ceiling/GF

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rsingler

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Thanks so much! This has been a fantastic thread!
I think it's time to fill it up with water and take my Perdix for a "dive" in the transparent pressure pot and check this all out. :D
20181113_202437.jpg
 

KenGordon

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Correct. GF low is only relevant for dives that require mandatory decompression stops.
So, if using 30/70 can I surface at a gf of 60 without having had a ceiling at any point? Is that an NDL dive in the planner?
 

leadduck

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Yes, that is possible and yes, it would be an NDL dive.

That's interesting. So you use GFhigh to determine NDL time, and start deco calculation with GFlow only after NDL time is over? That would cause some weird effect in a dive computer: just a second after NDL time is over, suddenly a long deco plan is displayed with many stops, starting at a depth below your current depth (if you've already ascended to a shallower place and GFlow is small); the diver immediately violates the deco plan and has to descend to his first stop.
So, the NDL times in the planner and in the dive computer are different for the same GF settings?
 

JohnnyC

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That's interesting. So you use GFhigh to determine NDL time, and start deco calculation with GFlow only after NDL time is over? That would cause some weird effect in a dive computer: just a second after NDL time is over, suddenly a long deco plan is displayed with many stops, starting at a depth below your current depth (if you've already ascended to a shallower place and GFlow is small); the diver immediately violates the deco plan and has to descend to his first stop.
So, the NDL times in the planner and in the dive computer are different for the same GF settings?

Nope.

Let’s say 30/80.

Remember HI is your surfacing GF, not your GF at depth. At X point in the dive, if you were to ascend, your HI would be 80. Any higher and you would have a mandatory decompression obligation. Which is why you can ascend and you “get back” more NDL.

If your LOW were driving your NDL, you would go into deco as soon as your fastest tissue hit 30, which depending on the depth could be a ridiculously short amount of time.

Coincidentally, this is why GF99 is such a powerful tool in the event of a “sporty” situation.
 

NAM001

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Well now that is interesting. Is your first stop calculated such that your tissues are 30% of the M value when you begin hovering at depth, or do you ascend to your first stop once your tissues are 30%? From the graph I would say the latter.

Don't you mean you descend from ambient?
You ascend t until you get to the GF low condition and that is your first stop.
 

NAM001

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That's interesting. So you use GFhigh to determine NDL time, and start deco calculation with GFlow only after NDL time is over? That would cause some weird effect in a dive computer: just a second after NDL time is over, suddenly a long deco plan is displayed with many stops, starting at a depth below your current depth (if you've already ascended to a shallower place and GFlow is small); the diver immediately violates the deco plan and has to descend to his first stop.
So, the NDL times in the planner and in the dive computer are different for the same GF settings?

NO Look at it like this you make a dive to 100 ft. depending on the tissue the tissues go to 100 feet at different rates. Lets say the fastest one is 5 minute rate. In five minutes (given the prior given of going to 100 ft.) your fast tissue is at 50 ft 5 minutes later it is at 75 feet 5minutes later it is at 87 ft. when you ascend at that time the process is reversed when you get above 75 ft the higher presure in the tissues leave the tissue at the same schedule rates but in reverse, gas moves half the depth difference every 5 minutes. It is imposssible (speekng in depths instead of pressure) to have your tissues at 150 ft when you have never been below 100 ft. So your statement of

starting at a depth below your current depth

can not happen. so reality again. you head up and the cmputer compares your calculated tissue depth with your true depth untill say a 30 ft difference exists and it tells you to stop until the tissue catches up or gets near to your real depth. that depth is GF low setting (tissue ambient pressure difference) then you head up again untill you reach teh limited difference again and you stop adn wait for the tissue to catch up again.
 
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