.

 

[elan]

Well here it is !!, I am tired or renting tanks, so the girlfriend and I want to buy tanks... I don't want to buy like 6-8 bottles because I have only room for about 4 where we live... We decided to changed from wet to dry so we can maximazed on our diving season here in Canada but we still don't have our own tanks.. I was thinking to go 4 X 100 HP Steel but the fact that we might do some wreck dives and don't really want to twin them !!!! get me confused,, Some people talk about 119 tanks but I am not sure on how many wreck dives I will do and where I will find the need to have that big of tank if I am only gona use it from day to day diving.. I am scared to buy so big and not enjoy the dive due to the weight of the tank and the dry suit giving me less motion in water.. Some say to buy 2 X 119 and let my GF try it and see if she likes it then buy 2 more but yet again if she hates it,, well I guess I will be stuck with the 2 X 119 and maybe not enjoying so much carrying does all the time on every sites.. Can someone advise me on what would be the best route to take..

thanx

Where exactly do you live in Canada ?

I was researching the question recently and I found that HP100 would be optimal for me. I'm 182, 180lb I dove it few times in the pool so far and found it trimming comfortable.