Calculating turn pressures (gas matching) for a pair of cave divers.

[tursiops]

Before I get to gas management calculations, I first calculate Rule #1.

Sometimes the Rules need to be revisited.
Team foxturd: DIR Rules. Explained

 

[wKkaY]

You really only need to know the tank factor for your common tanks.

I think I might add it as a page in my wet notes.

I'm a new cave diver who trained in Mexico and in metric.

When I visited Florida I found gas planning a lot more bothersome as there were different sizes and manufacturers of tanks, and it wasn't always clear what their internal volume is, and fill pressures differ from one shop to another or from one day to the next.

Tank factor was something new to me when someone flipped to the page in his wet notes. I don't think he's ever had much practice with them, as he struggled and gave up after one try In the end we just did it the metric way with our best estimation of our tank internal volume (that I took other peoples word for).

 

[Superlyte27]

I took my cave training in 1996. Gas matching was taught back then. Not sure how far back you have to go to find someone who wasn't taught gas matching by a good instructor.

 

[Manatee Diver]

When I visited Florida I found gas planning a lot more bothersome as there were different sizes and manufacturers of tanks, and it wasn't always clear what their internal volume is, and fill pressures differ from one shop to another or from one day to the next.

Tank factor was something new to me when someone flipped to the page in his wet notes. I don't think he's ever had much practice with them, as he struggled and gave up after one try In the end we just did it the metric way with our best estimation of our tank internal volume (that I took other peoples word for).

There are four common tank sizes and tank factors
AL80s 2.5/5.0
LP85s 3.2/6.4
LP95s 3.6/7.2
LP100ish 4/8 - Covers all in the LP100 range as you have 104, 105, and 108 each with their own tank factor.
HP100s 2.9/5.8

Yes metric is easier as the tank size is stamped on the tank, but these turn it into metric like calculations.

 

[wKkaY]

Yes metric is easier as the tank size is stamped on the tank, but these turn it into metric like calculations.

Just curious how did those numbers come about e.g. "3.2" for LP85? are they in gallons or some other kind of unit?

 

[O-ring]

Just curious how did those numbers come about e.g. "3.2" for LP85? are they in gallons or some other kind of unit?

cf3 per 100 psi

In the LP85 example (2650 is working pressure in an LP85)

2650/100 = 26.5
85/26.5 = 3.2

 

[Manatee Diver]

Just curious how did those numbers come about e.g. "3.2" for LP85? are they in gallons or some other kind of unit?

I figured the quote thread led to my post on page 1.

Calculating turn pressures (gas matching) for a pair of cave divers.

But as o-ring mentioned it is cubic feet per 100 psi.

 

[tursiops]

I figured the quote thread led to my post on page 1.

Calculating turn pressures (gas matching) for a pair of cave divers.

But as o-ring mentioned it is cubic feet per 100 psi.

To be even clearer, it is cubic feet per 100 psi of working pressure.

 

[Manatee Diver]

To be even clearer, it is cubic feet per 100 psi of working pressure.

Of air too, though the Z factors for nitrox are close enough.

Although these caveats also apply to metric calculations too.

 

[CptTightPants21]

Understand what you are saying, but with all things being equal, the differences will be small. To me gas planning out of the water is putting one of the most important things in a place where concentration can occur (less distractions), and then make tweeks in the water. You've seen it where two people who decided to do a dive, skip the S drill, and say my turn pressure ought to be "x", what is yours? Gas planning often is haphazardly mentioned in passing in the water- to me what is wrong with discussing at home, in the parking lot and tweeking it in the water?

It depends on the environment--diving off a boat or a shore dive with a lot of surge, do it before hitting the water. Cave, quarry, or easy walk in shore dive-do it in the water.

There are less distractions when you are all in water about to start the dive than on the surface--either people are still setting moving gear around, doing other checks, or you have everyone standing around overheating in their drysuit when this all could be done in the comfortable setting of in the water. In the water everyone is on the same page and can better control pace.

To go back to your example, a team that skimps on the s drill or is haphazard isn't going to be any better in or out of the water.

From a cave diving perspective, every dive has one or more stages. It's easier to the stage calculations when you have all your bottles ready to go then to try to remember back what you thought you said 20 mins ago in the parking lot.

Just curious how did those numbers come about e.g. "3.2" for LP85? are they in gallons or some other kind of unit?

It is cuft of volume per 100psi. so (100 psi/working pressure psi) * tank size

Working Pressure would be 2640 for LP tanks, 3000 for Al80s, 3442 for HP tanks.

LP85s = (100 / 2640) * 85 = 3.2 cuft (x2 for doubles)

HP 100s = (100 / 3442) *100 = 2.9 cuft

It's difficult to explain calculating turn pressures and volumes in metric vs imperial in a clear and concise way. I would just remember the below key numbers and use them as needed in your own calculation process.

For Metric =
AL80s = 11 liter tanks
hp100s = 12.2 liter tanks
LP85s = 13 liter tanks
LP108s = 17 liter tanks

28.3 liters in each cu ft
1 Bar = 14.7 psi