There are computers that will not show over 24hr desat or no fly?
28 Hours of No Fly Time?
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seeker242
Contributor
That's simply false and a failure of reading comprehension on your part.. I was staying in context while you were taking it out of context. Taking something out of context, and then arguing against that, is what is a schoolbook example of the straw man fallacy. So perhaps you should try to stay on topic next time and stop taking things out of context? Yea, good idea.
The answer is, in the context of the discussion, there isn't evidence to show it's unsafe. Therefore, you cannot say it's unsafe. Do you even know what the word context means? How could you not know this? Seriously?...Or, are you just intentionally ignoring it? Either way, doesn't matter I guess. Either way is ridiculous.
I would actually disagree with this. There is evidence to show that only waiting 24 hours and then flying is inherently less safe than a longer period, and this evidence is provided by DANs study showing bubbling occurring during flight in divers after a 24 hour wait. Enough evidence that it at least lead them to consider extending their recommendations for no flying. That is not to say that the evidence was significant enough to overcome their risk aversion, but that is a different statement than there is no evidence showing a reduction in safety.
It's important to keep in mind here that "safe" and "unsafe" is not a binary statement in this context. Everything is risk reduction step. The DAN recommendations may be that at 24 hours the risk is low enough to be a reasonable balance in their opinion.
All the computer is providing you is a calculated de-sat time based on the profiles it actually dove. You will never convince me that an arbitrary standardized number is more informed than a computer that actually tracked the data in the specific instance.
It is up to you to make the judgement call if your personal risk aversion is such to justify basing your decision to fly off of the DAN numbers or the more conservative, but informed data from the computer. At least you have two data points to make that decision off of instead of one.
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What's kinda interesting is that some people don't seem to grasp what a model is, and what it can be used for. Models are extremely useful when we want to control something, but can't make actual measurements to base our control on. Or if the measurements are so slow that we get the results long after we needed them for control. This is well known in science and industry, and many industries use model-based control with great success.
Now let's take a look at diving. We want to control our depth and time exposure so we have an acceptably low risk of having to be airlifted to the closest chamber and spend six boring and/or uncomfortable hours inside a small metal enclosure. How can we know when the risk becomes too big? We can't. Because the only way to know is to see if the diver is bent or not. We can get a pretty decent indication by grabbing the diver as they exit the water, throwing them on a stretcher and doing ultrasound measurements of bubbles in their blood, but those data are also only available after the fact. So, pretty useless for control.
But we can use a model, which, when we have put numbers into it and translated to math, becomes an algorithm, for controlling the dive. It's far from a theory, because it has abstract numerical simplified representations of reality instead of comprehensive descriptions of what we believe reality is. A model is flawed, a model is simplified, but if it's a good model it can be used - within limits - "well enough for government use". And very far from a "guess".
Perhaps the most used deco algorithm is Bühlmann ZHL-16. Not because it's so damned good (the US Navy uses another, for example), but because it's open source. If you want to, you can look it up and implement it in your own hardware or software. Bruce Wienke's RGBM, Suunto's "RGBM" or Yount's VPM aren't. Besides, ZHL is fairly elegant and simple, so it isn't too difficult to implement.
The ZHL-16 algorithm is based on a dissolved gas model. A dissolved gas model basically says that gas dissolves in our tissues under pressure If we ascend too fast and the pressure difference between our tissues and the blood becomes too big, gas will go from being dissolved to being a gas, and if those gas bubbles get too big we'll get bent. The basic premise has a good fundament in basic physics and physiology, so so far, so good. We also know that some tissues ongas and offgas quickly while other tissues ongas and offgas slower. Thanks, Haldane, for exploding those goats.
What Bühlmann did was - just like Haldane did - to represent these different tissues by so-called "compartments", in his case 16 of them. Then he assumed that on- and offgassing could be explained by an exponential decay curve, which also is a good assumption based on what we know about the physics. Then he assigned different time constants to those compartments and could set up a set of equations which described what happened "well enough". And that at some critical difference between "tissue pressure" (the amount of gas dissolved in the tissue) and ambient pressure, bubbles would form and the diver would be bent.
This is a very simplified representation of what's really going on in our bodies, but it can give decent predictions. IOW, a useful model. Then comes someone else and wants a bigger margin of safety against the pressure-driven offgassing, and gradient factors get tacked on. Or someone else wants to take into account faster ascent speeds, or skipped stops, and we get another variation of the algorithm. Add to this that others might prefer another type of model, like the bubble growth model which VPM is based on, and we get the mess we have where different 'puter brands - and even different models of the same brand can give different results. All because we don't really know what's happening. But every diver can - if they want to - get enough knowledge to decide whether they're comfortable with making their computer pissed off at them, and, if so, how pissed off.
Now let's take a look at diving. We want to control our depth and time exposure so we have an acceptably low risk of having to be airlifted to the closest chamber and spend six boring and/or uncomfortable hours inside a small metal enclosure. How can we know when the risk becomes too big? We can't. Because the only way to know is to see if the diver is bent or not. We can get a pretty decent indication by grabbing the diver as they exit the water, throwing them on a stretcher and doing ultrasound measurements of bubbles in their blood, but those data are also only available after the fact. So, pretty useless for control.
But we can use a model, which, when we have put numbers into it and translated to math, becomes an algorithm, for controlling the dive. It's far from a theory, because it has abstract numerical simplified representations of reality instead of comprehensive descriptions of what we believe reality is. A model is flawed, a model is simplified, but if it's a good model it can be used - within limits - "well enough for government use". And very far from a "guess".
Perhaps the most used deco algorithm is Bühlmann ZHL-16. Not because it's so damned good (the US Navy uses another, for example), but because it's open source. If you want to, you can look it up and implement it in your own hardware or software. Bruce Wienke's RGBM, Suunto's "RGBM" or Yount's VPM aren't. Besides, ZHL is fairly elegant and simple, so it isn't too difficult to implement.
The ZHL-16 algorithm is based on a dissolved gas model. A dissolved gas model basically says that gas dissolves in our tissues under pressure If we ascend too fast and the pressure difference between our tissues and the blood becomes too big, gas will go from being dissolved to being a gas, and if those gas bubbles get too big we'll get bent. The basic premise has a good fundament in basic physics and physiology, so so far, so good. We also know that some tissues ongas and offgas quickly while other tissues ongas and offgas slower. Thanks, Haldane, for exploding those goats.
What Bühlmann did was - just like Haldane did - to represent these different tissues by so-called "compartments", in his case 16 of them. Then he assumed that on- and offgassing could be explained by an exponential decay curve, which also is a good assumption based on what we know about the physics. Then he assigned different time constants to those compartments and could set up a set of equations which described what happened "well enough". And that at some critical difference between "tissue pressure" (the amount of gas dissolved in the tissue) and ambient pressure, bubbles would form and the diver would be bent.
This is a very simplified representation of what's really going on in our bodies, but it can give decent predictions. IOW, a useful model. Then comes someone else and wants a bigger margin of safety against the pressure-driven offgassing, and gradient factors get tacked on. Or someone else wants to take into account faster ascent speeds, or skipped stops, and we get another variation of the algorithm. Add to this that others might prefer another type of model, like the bubble growth model which VPM is based on, and we get the mess we have where different 'puter brands - and even different models of the same brand can give different results. All because we don't really know what's happening. But every diver can - if they want to - get enough knowledge to decide whether they're comfortable with making their computer pissed off at them, and, if so, how pissed off.
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Mhm. Maybe I'm dense, but when you quote me and then say
I kinda tend to believe that what you're saying is that I'm trying to claim that that 24 hours of fly time is unsafe. Please tell me how I've misunderstood and failed to read for comprehension.
Ok, since everyone else has already gone off the rails, I can happily add my piece of survivor bias. On my last tech dive trip, after 6 consecutive days of decompression diving to 50-52 meters (165-172ft), I was on a plane home some 8 hours after surfacing from the last dive. So was my buddy.
Neither of us died, nor got hurt, nor grew any extra pairs of eyes. I intend to do it again.
Now more seriously (disclaimer: dont take the advice literally, I’m not responsible for you).. commercial passenger aircraft must maintain a minimum of cabin pressure equivalent to 2400m altitude (approx 8000 ft). That equates to about 0.75 atmospheres. So you can do the math, or ask your deco program to do it. Let’s say you surface with the most loaded compartment at 85% of its m-value, and add a short surface interval of lets say 4h-6h, in my books you would probably be fine in any case, Thats because tissue half times are between 4 and 700ish minutes, and you”d have to do some hardcore stuff to get the slow tissues fully loaded, so likely some midway compartments will be leading. So the extra 4-6 hours will give you a big buffer to make it to 0.75atm. And unless you talk bush taxi or own plane, you wont make it faster than 4-6h from the water to the plane, incl drying off, packing gear, checking in and clearing security.
Now in the case of loss of cabin pressure at altitude it might be a different story, but first thats very unlikely and secondly you then have another set of problems at hand to begin with.
Neither of us died, nor got hurt, nor grew any extra pairs of eyes. I intend to do it again.
Now more seriously (disclaimer: dont take the advice literally, I’m not responsible for you).. commercial passenger aircraft must maintain a minimum of cabin pressure equivalent to 2400m altitude (approx 8000 ft). That equates to about 0.75 atmospheres. So you can do the math, or ask your deco program to do it. Let’s say you surface with the most loaded compartment at 85% of its m-value, and add a short surface interval of lets say 4h-6h, in my books you would probably be fine in any case, Thats because tissue half times are between 4 and 700ish minutes, and you”d have to do some hardcore stuff to get the slow tissues fully loaded, so likely some midway compartments will be leading. So the extra 4-6 hours will give you a big buffer to make it to 0.75atm. And unless you talk bush taxi or own plane, you wont make it faster than 4-6h from the water to the plane, incl drying off, packing gear, checking in and clearing security.
Now in the case of loss of cabin pressure at altitude it might be a different story, but first thats very unlikely and secondly you then have another set of problems at hand to begin with.
seeker242
Contributor
Sure, but nobody claimed there is no evidence to show longer isn't safer. "Safer" is a completely different thing from "unsafe". The question was "is 24 hours is unsafe". If the evidence is not significant enough to overcome risk aversion, the what I said is originally is true, it is hard to argue that it's unsafe. The fact that there is evidence showing it to be safer, does not even speak to whether or not it's unsafe.
seeker242
Contributor
Here is the failure of reading comprehension. I stated it before but I'll state it again...
Is this true or false? The answer is it's true. I am staying on the above topic and in the context of the above and you are not. Staying on topic and in context is not a strawman like you claim. To claim that staying in context and on topic, is a strawnman, is nonsensical. A strawman, by definition, is the opposite of that. The fact that you have gone off on some irrelevant tangent does not make me guilty of making a strawman. The idea that it is, is just plain ridiculous.
There is no inherently safe or unsafe in this argument though. Everything is in a grey area. It's up to you to decide how grey is too grey for you personally.
seeker242
Contributor
Sure, but who in the dive world would ever claim 24 hrs is unsafe? Unless you are talking about deco dives, etc. I have never heard any person or agency say that. Meanwhile, a 2 or 4 hrs Fly time, near everyone would say that’s unsafe.
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