- Reaction score
- New York City
- # of dives
- 1000 - 2499
Awesome! Scotch sounds terrific right now….
Here’s a way to think about it. Again, just look at a simple compartment for simplicity. All of these explanations are the same even if you consider all of the compartments, it’s just a lot more math. You also have to understand the concept of the “leading” or “limiting” compartment, which is the one most likely to cause trouble by bubbling, and the one that is limiting your dive by controlling your ascent. This would be the compartment where the supersaturation is greatest, and it’s not always the fastest compartment.
Looking at one compartment, at any point in the dive, that compartment can be assigned a number corresponding to how loaded it is with nitrogen compared to ambient - lets call that number the gradient (G). We normalize these numbers by plotting them on a scale ranging from 0% (ambient pressure) to 100% (the M-value of overpressure, or supersaturation). If you have a fully saturated compartment at depth (G=0%), as soon as you begin to ascend the compartment will become supersaturated (G>0%) relative to the new ambient pressure, even though the N2 loading of the compartment hasn’t changed much yet.
Remember, this number isn’t a measure of absolute N2 loading, but a degree of supersaturation compared to the M-value line. So if you go to 40 feet, sit there until your leading compartment is saturated, and then instantly shoot to the surface, you may well be over 100% even though your total N2 loading (based on time and PPN2) isn’t that great, and you risk DCS. On the other hand, if you go to 300 feet and then ascend 40 feet, you will have a much greater amount of N2 in your tissues, a much higher PPN2, but your gradient will be smaller because the ambient pressure at 260 feet is so much greater than at the surface.
GFLo isn’t something that you measure during a dive, it’s an arbitrarily chosen input to your decompression algorithm. The algorithm takes GF, mix, depth and time at depth to calculate your first stop. Change your GFLo from 30 to 50, and your first stop will be shallower.
So nothing “forces” anyone to ascend other than your dive plan. When doing tech diving, you figure out how long you can stay at your selected depth by working backwards. You pick a GF, time, depth and mix, and your software calculates for you (1) a safe ascent plan with all stops, and (2) how much gas it will take you to safely complete that ascent. Usually, you also run a scenario where you have to complete decompression on your back gas only (without a richer decompression gas - the lost gas scenario). If the ascent takes more gas than you are carrying (including a safety margin), then you need to start again with a shallower and/or shorter dive.