DIR- GUE Min Gas... LP108 vs HP130

[Ulfhedinn]

My math has failed me.. or I'm brain dead. I cant seem to dial in the numbers. I think Im in the analyses overwhelmed faze.

Lets say we are diving down to a wreck so we will be using 1/2.


HP130 @ 3500PSI @ 100' with a .75 sac

vs

LP108 @ 3600PSI @ 100' with .75 sac


Can someone write it out for both.

thanks

 

[Ulfhedinn]

MG = 2 divers x .75ft3 /min SCR x time needed to ascend x average pressure during ascent = MG

 

[rjack321]

Total time
1min on the bottom
30ft per min for 60ft
4 mins to the surface
= 7mins

Average depth = 40ft (halfway point of the time calc)
= 2.3 ATA

0.75 each = 1.5 total SAC

7 * 2.3 * 1.5 = 24.15cf

Tanks are functionally the same water capacity, working pressure is irrelevant
Tank factor is 3.75cf/100psi

24/3.75 = 640psi rounds to 700psi

3600 - 700 is 2900 useable. 1/2 of that is 1450.

So your 1/2s gas plan:
Start 3600

3600-1450 = Turn with no less than 2150

Use the second 1450 and You're back to the upline with 700

 

[Ulfhedinn]

Total time
1min on the bottom
30ft per min for 60ft
4 mins to the surface
= 7mins

Average depth = 40ft (halfway point of the time calc)
= 2.3 ATA

0.75 each = 1.5 total SAC

7 * 2.3 * 1.5 = 24.15cf

Tanks are functionally the same water capacity, working pressure is irrelevant
Tank factor is 3.75cf/100psi

24/3.75 = 640psi rounds to 700psi

3600 - 700 is 2900 useable. 1/2 of that is 1450.

So your 1/2s gas plan:
Start 3600

3600-1450 = Turn with no less than 2150

Use the second 1450 and You're back to the upline with 700

Thank you for helping. So I know some see the 130 as a TF of 3.5 and the LP108 as 4. I was struggling with looking past working capacity.

Thanks Again!

 

[elan]

To be precise TF for 130 is 3.77 IIRC doubles are 7.5 - they are in a 3:1 ratio to AL80 stage 108s are 8.

 

[tbone1004]

to be precise, @rjack321 is correct and assuming both are Fabers, they are identical in internal volume and as such are identical tank factors and will always hold the same volume of gas when filled to the same pressure, regardless of what that pressure is...

No one wants to remember the Z-factor when using low pressure tanks. That also means that the CF/100psi is higher when at low pressure than it is at high pressure by about 3% at 3000, 5% at 3442 working pressure, and 6% at 3600psi cave fill with percentages being applicable to air fills only at roughly room temperature.

Now, since we are being pedantic.
Faber LP108=FX133=17.0L
PST LP104=HP130=~16.6L *this sticks in my head for some reason, but they are a noticeably shorter than the Faber equivalents*
Worthington LP108=Faber LP108=17.0L
Worthington HP130<PST HP130=16.0L

You get to decide if that is significant enough of a difference.
100psi=6.9bar
16L=.565cf ->3.9cf/100psi
16.6L=.586cf ->4.0cf/100psi
17.0L=.6 ->4.14cf/100psi

Use 4cf/100psi/1.06=3.75 to account for full fill or straight 4cf on the bottom, the +3% is less than the precision of your pressure gauge.

 

[elan]

to be precise, @rjack321 is correct and assuming both are Fabers, they are identical in internal volume and as such are identical tank factors and will always hold the same volume of gas when filled to the same pressure, regardless of what that pressure

Good point, I fell into a trap of thinking about Worthingtons as I own them

 

[Ulfhedinn]

Thanks everyone.. they are Worthington's though


and @tbone1004 thanks for filling in the blanks.

I love info dumps! Pedantic be damnd...