Dissimilar tank volume calculation....made easy???

[prufrock]

Hey everybody,


I am scheduled to take apprentice next week and one of the BIG stumbling blocks that I have encountered is figuring out volumes/pressures when diving with partners with a dissimilar tank size. Does anyone have an easy, no-nonsense way of explaining how to do this for someone who is most definitely right brained and mathematically challenged? It would be much appreciated.


thanks


Rob F.

[ZuluMich]

After doing baseline calculations and looking at a standard spg a good 10% or 100 psi rule of thumb helps. Most cave tank setup a fall into main categories... Lp 85's/hp 100's, then 95's, then 104/108/hp 130's. As far as the accuracy of spgs are concerned, 95's are roughly 10% greater than 85's and 104/108/hp 130 are roughly 20% greater than 85's or 10% greater than 95's. Or even easier in water just use 100 to 200 psi subtraction from thirds for those that are not the controlling diver after pressure matching.

[PfcAJ]

I find it easiest to work in cubic feet when doing these calculations.

First, it helps to establish an easy way of dealing with volume calculations. I use a tool called "tank factors", which makes it easy to move from cubic feet to psi and back.

TF=(volume/fill pressure)/100.

Double Lp104s (and such, like X8-130s, LP108s) have a TF of 8
Double 95s (X8119s) are a 7
85s are 6
al80s are 5
And for you body builders out there, lp120s are 9

So how much volume does a set of full 104s have? 8(the tank factor) x36 (psi/100)=288cuft (For all you Albert Einstein E=MC^2 people out there, its scuba math. Calm down if its not exactly to the decimal point accurate).

A set of lp85s holds ~216cuft @3600psi (7x36).
A single al80 holds ~ 75cuft @ 3000psi (2.5x30)
A set of DOUBLE al80s @ 1500psi has ~75cuft (5x15). Its 5 because its a set of doubles, and its 15 because they're at 1500psi.

Make sense? I hope so! So now that we can easily get cubic feet from the tank size and the pressure, wtf do we do with it?

Simple. Take the smallest volume. THATS the volume that dictates the dive plan. It doesn't matter if you've got the worlds largest scuba tanks, the dude with the smaller tanks is the volume that the dive is planned with.

So you've got 104s @ 3600 and your buddy who always skips legs day has a set of al80s. You've got 288cuft, but he's only got 150cuft. Divide 150 by your gas plan (thirds, fourths, sixths, whatever) and that's the usable volume for YOU with your big fancy tanks.

150cuft/3=50cuft usable to penetrate.

How many psi is 50cuft in your 104s though? $#!* more math!? Don't sweat, its just as easy as the stuff we just did.

50cuft/8(your tank factor)=6.25

Move the decimal over 2 spots and you get 625. That's YOUR usable PSI to penetrate. 50cuft = 625psi in double 104s. Boom. Done.

Once you know the tank factor for common tanks, you're set. Here are your cheat sheet formulas:

TF x (psi/100)=volume in your tanks. (An easy way to divide by 100 is to just move the decimal over to the left 2 spots. 2400=24, 1700=17, and so on)
Volume/TF=PSI/100 Move that decimal over to the right 2 spots to get your actual PSI

That's it man. 2 simple formulas and you're in business.

[Max Speed]

Waterproof calculator watch from eBay $14.

Good to 130 feet or leave on your o2 bottle. Just don't push a button underwater.

[Greenwood_60]

Dive sidemount with more gas than your buddy.

[fno]

Rob, not to detract from the answers already given and they are worth listening to. I am curious why you wouldn't want to get this info from the person you are paying a lot of money to for the exact same information. Hopefully, said instructor will spend the requisite time needed to make sure that you probably will never forget how to do dissimilar gas calcs using method that suits your thought processes. Good luck with the class, by the way.

[ZuluMich]

Tank factor = baseline

[ZuluMich]

Rob who is your instructor?

[Moonfuzzy]

It's best if you memorize the ratios between your tanks and a few other standard tanks, and pull out a calculator for the rest.

Ex- I know LP 95s are about 20% bigger than my HP 100s. My buddies using 95s need to reduce their thirds by 20%. If my thirds 1200 they have to reduce their thirds to 1000


- Moonfuzzy

[nakatomi]

I'll take a shot at a “simple” explanation

First of all, the objective behind the calculation is as follows.
Imagine Mr. Air hog with huge tanks reaches thirds at the same time as his buddy Mr. Lightweight with small tanks.
Assuming Mr. Air hog loses his gas at that point, the remaining two thirds in the small tank of Mr. Lightweight wouldn’t bring both of them out of the cave.
That's the reason why we need to match tanks.

So how do we calculate who uses what?
First of all you figure out who carries the least amount of gas, by calculating how many cubic feet (or liter) of gas each diver has in his tanks.

The diver with the least amount of gas may use full thirds (or sixths) of his gas.
The second diver may use only the same amount of gas, even though his tanks may contain significantly more gas.

I think the math is not all that hard when the concept is understood.
Now here's a shot the math, in sidemount.

1. Mr. Air Hog is diving two large Faber LP120 tanks, which are rated at 2640psi.
This means at 2640psi they contain 120cuft of gas.

How his SPG’s say he has a cave fill of 3500psi in each tank, so we need to figure out how much gas this equals to in cuft.

Again, Mr. Air Hogs tanks contain 120cuft of gas at 2640psi (their rating).
To calculate the amount of gas at 3500psi, you simple divide 120 by 2640 and multiply by 3500.
The result is, Air Hog carries a whopping 159cuft of gas per tank for a total of 318cuft

2. Mr. Lightweight is diving two Faber LP80 tanks, which are rated at 2640psi
The SPG’s on his tanks show 3400psi.
Now 80 divided by 2640, then multiplied by 3400 is 103cuft per tank. Or 206cuft in total for both tanks

Obviously Mr. Lightweight carries less gas than Mr. Air Hog.
With only 206cuft of gas (in contrast to the 318cuft Mr. Air Hog has in his tanks) Mr. Lightweight, as the diver with the least gas, determines the usable gas.
Both divers may use no more than one third of 206cuft, which is 68.57cuft of gas

3. Calculating the turn pressures

Next we need to figure out the turn pressure of the individual divers.
Again, both of them may use a maximum of 68.57cuft of gas

For the diver with the least gas, Mr. Lightweight, it is simple. He may turn at 2/3 of his starting pressure.
In his case this would be 2/3 of 3400. which would be 2267psi.
2400psi would add a little safety margin and be a safer turn pressure.

->Mr. Lightweight must turn at 2400psi

For Mr. Air Hog, who may also use 68.57cuft of his gas, we can calculate his turn pressure as follows.

His total gas of 318cuft subtracted by 68.57 is 249.43cuft. Since he is diving in sidemount, we will divide this by two and he must turn when he has reached a level of 124.72cuft in each tank.

To calculate the turn pressure in psi, we remember that he has a starting pressure of 3500psi and 159cuft in each tank.

We divide the starting pressure of 3500psi by 159cuft and multiply that by 124.72cuft.

As a result, his turn pressure is 2745psi, rounded and with a safety margin, 2900psi

->Mr. Air Hog must turn at 2900psi

As you can see from the examples of others, there are different ways to do the math.