Figuring thirds....

[Sonofasailor]

So, I got my cavern cert. back in march and have been figuring thirds the way that I was taught in class. After a recent search online, I have seen in done more than three other ways than I was taught. Whats the best way to calculate thirds in water (without a calculator), once the water temp. has slightly reduced tank volume? I am curious to see the different methods people use.

 

[Dive-aholic]

I just drop the number down to one divisible by 3 and divide by 3. Of course, if you're diving with someone with a different tank size than you, then you have to account for that. I use a 10/100 rule - for every 10psi in capacity difference in tanks, I take 100psi off 1/3s. This works well for doubles. For example, if I'm diving double 95s and my buddy is diving double 85s, my turn is 1/3's minus 100psi. The actual number for gas matching is somewhere in the high 80s/low 90s, but it works well this way. If we have a third diver diving 108s, then that diver has to subtract 200psi from 1/3's to match the gas of the diver in 85s.

 

[KMD]

I just drop the number down to one divisible by 3 and divide by 3. Of course, if you're diving with someone with a different tank size than you, then you have to account for that. I use a 10/100 rule - for every 10psi in capacity difference in tanks, I take 100psi off 1/3s. This works well for doubles. For example, if I'm diving double 95s and my buddy is diving double 85s, my turn is 1/3's minus 100psi. The actual number for gas matching is somewhere in the high 80s/low 90s, but it works well this way. If we have a third diver diving 108s, then that diver has to subtract 200psi from 1/3's to match the gas of the diver in 85s.

So lets work this rule out:

Diver A has double 85's with 2700psi.
Diver B has double 95's with 3600psi.

Diver A's turn is 1800psi (2700 - (2700/3))
Diver B's turn is 2500psi (3600 - (3600/3 - 100))

Both of them have a cool dive and reach their turn pressure at the same time. (diver B has a poorer sac rate)

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 79cuft of gas and has 180cuft left

Then at max penetration Diver B looses his gas. The team needs 136cuft of gas to exit.... too bad they only have 115 in diver A's tanks.

 

[Mike Edmonston]

Here's a rule of thumb when you have MATCHING SAC RATES:

Diver A has 3600psi in 130's - turn at thirds = 2400psi
Diver B has 3000psi in 120's - turn at thirds = 2000psi

Now we adjust turn pressure in team to the member with the lowest pressure. Therefore, since diver B can only use 1000psi of her gas, diver A will also call thirds at 1000psi usage or 2600psi.

Hypothetically, If diver B could only use 500psi before turn, then diver A would turn dive when he used 500psi, even if his PERSONAL thirds were 1200psi gas use.

I hope I didn't confuse anyone...

 

[Mike Edmonston]

So lets work this rule out:

Diver A has double 85's with 2700psi.
Diver B has double 95's with 3600psi.

Diver A's turn is 1800psi (2700 - (2700/3))
Diver B's turn is 2500psi (3600 - (3600/3 - 100))

Both of them have a cool dive and reach their turn pressure at the same time. (diver B has a poorer sac rate)

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 79cuft of gas and has 180cuft left

Then at max penetration Diver B looses his gas. The team needs 136cuft of gas to exit.... too bad they only have 115 in diver A's tanks.

Diver B should turn the dive when he uses 900psi of gas, which is the same gas usage as diver A. Therefore, Diver B turns at 2700psi. according to your figures, and being that his sac rate is really horrible, diver B will reach thirds before diver A. Even with total gas loss at max penetration, they will have enough gas to exit (Theoretically)

Cheers

 

[KMD]

Diver B should turn the dive when he uses 900psi of gas, which is the same gas usage as diver A. Therefore, Diver B turns at 2700psi. according to your figures, and being that his sac rate is really horrible, diver B will reach thirds before diver A. Even with total gas loss at max penetration, they will have enough gas to exit (Theoretically)

Cheers

Ok so lets work this out:

Diver A Turns at 1800 (2700- (2700/3))
Diver B turns at 2700 (3600- (2700/3))

Both divers have a nice cool dive and they hit their turn pressures at the same time. (diver B's sac rate is worse that diver A's

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 64cuft of gas and has 195cuft left

At max penetration diver B looses his gas and they have to exit on diver A's tanks. They need 121 cuft of gas to make the exit... Too bad they only have 115 cuft.

 

[Mike Edmonston]

Ok so lets work this out:

Diver A Turns at 1800 (2700- (2700/3))
Diver B turns at 2700 (3600- (2700/3))

Both divers have a nice cool dive and they hit their turn pressures at the same time. (diver B's sac rate is worse that diver A's

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 64cuft of gas and has 195cuft left

At max penetration diver B looses his gas and they have to exit on diver A's tanks. They need 121 cuft of gas to make the exit... Too bad they only have 115 cuft.

According to your initial plan, They would hit thirds at the same time. But now diver B is using 300psi less air. Would diver A not have AT LEAST 300psi more gas in the tanks?

Not to mention that a sac rate difference that big would be addressed prior to the beginning of the dive, by putting the heaviest breather on the smaller tanks in order to have enough gas to get everyone out. The heavier breather would hit thirds WAYYY before anyone else on the team, therefore they SHOULD have enough gas to get out (anxiety and stress notwithstanding)

Cheers

 

[timle]

Ok so lets work this out:

Diver A Turns at 1800 (2700- (2700/3))
Diver B turns at 2700 (3600- (2700/3))

Both divers have a nice cool dive and they hit their turn pressures at the same time. (diver B's sac rate is worse that diver A's

Diver A has used 57cuft of gas and has 115 cuft left
Diver B has used 64cuft of gas and has 195cuft left

At max penetration diver B looses his gas and they have to exit on diver A's tanks. They need 121 cuft of gas to make the exit... Too bad they only have 115 cuft.

Couldn't agree more - convert to Cubic Feet and then turn is when either diver has used 1/3 of the smaller cubic feet number. In this case 57cuft.

I wouldn't count on making sure to put the diver with the highest SAC in the smallest tanks. Or other non foolproof methods. Do the math, live the math, and have enough to make it back.

 

[rjack321]

Not to mention that a sac rate difference that big would be addressed prior to the beginning of the dive, by putting the heaviest breather on the smaller tanks in order to have enough gas to get everyone out. The heavier breather would hit thirds WAYYY before anyone else on the team, therefore they SHOULD have enough gas to get out (anxiety and stress notwithstanding)

Unless the one with the smallest consumption and largest tanks loses all their gas and the hover and everyone else is exiting on those tiny tanks...


Jeesh what so hard about converting to cf using tank factors?
AL80s = 5cf/100psi
lp85s/hp100 = 6cf/100psi
lp95s/hp119 = 7cf/100psi
lp100/hp130 = 8cf/100psi

Each diver multiplies their gas in 100s by the tank factor. Divide by 3
Take smallest amount.

Covert smallest amount into psi for respective tanks
Subtract from start pressure
That's your turn pressure

Simple all these "rules of thumb" are bogus and I can't believe people who should know better are advocating dangerous shortcuts. Do the math right everytime.

 

[KMD]

According to your initial plan, They would hit thirds at the same time. But now diver B is using 300psi less air. Would diver A not have AT LEAST 300psi more gas in the tanks?

Not to mention that a sac rate difference that big would be addressed prior to the beginning of the dive, by putting the heaviest breather on the smaller tanks in order to have enough gas to get everyone out. The heavier breather would hit thirds WAYYY before anyone else on the team, therefore they SHOULD have enough gas to get out (anxiety and stress notwithstanding)

Cheers

Mike,

My examples were worst case examples. So no matter what we choose, we will always hit our turn pressure at the same time. This is what validates the gas plan.

One's sac rate is always subject to change during the dive. While one might start out and plan for the same sac rate it might not happen during the dive. With your meathod of planning a team can get to their max penitration and have exceeded their 1/3 gas limits.

The point I am trying to make with all of this, is that pressure does not equal volume. One's gas caluclations should always be based on volume and not on pressure.