Riding deco ceiling on ascent

[wnissen]

A New Measure of Decompression Sickness in the Rat - PMC

In this study we assessed the reliability of a tilting-board grip score as a measure of decompression sickness in rats. In experiments using a hyperbaric compression/decompression protocol, rats were observed for signs of decompression sickness and ...

www.ncbi.nlm.nih.gov

Thanks, that is very interesting. I know you didn't design the experiment, but it does seem cruel that 38% of the rats died from DCS before the experiment really even began. Presumably most interest in DCS these days is not hapless bridge workers dying like flies, but rather the predictive ability of decompression models that are rarely, if ever, going to lead to a fatal outcome of DCS. And to 90 m / 300 ft., that is getting into extreme depths.

 

[dmaziuk]

Presumably most interest in DCS these days is not hapless bridge workers dying like flies, but rather the predictive ability of decompression models that are rarely, if ever, going to lead to a fatal outcome of DCS. And to 90 m / 300 ft., that is getting into extreme depths.

IANA medical doctor but I think the prediction for 45 minutes at 90 msw on air for humans is: don't.

 

[wnissen]

IANA medical doctor but I think the prediction for 45 minutes at 90 msw on air for humans is: don't.

Right? At a PPO2 of 2.1, how can they tell if it's DCS or oxygen toxicity? I see this method is actually intended to be an improvement on the "binary" methods of testing DCS that rely on fatality as the metric.

 

[dmaziuk]

Right? At a PPO2 of 2.1, how can they tell if it's DCS or oxygen toxicity? I see this method is actually intended to be an improvement on the "binary" methods of testing DCS that rely on fatality as the metric.

Presumably they have a reason to believe oxtox doesn't work the same way in rats? On an actual dive there would also be WOB and C02 to consider I expect.

The relevant point is you want longer deco times for you trials, you can get them by going deeper and/or longer, but the latter obviously skews your experiment towards slow tissues while the former adds a whole 'nother dimension of variables with gas mixes and gas switches.

 

[Celt]

I don't really understand how or why people set conservancy before an actual real time dive, can this be done on a computer during a dive? I plan with no conservancy, 100/100 you might say, and adjust depending on how I feel and the actual dive conditions. Poor vis, cold or how hard I'm working. Would people use the same settings for a benign dive or difficult dive. For me the only saving trying to stay at a ceiling would be the time to go from stop to stop.

 

[boriss]

I don't really understand how or why people set conservancy before an actual real time dive, can this be done on a computer during a dive? I plan with no conservancy, 100/100 you might say, and adjust depending on how I feel and the actual dive conditions. Poor vis, cold or how hard I'm working. Would people use the same settings for a benign dive or difficult dive. For me the only saving trying to stay at a ceiling would be the time to go from stop to stop.

Assuming that even worked, ideally you'd want to do the opposite. Assume the worst, set your conservancy based on that and adjust up.

That's also the thinking behind planning for square dive profiles to maximum depth, but then following our computers so that the deco time is in our favor.

For example, I was diving GF 50/75, but while doing a deco dive a few weeks ago I decided that I'm comfortable with leaving the water when my SurfGF reached 80 and so I did. I adjusted my computer in the water while hanging on the line.

Some people run their backup computer in a much less conservative mode, so that if they "had to" get out right away (emergency), their computer would compute a plan to do that as fast as possible. Totally making this up, but let's say with a setting of GF 70/90 or GF 70/95.

 

[boriss]

Speaking of cognitive dissonance, I find it interesting that people have no issues with maintaining a stop, but can't ride a deco ceiling (e.g. maintain a higher stop). You don't have to "hug" the ceiling and be perfect within inches, but certainly it's not hard to. Just like you don't have to hug a stop.

I've said this before and I'll repeat it again: I'd love for Shearwater to give us an option to not be "penalized" by going a little past a stop as long as we're definitely below the CEIL.

Here's an example from last week (sorry about the reflection from the sun):

Note that my STOP is 20ft, CEIL is 11ft and I'm still hanging at 20ft. It's the same reason I set my last stop to 10ft, even though I don't usually do it at 10ft (especially in rough seas), so I don't get the darn flashing red screen!

Now that there's some literature on this, maybe Shearwater will change their stance and release a firmware update. I'll send them an email.

 

[BlueTrin]

Speaking of cognitive dissonance, I find it interesting that people have no issues with maintaining a stop, but can't ride a deco ceiling (e.g. maintain a higher stop). You don't have to "hug" the ceiling and be perfect within inches, but certainly it's not hard to. Just like you don't have to hug a stop.

I've said this before and I'll repeat it again: I'd love for Shearwater to give us an option to not be "penalized" by going a little past a stop as long as we're definitely below the CEIL.

Here's an example from last week (sorry about the reflection from the sun):
View attachment 740548

Note that my STOP is 20ft, CEIL is 11ft and I'm still hanging at 20ft. It's the same reason I set my last stop to 10ft, even though I don't usually do it at 10ft (especially in rough seas), so I don't get the darn flashing red screen!

Now that there's some literature on this, maybe Shearwater will change their stance and release a firmware update. I'll send them an email.

When you say penalty, you mean that you get a warning?

I don’t think the perdix will have any other penalty than just telling you that you went over the stop.

 

[boriss]

When you say penalty, you mean that you get a warning?

I don’t think the perdix will have any other penalty than just telling you that you went over the stop.

Good question, it starts flashing the CEIL and TIME in red, which according to their color coding means this "could be life threatening if not immediately addressed"[1]:

Here's more from the user manual about missing deco stops:

Now, the nice thing about Shearwater computers is that they won't lock you out for missing a stop, but given the severity of the alarm we don't want "normalization of deviance."[2] So even though it may not punish me (I haven't tested ), I heed all alerts.


[1] https://www.shearwater.com/wp-content/uploads/2020/07/Perdix-UserManual-RevE.pdf
[1] Defensive Dive Profile Planning - Divers Alert Network

 

[BlueTrin]

@boriss so I don’t know for this one, but everytime I emailed Shearwater they replied fairly promptly and they seem to not add arbitrary penalties, so please ask them, but it’s possible that you can just ride the ceiling except you’ll get this red warning.

(probably worth asking them to add a mode to just allow to follow the ceiling but I suspect they may be reluctant due to legal implications)

Suunto seem to add penalties in their algo for violations as a counter example.