what sidemount tanks do you dive with?

[JBFG]

I use HP100s with my drysuit in fresh water. I only need 4lbs extra for weight.
I’m 5’6” and the length is perfect for me.
in tropical or wetsuit situation I dive AL80s

 

[SJT1961]

This isn't true. At 3,600 I think you will get around 105 cu ft. They are the virtually the same size internally. <1% difference.

I use LP85s. Sold the HP100s.
I sometimes only fill them to 3,600. Usually a bit north of there. Depends on the dive.

A LP85 holds 85 ft^3 only at +10% pressure of 2640. At 3600 psi: 3600/2640 *85 = 115 ft^3.
A HP100 holds 100 ft^3 at rated pressure of 3442.
So I believe the difference is 15%.

 

[Andrew Dawson]

I love my LP72 steels with normal fills...I do open water dives only...within rec limits...in cold water with a drysuit. They are just the right size and weight for me. Not too big to get me into too much deco...not too small to limit me...and they are cheap!

 

[RayfromTX]

A LP85 holds 85 ft^3 only at +10% pressure of 2640. At 3600 psi: 3600/2640 *85 = 115 ft^3.
A HP100 holds 100 ft^3 at rated pressure of 3442.
So I believe the difference is 15%.

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

LP85: Steel BS85 (Blue Steel/Faber) † 13.0 L 2640 psi 82.5 ft3
HP100: Steel X7-100 HDG (XS Scuba/Faber) 12.9 L 3442 psi 106.7 ft3

These are the real numbers. The important one is the capacity. The LP85 is 13.0 liters. The HP100 is 12.9 liters That is a difference of 0.77% which is less than 1%

Three things confuse the issue; the inaccuracy of labeled capacity, the ideal gas laws and the compressibility of gas being non linear which creates a diminishing return.

You have to look at the actual numbers rather than the labels. The LP85 is rated at 81.1 cu ft at it's rated pressure of 2640 due to the Z factor of 1.0173. The hp100 holds 101.3 cu ft at 3442 psi. The Z factor of air at 3442 is 1.0532. The Z factor of air at 3,600 psi is 1.059.

The LP85 ideal gas capacity is 82.5 cuft. at its service pressure of 2,640. If you fill it to 3,600 psi the equation goes like this. 3,600/2640*82.5=112.5/1.059(Z factor)=106.23 cuft.

The HP100 ideal gas capacity is 106.7 cuft. at its service pressure of 3,442. If you fill it to 3,600 psi the equation goes like this. 3,600/3,442*106.7=111.6/1.059(Z factor)=105.38 cuft.

You get less than one cuft additional gas into the LP85 at 3,600 psi than the HP100 at 3,600 psi.

The Z factor is an expression of the fact that at higher pressures gas molecules don't compress as tightly and so the higher you compress the less each additional 100 psi adds to the volume of compressed gas.

Here is a link to a handy chart. BAUE Z Factor Table

Of course if you cave fill your LP85 you will get much more than 85cuft but you won't get more than your HP100. This matters if you are really counting on having 15 additional cuft and really have only .85 cuft additional.

I hope I didn't explain this in a clunky fashion.

 

[Bierstadt]

Calculating SCUBA Cylinder Capacity | Dive Gear Express®

LP85: Steel BS85 (Blue Steel/Faber) † 13.0 L 2640 psi 82.5 ft3
HP100: Steel X7-100 HDG (XS Scuba/Faber) 12.9 L 3442 psi 106.7 ft3

These are the real numbers. The important one is the capacity. The LP85 is 13.0 liters. The HP100 is 12.9 liters That is a difference of 0.77% which is less than 1%

Three things confuse the issue; the inaccuracy of labeled capacity, the ideal gas laws and the compressibility of gas being non linear which creates a diminishing return.

You have to look at the actual numbers rather than the labels. The LP85 is rated at 81.1 cu ft at it's rated pressure of 2640 due to the Z factor of 1.0173. The hp100 holds 101.3 cu ft at 3442 psi. The Z factor of air at 3442 is 1.0532. The Z factor of air at 3,600 psi is 1.059.

The LP85 ideal gas capacity is 82.5 cuft. at its service pressure of 2,640. If you fill it to 3,600 psi the equation goes like this. 3,600/2640*82.5=112.5/1.059(Z factor)=106.23 cuft.

The HP100 ideal gas capacity is 106.7 cuft. at its service pressure 3,442. If you fill it to 3,600 psi the equation goes like this. 3,600/2,640*106.7=111.6/1.059(Z factor)=105.38 cuft.

You get less than one cuft additional gas into the LP85 at 3,600 psi than the HP100 at 3,600 psi.

The Z factor is an expression of the fact that at higher pressures gas molecules don't compress as tightly and so the higher you compress the less each additional 100 psi adds to the volume of compressed gas.

Here is a link to a handy chart. BAUE Z Factor Table

Of course if you cave fill your LP85 you will get much more than 85cuft but you won't get more than your HP100. This matters if you are really counting on having 15 additional cuft and really have only .85 cuft additional.

I hope I didn't explain this in a clunky fashion.

Good job, I was thinking of doing this calc but didn't have time. You flipped your hp100 3442/3600 numbers, but good other than that.

 

[isr_tt89]

I use steel 12L cylinders that are rated to 200 bars. I usually stay within the 200bar, but sometimes when there is a need I'll go up to 220 bar. The tare weight is 12.2kg so they are almost neutral when empty.

They weight 2.2kg / ~5lbs less than AL80 and has a bit greater volume.

And as freedom units
Volume per cylinder is 84 cu.ft. at rated pressure of 2900psi ( 93 cu.ft. when overfilled to 3190psi)
The tare weight is about 27lbs.


PS. I'm rather new to sidemount, had these tanks laying around unused and figured that they would suit this purpose just fine. My dives don't require SM system but i just like to experiment with different things.

I dive the same size of tanks and 99% time they are at 200 bar.
But they are rated for 232 bar.

 

[RayfromTX]

Good job, I was thinking of doing this calc but didn't have time. You flipped your hp100 3442/3600 numbers, but good other than that.

Thanks, I see I typed 2640 instead of 3442 but I entered the numbers correctly in the calculator so the results are correct. I have edited the typo.

 

[Jim Lapenta]

I use LP 85's, LP 50's, LP 72's, al 80's, LP95's (rarely), and my favorites are the LP75.5's I have. I fill myself so no worries getting good fills. The 75.5's are good for anything up to around 130 ft for me with bottom times up to 35-40 minutes. Over that the 85's or 95's get called into play.

 

[vinegarbiscuit]

I had a set of Worthington 85s and a set of Faber 95s, as well as LP45s and 50s as bailout for small-ish rebreather dives. I loved them all, albeit for different reasons; the Faber 95s were my favourite, since I am petite and short-torsoed, and the tanks fit me perfectly. I also loved their buoyancy characteristics - they were rock solid, even when purged down to virtually nothing as an experiment (the Worthington 85s were also marvellous in that regard). Converting to aluminium 80s, which is the default tank here in Mexico, was a shock to the system!

 

[_Ralph]

Used to be nicely filled 85’s, but that shop has closed for ‘exploration’ reasons...

Now I get just ‘can’t argue with the filler’ filled 85’s until I get a booster (use them for DILOUT)

_R