LP vs HP

[raymond phule]

2. In most cases if you take a 3442 psi tank and a low pressure tank of the same external dimensions and similar weight and fill them to the same tank pressure, you get nearly identical gas volumes.

This doesn't seem to be correct in general... You are saying that the inside-, outside volume and the weight are the same for both a low- and high pressure tank. A 10 liters 300 bar steal tank is heavier compared to a 10 liters 200 bar steal tank.

Am I missing something? Like are all LP tanks aluminium or something?

Your #2 would suggest that the LP and HP tanks are identical for all practical purposes (except the max pressure) and the obvious choice would be to go with the HP tank.

 

[Jimmer]

Your #2 would suggest that the LP and HP tanks are identical for all practical purposes (except the max pressure) and the obvious choice would be to go with the HP tank.

Yeah they essentially are. There might be a very tiny size difference like 1/4", or 1 pound in weight, but for all intents and purposes they are the same.

Edit: The buoyancy characteristics are a little bit different as well, which is one reason some people prefer one over the other.

 

[hudson]

Jimmer said it, but in case you needed more affirmation, for a given physical size of tank, volume in liters and weight characteristics are largely the same regardless of LP or HP...for your average diver LP doesn't make much sense (there are fringe issues of course - cost, pressure your equipment runs at, you may want more weight/bigger size for trim purposes etc)

 

[raymond phule]

Yeah they essentially are. There might be a very tiny size difference like 1/4", or 1 pound in weight, but for all intents and purposes they are the same.

Ok, then should HP tanks be better in general (if you buy a tank with a specific weight and outside size).

I see a differene between europe though. We have 200, 232 and 300 bar tanks, (2900, 3362, 4350psi tanks) and a high pressure tank are much heavier compared to a lowpressure tank at the same inside volume.

 

[Jimmer]

Ok, then should HP tanks be better in general (if you buy a tank with a specific weight and outside size).

I see a differene between europe though. We have 200, 232 and 300 bar tanks, (2900, 3362, 4350psi tanks) and a high pressure tank are much heavier compared to a lowpressure tank at the same inside volume.

In my opinion, yes HP tanks are generally the better choice, but some people will choose LP for some specific criteria. I own a set of double Lp 108's, and bought them used because the price was right. If I purchase another set of big doubles in the future, I will buy Hp tanks this time.

That makes sense to me that your 300 bar tanks would be very heavy, I would expect there to be much thicker walls on those cylinders.

 

[Divin'Papaw]

This doesn't seem to be correct in general... You are saying that the inside-, outside volume and the weight are the same for both a low- and high pressure tank. A 10 liters 300 bar steal tank is heavier compared to a 10 liters 200 bar steal tank.

Am I missing something? Like are all LP tanks aluminium or something?

Your #2 would suggest that the LP and HP tanks are identical for all practical purposes (except the max pressure) and the obvious choice would be to go with the HP tank.

What he is saying is that, for example, a LP104 is just about idential externally (height, diameter, weight) to a HP130. PST LP104 is 26.88" high, 8" in diameter, weighs 46 lbs empty, is 3.3 lbs negative empty and 11.27 lbs negative full. A Worthington X8-130 is 25.5" high, 8" in diameter, 43 lbs empty, is 2 lbs negative empty and 11.7 lbs negative full.

If you took a HP130 a filled it to just 2640 psi, you'd have around 100 cf of air. But if you fill it to 3442, you'd have 130 cf of air. If you took a LP104 and filled it to 3442, you'd have around 135 cf of air. The capacity of the two tanks is pretty darn similar.

The key difference is that taking a HP130 and underfilling it to 2640 is an easy thing to achieve at pretty much any dive shop. You also can very easily get it filled to 3000 psi (giving you 113 cf) and most places can fill it to 3442 psi (130 cf). However, with a LP104, the only way you'll be able to get it filled to 3442 psi (or higher) is to (1) go to cave country in N Central FL or (2) setup your own home fill station and fill it yourself. Those two options are not really possible for the majority of folks.

It's also important to remember that with LP tanks, you must go through the extra effort every 5 years to find a cylinder test facility who will retest your cylinder to the level required to maintain the + exemption. If you don't do that, once your tank is over 5 years old you can only get it filled to 2400 psi (again unless you live in N FL or fill it yourself) and the LP104 then becomes essentially a LP95.

It's this kind of reasoning that has lead me to the HP tanks. If I wish, I can underfill them and they are therefore essentially LP tanks. I can also fill them to rated pressure without breaking any "rules." I also don't have to worry every 5 years about maintaining the + rating.

 

[cummings66]

A 10 liters 300 bar steal tank is heavier compared to a 10 liters 200 bar steal tank.

I think you got it backwards, a LP tank of the same volume will be heavier than the HP thank will be.

 

[redacted]

I think you got it backwards, a LP tank of the same volume will be heavier than the HP thank will be.

No, I believe Rayhmond is correct. The 10 liters is a measure of internal volume of the empty tank.

 

[vshearer]

I picked up a new Faber LP108 at a REALLY good price and it looks really cool so I am very happy. Since I can get it filled how I want it works out as a good 120cf.

 

[raymond phule]

I belive we missunderstood each other here because we are used to different ways to give the size of the tank.

I have found a table
Scuba Cylinders

We have a tank with an inside volume V_i and weight W. Fill it to a low pressure, say 2640 psi and we get a volume of V_l of air. Fill it to a high pressure say 3500 psi and we get V_h of air.

In the US the is the tank size V_l or V_h depending on the rated max pressure. In europe is the tank size the volume V_i.

2. In most cases if you take a 3442 psi tank and a low pressure tank of the same external dimensions and similar weight and fill them to the same tank pressure, you get nearly identical gas volumes.

This is the same as saying that a LP and HP tank of the same outside volume and weight has the same inside volume V_i, i.e. almost identical physical tanks. DA aquamaster then argues that you because of this always can use a HP tank with a lower pressure without losing anything.

This doesn't seem to be true though. A HP tank with the same inside volume V_i as a LP tank is heavier compared to the LP tank for the steal tanks in the table linked to above (you need to do some calculations) and for 200 vs 300 bar tanks used in europe.

HP and LP tanks with the same volume of air, V_l and V_h seems to have about the same weight but the HP tank is more negative, i.e. smaller outside volume.

Bying say a 120 HP tank and fill it to 2770 psi is going to give the same volume of air as a LP 95 at normal pressure but the HP tank is going to be 8 pounds heavier compared to the LP tank. This contradics DA aquamaster above.