What's the point of GUE Factors?

[deadly_risk]

I am reading “Getting Clear on the Basics: The Fundamentals of Technical Diving” by Jarrod Jablonski, and have a question on the GUE Factor. What was the point? Is it easier for most people to remember the GUE Factor when doing dissimilar tank conversion? Chapter 11 section 7 page 126.

I'm going to take the 95cf cylinder at 3600psi example from the book:

Makes sense to me:

95cf/2640psi = 0.03598cf/psi * 2 (for doubles) = 0.07197cf/psi * 3600psi = 259.092cf

I don't get:

(95cf/2640psi) * 100 = 3.5985cf/100psi * 2 (for doubles) = 7.1970cf/100psi rounded to 7cf/100psi

then: 3600psi/100 = 36 * 7cf/100psi = 252cf

Note: I'm skipping the rest of the calculations in 11.7 because they are not relevant to my question...I think.

The latter seems much less straight forward to me, so what am I missing?

 

[amascuba]

Tanks factors are SUPER easy and can be done on the fly.

For example: Last weekend, I was diving with three other guys and all of us had different tank sizes. One guy had double HP 120's, the other had double LP 108's, the other had double AL80's, and I had double LP72's. We were sitting on the surface and discussing our dive plan to 100 ffw for 30 minutes. Using tank factors we were all easily able to determine our rock bottom in psi in our heads.

It's much easier to do calculations in your head when you know how much gas (in volume) a tank holds per 100 psi instead of the volume per psi.

For example - My double LP72's rated at 2250 psi.

72/2250 = .032 * 2 (because of doubles) = .064 cubic feet per psi.

To make things easy to do in the head on the fly, I used a tank factor of 6. Which means that my tanks hold 6 cubic feet per 100 psi. In reality it's 6.4 cf3 per 100 psi, but that's not as easy to calculate on the fly.

For our 100 ffw dive, we needed to reserve 40 cf3 for rock bottom. All I needed to do was find the closest number that would divide 6 into 40. I went with 6 on the fly, which basically 36 cf3 at 600 psi. That's technically less than 40 cf3, but I also had the .4 fudge factor per 100 psi.

 

[coldsmoke]

yup - it's just easier.

You can always do it the other way, but I find it easier to remember the tank factors. It makes the head maths easier.

Hunter

 

[Peter Guy]

IF you are talking about "Tank Factors" (which it appears you are), the purpose is to make gas management calculations easily -- aka "Scuba Math."

For example, the AL80 has a TF of 2.5 as a single or 5 as a double (which in Mexico cave diving is important). That means there are 5 ft3 of gas for every 100 psi -- makes it easy to do calculations. Or the old LP 72 and the HP 100 both have TFs of 3 (or 6 for doubles) -- and again, makes calculating gas needed or gas remaining easy.

IF you were referring to something else, then "Never Mind."

 

[deadly_risk]

Ok, got it. Easier to do in the field and on the fly. The side effect of just reading and not doing is I miss the practicalities.

 

[DA Aquamaster]

So if a GUE factor is a tank factor does that mean JJ tried to make it sound like it was a GUE concept?

 

[deadly_risk]

So if a GUE factor is a tank factor does that mean JJ tried to make it sound like it was a GUE concept?

That's what it looks like, the table listing these factors for various cylinders in the book is labeled "GUE Factor = ft³/100psi, rounded to the nearest whole number."

There is no discussion about what exactly using the factors is supposed to do, make easier, or simplify. I would assume it would be explained in an actual GUE class.

 

[StreetDoctor]

Here we go...

 

[amascuba]

So if a GUE factor is a tank factor does that mean JJ tried to make it sound like it was a GUE concept?

Erm, No. It's been a while since I've read the Fundamentals of Recreational Diving book, but when I took the fundamentals class, it was just called tank factors, with nothing about it being a GUE concept. I think that you will find very little where GUE claims owner ship over an application of information or concepts, but more of "We do things this way because ____________."

 

[Anti-Hero]

GUE, UTD, NTEC - all have tank factors in their training. I don't think I've seen any of them try to claim simple calculation as their own creation.