Dissimilar tanks

[SLIM]

K lets start a new thread.

Most of you all are all kind of right I think but to be able to figure D T is more inportant then you may wish to think but lets look at it like this and then make your own decisions.

Diver A has lp 85's filled to rated PSI that gives him 170 cft
Diver B has lp 104's filled to rated PSI that gives him 208 cft

Diver B has 38 CFT more then Diver A. Dive is planed for 1/3 well if Diver B hits his 1/3 before Diver A does and they are at Max penatraion then how much gas is left for them both to exit with?

It is way to easy to figure out and even esyer if you do 1 of 2 things. Both dive same size ( LMAO right, I have to many of many differant sizes myslef and take what I have ready to go) then just be extra nice and invite a 3rd diver along and never wory about it.

We can also throw in that it would only have to be to get back to a deco cylindar or a stage cylindar. There are so many variable tha tcan be thrown in to change it all one way or the other.

I know that many who do not like to teach dissimilar tanks but it is easyer then many make it. It can be done in minutes but it is a simpal thing to figure out and remmeber so RMV or SAC is important.

Just remember if you dont figure it out right, then it iwll be going into both divers gas supply. Will the end be reached. Does anyone wish to make some test and see if does work or not?


SLIM

[skip]

I'm unlcear on just what your post means or if you are asking a general question. But I'll toss in my thoughts anyway.

First, attached is a table that shows typical tank sizes and volumes and thirds across the dissimilar tank volumes. One of the nice things about tank volumes is that once you find the difference between two (or three), it stays the same. For example my buddy dives 119's and I dive 130's. There's 100 psi difference meaning that to match his turn volume I simply deduct 100 psi from my turn pressure. He may have 800 psi usable, which means I can use only 700 psi in my larger tanks. And this applies for all pressures....he has 900, I get 800...he has 1200, I get 1100. When I dive with a new buddy, I just look up the new values in the table and we go diving.

Second, RMV makes no difference whatsoever. This one took me awhile to figure out, cause it makes no sense. You'd think the diver with bigger lungs would use gas faster...and that's true. But you've already calculated thirds and no matter how much gas is consumed, it's always just one third, leaving two thirds for exit. And since the big-lung is the one who turns the dive, the buddy will always have that volume for both to exit (which actually means that the little-lung diver will have extra gas). Of course let's make sure we calculate team thirds and no one panics, viz is good, current is in your favor, etc. If not, then the dive plan should be adjusted to fourths or fifths, not thirds.

Third, stages and deco are irrelevant. You figure those into the dive plan to start with and are diving thirds to include the gas in the stage. The deco bottle is at the end, near cave entrance or hanging at 20 feet. If carried, it's still not usable until 20/30 feet.

After years of diving and calculating thirds, some divers no longer see the need. You can ask your buddy what their spg reads, see their tanks, and know whether to add or subtract and how much from your psi for team thirds. Of course the point of a buddy (one point) is to have that second brain working for you, so open discussion is always best.

skip

[steve2281]

K lets start a new thread.

Most of you all are all kind of right I think but to be able to figure D T is more inportant then you may wish to think but lets look at it like this and then make your own decisions.

Diver A has lp 85's filled to rated PSI that gives him 170 cft
Diver B has lp 104's filled to rated PSI that gives him 208 cft



SLIM

I'm running late. I've got to get to bed, so I'll probably screw this up.

In the example it looks like both sets of doubles filled to 2640.
The LP 85's are the control. EACH diver can use 1/3 of the 85's gas. In this case 56 cu ft.

Without any rounding (like we do in the water before a dive)
Diver A has 875 psi to use and will turn at 1765.
Diver B has 717 psi to use and will turn at 1923.
Both divers have 56 cu ft to use before turn.
The tank factor is all you need to do the figuring.

Skips example of 100 psi difference as you move from 85 to 95 to 104 comes out pretty darn close every time.

[scububa]

A new thread was a good idea. I knew I was pulling the END thread off.

Steve,
First of all, I completely glossed over your OP, probably because of the context of IANTD table exercises, I 'read' into your post 1/3 control by psi. You definitely stated cuft. And, so I see the logic with your statement about controlling volume of the smaller set.

As demonstrated by the example, you have to ratio this to the tank psi's. Which is what the tables are doing. If you are comfortable getting there with calcuations, great, that's how the tables are developed right Anyway, sorry, I got sidetracked in reading your post and came away with pressure instead of volume.

All,
I agree that RMV, SCR, SAC doesn't factor in until, as mentioned previously, you are confirming sufficient gas for the dive (distance, depth, etc.) The reason I mentioned it is because it is relavent to explaining why a dangerous situation occurs when the biggest tank/biggest breather combination is treated with 1/3 turn psi plan. (Again, I misread Steve's post that said cuft not psi.)

[SLIM]

Eye but I have made not only you all think about this but anyone who reads it and they can now see how easy it is to figure out.

Why is it so many Tec instructors are scared to teach it and make it so much harder then what it is?

Remember I just put thoughts out there and make you all think about it and you all can disscuss about it and everyone learns from the many on here that have the real world knowledge.

SLIM

[hudson]

When I read the IANTD manual, it left me scratching my head..

There is another way to look at it, which may be even simpler: instead of knowing the tank factors, all you need is the capacity ratio of the two tanks at rated psi (needs an extra step to normalize the pressure when you are comparing HP vs LP)

So let's take this example:

Step 1: 104/85=1.22 - I'm going to simplify so say 1.2

Step 2: Take diver A's straightforwards 2640/3=880psi usable, divide it by 1.2
880/1.2=733psi usable for diver B

Done. No tank factors necessary.

[b1gcountry]

When I read the IANTD manual, it left me scratching my head..

There is another way to look at it, which may be even simpler: instead of knowing the tank factors, all you need is the capacity ratio of the two tanks at rated psi (needs an extra step to normalize the pressure when you are comparing HP vs LP)

So let's take this example:

Step 1: 104/85=1.22 - I'm going to simplify so say 1.2

Step 2: Take diver A's straightforwards 2640/3=880psi usable, divide it by 1.2
880/1.2=733psi usable for diver B

Done. No tank factors necessary.

Well, kinda. Not all tanks are rated for the same pressure. The tank factor is just a way of normalizing all tank sizes based on a common denominator. Your example works because the two tanks you are talking about are both rated for the same pressure. If they weren't then you will be making a potentially dangerous math error. You really need to divide the tank factor of the two tanks, not their rated capacity.

Consider a LP104 vs a HP100.





If you want to get really easy when calculating dissimilar tanks, take the worst case differential in pressure per third, and always add/subtract that from the non-controlling tank.

For your example, a third of a pumped LP 85 pumped to 3600psi is 1200psi/76cf. That same 76cf is going to be 950psi in a LP104. The difference for any pressure lower than 3600 will always be 250psi or less.

So if the LP85s are controlling, then take thirds for the 85s, and subtract 250psi. This gives you thirds for the LP104s. Because you round down, thirds in the LP104s will always be 300psi lower.

When the LP104s are controlling, then take thirds for the 104s, and add 250 psi. This gives you thirds for the LP85s. Because you round down, thirds in the LP85s will always be 200psi higher.

Which tank is the controlling tank, calculate simple thirds for each set. If the thirds on the 85s are more than 200 psi higher, then the 104s are the controlling tanks this dive.

So:

3000psi in LP104s; 3000psi in LP85s
thirds of either is 1000psi. LP85s are in charge.
LP85s turn at 2000psi. LP104s turn at 2300psi

2500psi in LP104s; 3000psi in LP85s
thirds of 104s is 800psi; 85s is 1000psi. LP85s still in charge
LP85s turn at 2000 psi; 104s turn at 1800psi.

2000psi in LP104s; 3100psi in LP85s
thirds are 600psi, and 1000psi. LP104s are in charge since 1000-600 > 250psi
104s turn at 1400psi. 85s turn at 2300psi.

[hudson]

I see, interesting.

But back to the capacity ratio method: that's why I said if you are comparing LP vs HP you need to normalize the pressures. All you need to know is an LP104 is 138cuft at 3500psi (or an HP138 if you will). Just multiply or divide by ~1.32 (3500/2640), whichever you prefer, that's the extra step, and you now have "equivalent" pressures. You still don't need the tank factors or have to do a psi/cuft conversion. An HP130 vs LP85 is roughly HP130 vs HP112, for example.

[skdvr]

Orchun, you have me confused. I am going to work this out how I think you are saying. Please tell me if I screwed it up (Very Likely)

Diver A has P85's filled to 2640
Diver B has HP130's filled to 3442

We are going to turn at 1/3 of the 85's

2640/3 = 880psi so diver A turns at 1760

To figure the ratio between the two 3442/2640 = 1.3

880/1.3 = 676psi

So diver B will turn at 2766

Did I do that correctly?

If so then diver B is not using 1/3 of the LP 85's

676*3 = 2028psi

The Baseline of a HP 130 is 3.8

(2028*3.8)/100 = 533.68/100 = 77cf

Not that is necessarily a bad thing because that diver is on the more conservative side.
__________________________________________________ _______________________


Now onto my way of thinking for the same two divers

Diver A with LP85's at 2640
Diver B with HP130's at 3442

Need to figure out at what PSI a 130 holds 85cf
Again the basline of the 130 is 3.8
(85/3.8)*100 = 22.36*100 = 2236psi

So the LP 85's at 2640/3 can use 880 psi so they turn at 1760.
The HP130's at 3442 have to pretend that they only have 2236psi to calculate thirds.
2236/3 they can use 745psi so they turn at 2697 (745-3442 = 2697)

Is that right or am I way off base here???

Phil

[b1gcountry]

I see, interesting.

But back to the capacity ratio method: that's why I said if you are comparing LP vs HP you need to normalize the pressures.


I'm sorry, I totally didn't even read that part. Yes, that would work just fine. It's kinda like a tank factor times 35.

Tom

[b1gcountry]

Orchun, you have me confused. I am going to work this out how I think you are saying. Please tell me if I screwed it up (Very Likely)

Diver A has P85's filled to 2640
Diver B has HP130's filled to 3442

We are going to turn at 1/3 of the 85's

2640/3 = 880psi so diver A turns at 1760

To figure the ratio between the two 3442/2640 = 1.3

880/1.3 = 676psi

So diver B will turn at 2766

Did I do that correctly?

If so then diver B is not using 1/3 of the LP 85's

If I understand Orchun correctly, he's converting all tanks to HP capacities. So at 3500psi, a LP85 holds {85 * 3500/2640 = } 113cf. So the ratio between a LP85 and a HP130 is 130/113 = 1.15.

[b1gcountry]

Now onto my way of thinking for the same two divers

Diver A with LP85's at 2640
Diver B with HP130's at 3442

Need to figure out at what PSI a 130 holds 85cf
Again the basline of the 130 is 3.8
(85/3.8)*100 = 22.36*100 = 2236psi

So the LP 85's at 2640/3 can use 880 psi so they turn at 1760.
The HP130's at 3442 have to pretend that they only have 2236psi to calculate thirds.
2236/3 they can use 745psi so they turn at 2697 (745-3442 = 2697)

Is that right or am I way off base here???

I think that's right, except you normally round your turn pressures off in 100psi increments.

The other thing to consider is that the LP85s aren't always going to be at 2640psi. There's a good chance they were overfilled, or you're on your second dive with them.

Tom

PS, I should also note that I'm not full cave. My education on thirds came from Fundies/Trimix/Cavern/personal study.

[SLIM]

Problem I see is the 130's at 3442 are not HP tanks. HP tanks are 3500 or greater and requier a 300 bar DIN attachment point only. The neck of the valve is also a differant size.

130's at 2442 are MP tanks and are still under LP tanks. Confusing isnt it.

The 2640 is for tanks that have the 10% overfill rateing.

We can play with psi all we want but you will never be guaranteed th have the over fills since DOT does not like that so we must work and figure it out with the working psi.

Have you all seen how easy it is to figure out and wondered why so many instructors do not like to teach it. Why do you think some instrucotrs requier you have specific size tanks. Yes there are sever out there that specify what size tanks you need for class.

SLIM

[skdvr]

If I understand Orchun correctly, he's converting all tanks to HP capacities. So at 3500psi, a LP85 holds {85 * 3500/2640 = } 113cf. So the ratio between a LP85 and a HP130 is 130/113 = 1.15.

Ahh, That makes more sense and seems to work out in the calculations.

Phil

[skdvr]

The other thing to consider is that the LP85s aren't always going to be at 2640psi. There's a good chance they were overfilled, or you're on your second dive with them.

Tom

PS, I should also note that I'm not full cave. My education on thirds came from Fundies/Trimix/Cavern/personal study.

The fill would not really matter although myway of calculating does get a bit more complicated.

LP85 at 2000psi
HP130 at 2200psi

2000/3 = 666psi (dont like that one)
600 (rounding down)-2000 = 1400 psi turn pressure
The base line for a LP85 is 3.1
(2000*3.1)/100 = 6200/100 = 62cf

so now we have to find out at what PSI the 130 will hold 62cf
The base line for a HP130 is 3.8
(62/3.8)*100 = 16.31*100 = 1631psi
1631/3 = 543
500 (rounding down)-2200 = 1700 psi turn pressure

I think that after looking at the other posts myway is much more complicated. It works but the other ways seem much more practicle.

PS. I should note that I am not cave/trimix/ or any other type of tech cetified. Just tyring to learn all I can before going that way...

Thanks
Phil

[skdvr]

Problem I see is the 130's at 3442 are not HP tanks. HP tanks are 3500 or greater and requier a 300 bar DIN attachment point only. The neck of the valve is also a differant size.

130's at 2442 are MP tanks and are still under LP tanks. Confusing isnt it.

The 2640 is for tanks that have the 10% overfill rateing.

We can play with psi all we want but you will never be guaranteed th have the over fills since DOT does not like that so we must work and figure it out with the working psi.

Have you all seen how easy it is to figure out and wondered why so many instructors do not like to teach it. Why do you think some instrucotrs requier you have specific size tanks. Yes there are sever out there that specify what size tanks you need for class.

SLIM

Really the HP/LP thing makes no difference. All you need to know is what tanks you have and what their rated fill pressure and capacity is compared to other teammates tanks.

For all practicle purposes 3442 tanks are considered HP tanks by the scuba industry, although some people call the 3500psi tanks "True HP"

As far as the DOT is concerned anything over 15 or 20 psi (can't remember off the top of my head) is a High Pressure Vessel.

Phil

[hudson]

Have you all seen how easy it is to figure out and wondered why so many instructors do not like to teach it. Why do you think some instrucotrs requier you have specific size tanks. Yes there are sever out there that specify what size tanks you need for class.
SLIM

Really, wow.. that's telling the student to deal with it on his own when he encounters the situation. In my basic cave class, we did dissimilar tank matching every dive and that helped a lot. I do think books make it look unnecessarily complicated though...I was REALLY confused initially, after 6 months of no diving and then trying to refresh things reading the IANTD cave book...NSS-CDS doesn't even have it spelled out in their material, and searching for it on the web is very hit and miss.

In fact, I've never seen it stated anywhere the way Tom described it. Once you have enough experience with different tanks (or you sit down and work out what the maximum differentials in psi would be) that's super simple, very little math involved. My way of doing it works better if you are clueless and too lazy to refer to a table :)

[SLIM]

Yes Tom stated it very good and kept it simple, that is what it is. It is not heavy math but likeyou said once you get it, it is so easy to do with all kinds of differnat tanks.

It is true that many have a hard time teaching it since it can look confusing and get strange looks, it is the way you present it. I have seen several and no names will be given that think it is hard to explain.

As you have seen on this thread, there are many that understand it and see how easy it is to figure out. Just keep it all basic and do not put to many variables in the equation.

Remember that even in diving and it be cave or mix KISS is the easy way to go and work things out.

SLIM

[scububa]

When I read the IANTD manual, it left me scratching my head..

There is another way to look at it, which may be even simpler: instead of knowing the tank factors, all you need is the capacity ratio of the two tanks at rated psi (needs an extra step to normalize the pressure when you are comparing HP vs LP)

So let's take this example:

Step 1: 104/85=1.22 - I'm going to simplify so say 1.2

Step 2: Take diver A's straightforwards 2640/3=880psi usable, divide it by 1.2
880/1.2=733psi usable for diver B

Done. No tank factors necessary.


Ummm...IMO those are tank factors. Very interesting conversations which probably never would have started if I had correctly READ Steve's post on the matter and paid attention that he said cuft, where I 'assumed' he meant psi. :o

Everything since that has said about the same thing. There are several ways to get there, but you are doing the same thing. So, whether by tables or by math, by direct or relative difference, fundamentally you are doing the same thing. Heck, all these super Trimix computational units should have a program for this!

I do appreciate the educational value of this and I do like the relative difference at thirds in Skips post. I do have some followup questions on that...first I'll make sure it wasn't covered in the recent posts I've skimmed.

[skip]

But why oh why do the calculations when there is a look-up table!? AND you only need to look it up once to see the difference in volume and psi in two different size tanks. once you know the difference, it's good for almost all psi's (until you or buddy gets different size tanks). Trying to do those calc's in the water after drills is a real pita. if i know i have to subtract 100 psi from my turn pressure to equal my buddies turn pressure...we can do that on the surface before the dive, heck, even before we get fills, and know that whatever or psi in the water after drills, it's still the same 100 psi (or whatever).

Forget the math. My former buddy made a math error at Ginnie a few years back while in the parking lot and got bent pretty bad (I was not his buddy on that trip). He's no longer diving and after three years of pain meds, etc., he's finally getting hip replacement surgery! Bone DCI. Math is not your friend.

The math is fun and entertaining and doing it shows you how all this works, and with luck you will see the patterns and verify the table entries for yourself (see my earlier post in this thread for the table). I rounded entries for conservation (no need for 77, or 328...just call it 100 and 350).

-skip

[skdvr]

I hope that Skip does not mind but I took his .PDF and made a excel doc of it. You can change what ever info if you want. To change a tank all that has to be changed is the actual volume and the rated pressure and everything else will update.

OK, well apparently you cannot upload excel documents so if anyone should want it just PM me your email address and I will send it to you...

Phil

[b1gcountry]

But why oh why do the calculations when there is a look-up table!? AND you only need to look it up once to see the difference in volume and psi in two different size tanks. once you know the difference, it's good for almost all psi's (until you or buddy gets different size tanks). Trying to do those calc's in the water after drills is a real pita. if i know i have to subtract 100 psi from my turn pressure to equal my buddies turn pressure...we can do that on the surface before the dive, heck, even before we get fills, and know that whatever or psi in the water after drills, it's still the same 100 psi (or whatever).

Forget the math. My former buddy made a math error at Ginnie a few years back while in the parking lot and got bent pretty bad (I was not his buddy on that trip). He's no longer diving and after three years of pain meds, etc., he's finally getting hip replacement surgery! Bone DCI. Math is not your friend.

The math is fun and entertaining and doing it shows you how all this works, and with luck you will see the patterns and verify the table entries for yourself (see my earlier post in this thread for the table). I rounded entries for conservation (no need for 77, or 328...just call it 100 and 350).

-skip

All tables are based upon a calculation somewhere along the line. I make it a personal habit never to use something unless I understand how it works. Understanding how thirds are calculated for dissimilar tanks is important beyond just memorizing a formula, or using a table. Once you understand the concepts, then applying them using whatever method you choose becomes easier, and you are more likely to spot a math error.

From what it sounds like, your tables are telling you the same thing as my method is. If that's the case, it doesn't really matter if you look it up in a table or if you sit down and do the math. It should give you the same results. Either way, you need to memorize that differential, and correctly apply it to your turn pressure.

We can't get around human error. At the end of the day, we're all humans underwater, and like it or not, our lives depend on us not making certain errors. We just need to try to be as reliable as we can be.

Tom

[b1gcountry]

I rounded entries for conservation (no need for 77, or 328...just call it 100 and 350).

I'm not completely sure you're using the same method as me, but with the way I described, it's actually important not to round the differential. In my example, the difference in thirds for 130's and 85's was about 250psi. Which way would you round 250?

1000psi in 130's gets adjusted to 1250psi in 85's, which, as a turn pressure, gets rounded down to 1200psi.

1000psi in 85's gets adjusted to 750psi in 130's, which gets rounded down to 700psi.

So you get differentials in turn pressure of 200, or 300psi depending on which way you are converting.

Tom

[skip]

i only rounded when it made sense...like when I get 387psi...just call it 400. sometimes even 360 was rounded to 400. The table values never violate thirds, instead they provide a bit of extra safety margin (a bit more gas, turn a tad before reaching thirds).

And I agree completely about doing the math and understanding what and why, not just blindly following someone's table or tank factors or differentials, but figuring out how they got there, seeing for yourself how it all works. That understanding to me is the very heart of a safe diver.

skdvr: sure take the table and do as you please - and everyone else too. It was originally published in the NSS/CDS U/W journal with accompanying article, so I don't think you can take it and mass market for selling....but that would be between you and them. I release all claim and rights, if I haven't already!

And do send me your excel version - psyskip@comcast.net

thanks!
skip

[scububa]

I did the same, creating an excel table. My questions were related to what the Working Pressure of the various tanks were. As I back computed, I wasn't seeing expected pressures. I tried to consider some rounding, but had to set it aside til later.

I was going to play around with a nomograph. I didn't think we had enough ways to confuse ourselves yet :confused:

And, I gotta believe PADI has a wheel for it :2137:

[skip]

if PADI didn't, they will soon; you just planted the seed!

-skip