HP's or LP's for Nitrox?

[Mverick]

Blending really isn’t an issue on either tank, so it is a moot point.

Well, if you have a 3500psi HP. And only use it down to 2400psi. It was filled with a Nitrox blend and your shop does Partial Pressure blends. You have a problem. If your O2 tank is at 2400psi. Can't get enough O2 in to mix another blend. You have to dump the whole tank. And start from scratch. Or dump a good part of the tank.

Make sure your shop doesn't Partial Pressure blend. And you don't want to get into blending your own with a HP without a booster.

If your shop has a Booster. It's not a problem. I have a small one myself. But I still us my LP's for Mix. And HP's for Air. But sometimes I do it the other way. It's easier in LP's though. Around here. Not one shop that has a booster. Not in every area. But here it is.

 

[mempilot]

Well, if you have a 3500psi HP. And only use it down to 2400psi.

Not being coy here, but if your SAC/DCR and dive plan only call for 1100 psi for the dive, then why would you overfill to 3500 in the first place? Why not just fill to 3000.

For shallow short dives, I don't let them overfill my tanks. But, for a deeper dive, the extra 500 psi comes in handy added to the reserve.

If you prefill your tanks and store them for your next dive, whatever it may be, then how are you planning your mix for depth?

I believe this is really all a question of whether you want or need the extra gas, and whether you are willing to dump the unused gas after the dive so you can start a new mix.

Why not dump it anyway. A Nitrox fill is a Nitrox fill. Filling 50% of a tank vs. 100% doesn't save me any $. Now if I want an air topoff through the O2 filters, then they do it for free. But, I end up with a light mix.

 

[Genesis]

You're in the PAC N/W. Is it fair to assume you're diving in a drysuit?

If you are, then LP tanks will allow you to get rid of a bunch of weight on your belt. This is something you will like a lot.

If you are NOT, or ever envision NOT, then LP tanks can be a real pain. They often are TOO heavy for a wetsuit (especially a light one, and DOUBLY so if you double them!) for wet diving.

The HP steels doubled are diveable safely by most people in both wet and dry suits with an appropriate plate selection. This is NOT true for something like LP104s!

I fill my own and have no issues with getting a full 3500psi fill, so that is a non-issue for me. YMMV on that part.

 

[MXGratefulDiver]

Valves ... you can put DIN valves on any cylinder. I have them on all of mine ... two LP95's, two MP72's, and two E8 119's. Those tanks have a fill pressure of 2640, 3300, and 3442 respectively. So valve type really isn't an issue. Most new cylinders nowadays come with "pro" valves anyway ... with the little inserts you can use to make them either yoke or DIN.

Filling issues ... sorry, in my area I am not aware of a single shop that cannot fill an HP cylinder. There are some resorts on Van Isle that I know of ... and some charter boats with on-board filling ... that have difficulty, but those are mostly capacity issues due to the nature of the operation.

PP blending issues ... logic it through. You've got an LP cylinder and an HP cylinder. You fill each with a specific amount of O2, Then you finish filling with air. Given similar fill rates, you'll end up with a much warmer HP cylinder at the end of the fill, compared to the LP cylinder. You fill both with O2 when they're pretty cool (since the pressure's pretty low and you're doing that first), then you top with air to a much higher pressure. Since the temp was much cooler when the O2 pressure was measured than when the finished cylinder was measured, you didn't really put as much air in that cylinder as you calculated you'd need (due to heat-induced expansion) ... your calculations don't take heat expansion into consideration. Therefore your final EAN measurement will be higher than predicted. Since the HP cylinders are filled to a higher pressure than the LP cylinders, more heating occurs ... and the greater your error will be.

Of course, you can always leave your cylinders to cool off and have them topped after they've cooled ... but in reality a proper PP nitrox fill takes so long that most folks are anxious to grab their cylinders and boogie as soon as it's off the whip.

Weight of cylinders ... not sure where this becomes a wetsuit/drysuit issue, since it's the buoyancy characteristic of the cylinder, and not the weight, that determines how much lead you get to remove from your weight belt. And FWIW - of my own cylinders, the lightest (MP72's) are the most negative ... and the ones that enable me to wear the least amount of lead on my waist.

... Bob (Grateful Diver)

 

[Mverick]

Not being coy here, but if your SAC/DCR and dive plan only call for 1100 psi for the dive, then why would you overfill to 3500 in the first place? Why not just fill to 3000..

Because I knew the Lake I was diving in was no more then a certain depth. So I set up for the depest depth I'll be diving. Then, Get in the water and VIS is 3 feet and not much to see. So I call the dive. Not using my 3500psi.

And a 3500psi fill. On a HP tank. ISN'T a OVERFILL. Yea, I like to go down with the most gas I can. If I see something I like. I stay awhile. If not. I don't.

For shallow short dives, I don't let them overfill my tanks. But, for a deeper dive, the extra 500 psi comes in handy added to the reserve..

I fill my tanks. And on a 3500PSI tank. A fill to 3500PSI isn't a overfill.

If you prefill your tanks and store them for your next dive, whatever it may be, then how are you planning your mix for depth?.

Never said I prefill my tanks and store them. I said. They LP's are easier for me to Mix then HP's. Don't have to use my booster.

I believe this is really all a question of whether you want or need the extra gas, and whether you are willing to dump the unused gas after the dive so you can start a new mix..

Why not dump it anyway. A Nitrox fill is a Nitrox fill. Filling 50% of a tank vs. 100% doesn't save me any $. Now if I want an air topoff through the O2 filters, then they do it for free. But, I end up with a light mix.

Well if you'd of just dumped it. Thinking you wanted another mix. You would have to pay for air.

I mix my own. LP's are just easier. If you have a $40,000 mix set up. Yea, it's not a big deal. Not all shops have them. I even know shops that don't mix Nitrox Period. And they are the only shop in the area.

 

[mempilot]

1. And a 3500psi fill. On a HP tank. ISN'T a OVERFILL. Yea, I like to go down with the most gas I can. If I see something I like. I stay awhile. If not. I don't.

2. Never said I prefill my tanks and store them.

3. Well if you'd of just dumped it. Thinking you wanted another mix. You would have to pay for air.

4. I mix my own. LP's are just easier. If you have a $40,000 mix set up. Yea, it's not a big deal. Not all shops have them. I even know shops that don't mix Nitrox Period. And they are the only shop in the area.

1. My bad, I mean't to use 3443 and up. My brain wasn't firing on all cylinders earlier.

2. I was just asking on the prefills. I do it occasionally if I know I'm not going to need a specific mix in the next dive. ie. shallow.

3. My shop tops off my used nitrox with filtered air for free. I may start with a 36 before a dive, breath it, top it off and get a 30 out it for free. When I start with a 32 or less, breath it, the top off isn't worth it since the O2 drops too much to be useful on another dive.

4. I don't mix my own. Don't have a garage or basement, so I'm stuck with the LDS. I'd love to own a filling station though.

Sorry for the mix up.

 

[d33ps1x]

Buy HP tanks and fill em LP when and if you feel like it. For singles I use PST HP E7-100's and even with a crap fill I get a 90 cf tank thats smaller than an aluminum 80. If I fill it low pressure its a nice small 80.

DIN valve with yoke doughnut insert for quick conversion either way.

It's win win umm win and er...win situation.

 

[NEWreckDiver]

Well, if you have a 3500psi HP. And only use it down to 2400psi. It was filled with a Nitrox blend and your shop does Partial Pressure blends. You have a problem. If your O2 tank is at 2400psi. Can't get enough O2 in to mix another blend. You have to dump the whole tank. And start from scratch. Or dump a good part of the tank.

Make sure your shop doesn't Partial Pressure blend. And you don't want to get into blending your own with a HP without a booster.

If your shop has a Booster. It's not a problem. I have a small one myself. But I still us my LP's for Mix. And HP's for Air. But sometimes I do it the other way. It's easier in LP's though. Around here. Not one shop that has a booster. Not in every area. But here it is.

If the only way to partial blend a tank was to use straight O2 froma K bottle, then you could be right.

But we know that is not the case. I bank 32% and 50%. I can then blend anything I need regardless of the remaining gas in a tank. Now you do have to have High Pressure Banks. Ours run to about 4500psi.

With having so many different tank sizes, LP/HP, Singles/Doubles, You need to be prepared for anything.

 

[Mverick]

If the only way to partial blend a tank was to use straight O2 froma K bottle, then you could be right..

In my post. I mentioned using a Booster.

But we know that is not the case. I bank 32% and 50%. I can then blend anything I need regardless of the remaining gas in a tank. Now you do have to have High Pressure Banks. Ours run to about 4500psi. .

Banking a 50%? On a Rix I hope. Others aren't supposed to go that high. But your risk your boom.

Can you Blend me a 80/20 deco mix? On that 50% bank? With the High Pressure bank? Nope... Gotta use a booster. Unless you bank 100% O2 at 4500psi. Which is possible. Just need a booster. Or a Rix O2 Compressor.

Never said it was the only way. Just the easiest and cheapest. I don't want to invest $40,000 in a panel.

With having so many different tank sizes, LP/HP, Singles/Doubles, You need to be prepared for anything.

Yea, total count I have is 29 tanks. LP/MP/HP's. Singles and duals. And 2 Rebreathers. With corresponding tanks.

 

[NEWreckDiver]

In my post. I mentioned using a Booster.

Banking a 50%? On a Rix I hope. Others aren't supposed to go that high. But your risk your boom.

Can you Blend me a 80/20 deco mix? On that 50% bank? With the High Pressure bank? Nope... Gotta use a booster. Unless you bank 100% O2 at 4500psi. Which is possible. Just need a booster. Or a Rix O2 Compressor.

Never said it was the only way. Just the easiest and cheapest. I don't want to invest $40,000 in a panel.

Yea, total count I have is 29 tanks. LP/MP/HP's. Singles and duals. And 2 Rebreathers. With corresponding tanks.

Well we both know that there is more than one way to skin a cat.

No, I don't and won't dream of storing O2 at such a high pressure. You just lay in the O2 first and then add the appropriate top off gas. And I agree. All of this would be hard without a fill panel.

This tread start as “Which is better, LP or HP”. I think we would both agree. It is completely a personal preference.