Double LP85's or HP100 - Diving Wet-Steel Backplate

[StefinSB]

@macado where are you getting those numbers from? That is only true with the Worthington LP85's against normal HP100's or the newest FX100's *which are closer to 120's than 100's* from Faber against their LP85's. "normal" HP100's are 2" shorter and bigger around than LP85's, but anyway, that's neither here nor there.

To the OP, your configuration is not a safe diving configuration. I'll just go straight to the point, and here is why. 7mm suits have no business doing deep diving due to the suit compression, especially if it is a farmer john. You also shouldn't dive tanks that are that big in a wetsuit because at depth the odds of you being able to kick them up from the bottom in the event of a wing failure are basically 0. Compound that with the fact that a steel backplate will grossly overweight you to the point that it is idiotic, you're asking for trouble. Your wing has to do 2 functions. Hold the rig at the surface, and compensate for buoyancy. Let's do some math.
Bands-4lbs
Manifold/valves-6lbs
Backplate-6lbs
Regulators-4lbs
Weight of gas-roughly 16lbs
Buoyancy of tanks-anywhere from -7 with Worthington HP100's to 0 with LP85's
You are at a minimum of -36, likely closer to -40lbs at the surface.

Wetsuits have a buoyancy compensation of roughly 3:1. I.e. 1lb of wetsuit at the surface requires about 3lbs of lead to sink. 7mm's are roughly 5-7lbs for most people, so 15-20lbs of lead to get it to sink. You are now at a minimum of 51lbs, potentially up to 60lbs lift requirement out of your wing. You are starting the dive naturally overweight because the the ballast of the empty rig exceeds the requirements of your exposure protection unless you're diving LP85's in a 7mm farmer john, in which case it is still not a safe configuration due to the buoyancy swing. IF you're going to do this anyway, at least do it with a neutral or close to it plate instead of stainless steel, especially if you go with HP100's where you are going to be close to 5lbs overweighted even with a kydex/aluminum plate.

Also remember your exposure. You define cold water, but don't say how cold. 100ft with HP100's on a normal diver, in open water, will net you well over an hour of bottom time, which at depth in a 7mm means you will get COLD, and quite possibly dangerously cold increasing your likelihood of getting DCS. Drysuit in deep cold water diving is a safety concern so please do yourself a favor and consider purchasing one....

I am not sure I understand where your 51-60# of lift requirements come from. You list fixed weight of rig (using LP85s) as 20#. You also state the 7mm suit needs roughly 20# to sink. So no additional lead is required in this scenario. The gas adds 16# making the required minimal lift 20+16=36. What do I miss?

 

[tbone1004]

One thing that the OP asked that we haven't touched on much is HP vs LP. Each has its place. If I am diving in a remote area we sometimes have to top off via crossover from AL80's. In this case the LP tanks have an advantage since you can get closer to a full fill than an HP tank. I have doubles that are LP95s, HP100s, HP120s, AL80s and AL40s and select based upon the dive, environment and logistics.

With that said though, I concur that if you're wet, go with AL80s.

please stop propogating this quite simply untrue statement. This myth stems from tanks in the US being measured indirectly, i.e. how much gas they hold at working pressure. The simple fact of why your statement is wrong, and we'll use Faber because they are the only tanks available in the US currently being manufactured is that a FX117, and a LP95 are the exact same water volume. I.e. it's the SAME tank regardless of what pressure you fill it to. If you fill that 117 to 2640, it will hold, wait for it, 95cf. Conversely, if you fill that lp95 to 3442 it will hold 117cf. So HP and LP are completely stupid, buy HP always unless you can guarantee cave fills of the low pressure tanks, and just plan accordingly based on the water volume of the tanks, not the stupid indirect measurements.

Stefin,
correct, my apologies, apparently I shouldn't math while travelling...

 

[kensuf]

Not that I have any cave training, either.

You know the risks, techniques, and requirements and can finish the first step any time you want to come down for a day. Most importantly, you have maturity and respect for the environment.

 

[stuartv]

Stefin,
20lbs rig+16lbs gas=36lbs
you now have to add back in the 20lbs of the wetsuit because it will compress at depth which means it loses it's positive buoyancy. It won't lose all of it, but 3/4 is quite reasonable to expect which means it will only take about 5lbs to sink, so 36+15=51lbs required at depth

I'm lost, too.

-20# rig + -16# gas + 20# suit = -16# at the surface. I.e. perfectly weighted to be neutral with empty tanks.

If you descend and your suit loses 15# of its buoyancy, you'll hit the bottom 31# negative, not 51, right?

 

[tbone1004]

edited, you're right. I apparently can't math properly when I'm not paying attention...

 

[IDS_Bill]

please stop propogating this quite simply untrue statement. This myth stems from tanks in the US being measured indirectly, i.e. how much gas they hold at working pressure. The simple fact of why your statement is wrong, and we'll use Faber because they are the only tanks available in the US currently being manufactured is that a FX117, and a LP95 are the exact same water volume. I.e. it's the SAME tank regardless of what pressure you fill it to. If you fill that 117 to 2640, it will hold, wait for it, 95cf. Conversely, if you fill that lp95 to 3442 it will hold 117cf. So HP and LP are completely stupid, buy HP always unless you can guarantee cave fills of the low pressure tanks, and just plan accordingly based on the water volume of the tanks, not the stupid indirect measurements.

Stefin,
correct, my apologies, apparently I shouldn't math while travelling...

You are correct - math is not your strong suit.

HP vs LP on a crossover. This is about the equalization of pressure. The most basic example of this would be if you have LP95 doubles at 2640 and HP100s at 3442.

Breathe 25cuft out of each

Now you have 2293 in the LPs and 3012 in the HPs

Now crossover with an AL80 with 3000 psi in it

You will get 25 cuft in the LPs (equalizes at 2646 actually)

You get nothing into the HPs because they are already above 3000

Oh and if you fill LP95s to 3442psi you get 124cuft

 

[tbone1004]

i was going to leave this alone, but you are so far gone into some made up fantasy land with tank volumes that this has to be corrected. I'm not sure what they taught you about tanks in your technical instructor course, but it clearly had no bearing on reality since you are trying to make an argument for LP tanks when you are comparing an apple to cabbage.


For this, we are ignoring the fact that gas doesn't behave linearly as pressure goes up because that's a lot of math that isn't really important to this argument, because unlike your LP vs. HP argument, we are going to compare similar tanks.

Please compare the same tanks if you're going to continuing spewing this nonsense. Your LP95 vs HP100 argument is comparing a 15l tank to a 12l tank. They share a nominal length, and a nominal capacity when filled to their working pressure, at the same temperature, that's it. This rating is about as useful as a SAC rate expressed in psi/min. If you were comparing a LP95 to a Faber FX100 it would be a 15l against a 13l which is slightly closer, but still quite a ways off from being a useful comparison.

Yes, the 95's filled to 3442 hold 124, but the 117's from Faber hold 125 if you don't factor in the non-linear capacity at high pressure. You know why this is true? They hold the SAME water volume of 15 liters. The cubic footage numbers are not accurate for hardly any tanks because they are all nominal values. Fabers new FX100 holds 109cf because it is a 13l tank instead of most HP100's which are 12l. You have to go off of the direct measurements not nominal indirect measurements, and if you are going to continuing spewing this nonsense, please at least know what you're talking about.

Perfect case, an AL80 only holds 77.4cf. If you actually did the math based off of the fact they hold 11.1l everything is fine, but if you base the math off of their nominal value, then everything is off by around 3% against your for consumption calculations because the tank is smaller than you think it is, or around 8% in your favor if you think the Faber FX100 actually holds 100cf. 8% isn't a whole lot, but in technical diving it is rather important if you're planning real dives.

TLDR: there is no reason to ever not choose a HP tank unless you need specific buoyancy characteristics of the LP tanks which are less negatively buoyant in general *with some exceptions like the PST LP104's*

 

[IDS_Bill]

i was going to leave this alone, but you are so far gone into some made up fantasy land with tank volumes that this has to be corrected. I'm not sure what they taught you about tanks in your technical instructor course, but it clearly had no bearing on reality since you are trying to make an argument for LP tanks when you are comparing an apple to cabbage.

Please compare the same tanks if you're going to continuing spewing this nonsense. Your LP95 vs HP100 argument is comparing a 15l tank to a 12l tank. They share a nominal length, and a nominal capacity when filled to their working pressure, at the same temperature, that's it. This rating is about as useful as a SAC rate expressed in psi/min. If you were comparing a LP95 to a Faber FX100 it would be a 15l against a 13l which is slightly closer, but still quite a ways off from being a useful comparison.

Yes, the 95's filled to 3442 hold 124, but the 117's from Faber hold 125 if you don't factor in the non-linear capacity at high pressure. You know why this is true? They hold the SAME water volume of 15 liters. The cubic footage numbers are not accurate for hardly any tanks because they are all nominal values. Fabers new FX100 holds 108cf because it is a 13l tank instead of most HP100's which are 12l. You have to go off of the direct measurements not nominal indirect measurements, and if you are going to continuing spewing this nonsense, please at least know what you're talking about.

Perfect case, an AL80 only holds 77.4cf. If you actually did the math based off of the fact they hold 11.1l everything is fine, but if you base the math off of their nominal value, then everything is off by just over 3% against your for consumption calculations because the tank is smaller than you think it is, or 7.5% in your favor if you think the Faber FX100 actually holds 100cf. 8% isn't a whole lot, but in technical diving it is rather important if you're planning real dives.

Okay, according to tbone math then if you breathe 25 cuft from a full set of HP100's and then crossover from an AL80 with 3000psi in it, how much air will you wind up with in the 100s?

Not sure why you are taking this so personally but maybe you're just 'that guy'

 

[StefinSB]

edited, you're right. I apparently can't math properly when I'm not paying attention...

No worries. I thought there was some gravitational change between the west and east that caused that ;-)

 

[tbone1004]

ok Bill, someone has clearly told you some very wrong stuff, it isn't necessarily your fault, but you have to come around to this, especially as a technical instructor because this is literally the basis of the ideal gas law where PV=nRT

We will use Faber tanks because they are the only new ones available, and we will assume a linear capacity increase regardless of fill pressure.

You mention HP100's with 25cf taken out of them, so that means they are at 3030 PSI, and now hold 193cf. Crossing over from an AL80 will do nothing which was your point.

Unlike you, we will actually compare that to the low pressure equivalent, which are the LP85's and hold 83.5cf at working pressure. If we fill those tanks to 3030 psi we get 83.5*2*3030/2640= roughly 215cf. Some rounding errors converting from imperial to metric, but they are the same nominal capacity of 108cf, and they have to be because they hold the same water volume. Please also remember that when below the working pressure of that AL80, because the LP95's are bigger tanks volumetrically, they will have a lower increase in pressure when transfilling.

You are also carrying a tank that is 3lbs heavier than the HP100, and if you are having to wear lead, you have an extra 2lb buoyancy disadvantage which means your total rig weight is 5lbs heavier for the same nominal gas volume at working pressure. If you compare the LP85 to the HP100 which is the fair comparison, you realize that while the HP100 is 3lbs heavier on land, it is also 3lbs heavier in the water so the total rig weight is identical for both of the tanks. The advantage you have of the HP100 is never having to fight a dive shop to fill over the working pressure to get some extra volume. Same math applies for the LP95 vs the FX117 which are nominally identical tanks where the FX117 is 1.5lbs heavier on land, but is also 1.5lbs more negative in the water so net weight change is 0.

To follow your example set above. A set of lp85's filled to 3442, with 25cf removed will be at the exact same pressure as the FX100's. Thus, the only reason to ever choose the LP85's over the FX100's is if the 3lbs/tank more negative of each FX100 is problematic either on land, or in the water. Many cave divers will continue to choose the LP85's because of their buoyancy characteristics, but there is also no problem getting 3600psi cave fills. If I was diving in cold water where I needed that extra ballast, or diving in a location where cave fills weren't possible, then there would be absolutely nothing that would convince me to ever purchase a LP tank if I had the HP equivalent available. The problem is instructors and divers will often talk about the same math that you just did, but since you aren't comparing equivalent tanks, the argument doesn't matter. Tanks of the same volume will hold the same amount of gas at the same pressure regardless of what their nominal capacity is at whatever working pressure the manufacturer determines. That is the absolute basis of the ideal gas law. You are not comparing vessels of the same volume, and because of that the argument is irrelevant.