Turn Pressure Tec Deep

[Divetech99]

Is this correct ?

METRIC

turn pressure = start pressure - (bottom volume ÷ cylinder capacity)

It doesn't seem right. The lower your sac rate, the higher the turn pressure.

Example:
Start pressure = 200 bar
Depth = 30 meters
B Time = 10 minutes
RMV = 25 liters/min
ATA = 4
Bottom vol required = 1000 liters
Turn pressure = 109 bar (using above formula)

However if a diver's RMV is lower at 15 ltr/min bottom gas required is only 600 liters of air BUT Turn Pressure becomes 146 Bar???

So using the above TP formula, a diver with with a lower RMV (15 lpm) has to turn the dive sooner (at 146 bar spg reading) than someone with a 25 liters per min RMV (TP of 109 bar)?

Doesn't look right.

 

[tbone1004]

well the lower your sac rate, the higher the turn pressure using that formula makes sense because your bottom volume is much lower at the same time. With a lower sac rate if you have the same turn pressure you have more bottom time. Run a standard rock bottom calculation and have that as your turn pressure

 

[Divetech99]

Thanks tbone. If my sac rate is low, it SHOULD follow that my turn pressure should be at a lower spg reading thus giving me more bottom time. But the turn pressure formula above doesn't do that.

It's telling a high sac rate diver that his TP is at 109 bar and telling a low sac rate diver to turn the dive at 146 bar. Doesn't make sense.

 

[tbone1004]

not quite. the initial formula that you presented is essentially starting pressure-planned gas usage=turn pressure which is not how I have ever seen anyone do their gas planning.

we'll switch back to imperial for easy math with theoretical tanks
100cf tanks with a working pressure of 4000psi
what you are saying is that you are planning on being down for 10 minutes, and if you're sac rate is 0.5 at 100ft, that requires 40cf of gas. If you start with 4000psi, and you are using 40% of it, that equates to 1600psi, so your turn pressure is 4000-1600=2400. If you have a worse sac rate, say 1cfm, now you're using 80cf of gas so you'll burn 3200psi, so you're turn pressure would be 800psi.


Now, the better way to do this gas planning is to plan your reserve gas and not fix the time. At 100ft with a sac rate of 0.5, each diver needs roughly 10cf of gas to get up, you should at least double that, and I put a 500psi buffer in for IP and various other things.. Now the formula becomes 20cf out of a 100cf tank at 4000psi, your turn pressure is 1000psi regardless of what pressure you start.

Out of curiosity, why did you choose to fix the dive time as opposed to fixing the reserve gas?

 

[JohnnyC]

Understand you're planning a dive based on bottom time so if you have a lower SAC, you will consume less gas during your bottom time. Because of the way they do gas planning, you base your turn pressure on your predicted gas consumption for the defined time, and you turn the dive on time or pressure, theoretically this should happen at exactly the same time if your planning was correct. If you're consuming less gas in a specific amount of time, you will have more gas left over in your tank after that given amount of time than someone with a higher SAC.

It's just a way to plan the dive so that you're turning on whichever comes first, and theoretically at the same time. It's one of those "plan your dive, dive your plan" things in tech diving where even if you've got more gas than predicted, you're still off the bottom when you hit your run time. Because of how extra time at depth can affect your decompression obligation, it may not be practical to hit your gas turn pressure instead of your planned bottom time.

 

[Divetech99]

Thanks tbone. I agree with you. The latter way you determined turn is the way I do it too. The dive time is just an example to use with the DSAT TP formula since the formula requires computing for total required bottom gas volume.

 

[Kevrumbo]

Is this correct ?

METRIC

turn pressure = start pressure - (bottom volume ÷ cylinder capacity)

It doesn't seem right. The lower your sac rate, the higher the turn pressure.

Example:
Start pressure = 200 bar
Depth = 30 meters
B Time = 10 minutes
RMV = 25 liters/min
ATA = 4
Bottom vol required = 1000 liters
Turn pressure = 109 bar (using above formula)

However if a diver's RMV is lower at 15 ltr/min bottom gas required is only 600 liters of air BUT Turn Pressure becomes 146 Bar???

So using the above TP formula, a diver with with a lower RMV (15 lpm) has to turn the dive sooner (at 146 bar spg reading) than someone with a 25 liters per min RMV (TP of 109 bar)?

Doesn't look right.

Looks right and makes sense. (Given an AL80 tank which is 11 Liters/bar cylinder capacity rating).

For RMV of 25 liters/min:
(25 liters/min*ATA)(4 ATA)(10 min) equals 1000 liters;
200 bar minus (1000 liters/11 liters per bar) equals 109 bar Turn Pressure;

For RMV of 15 liters/min:
(15 liters/min*ATA)(4 ATA)(10 min) equals 600 liters;
200 bar minus (600 liters/11 liters per bar) equals 145 bar Turn Pressure.

Don't forget to state your tank(s) metric rating next time. . .

 

[Kazeone]

Gas needed/11.1=X
Start pressure/3=Y
X+Y=TP

 

[RainPilot]

Gas needed/11.1=X
Start pressure/3=Y
X+Y=TP

Just to note for others, that will work for metric units and an AL80. Other sizes and imperial units will not.

Is "Gas needed" the gas for sharing ascent?

 

[Mike1967]

we'll switch back to imperial for easy math

LOL