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Custom Fuction CF16
Posted by: mobu ()
Date: July 29, 2019 10:18AM

Hi,

My dive partner has an OSTC 2C. She says that when It marks a deco stop she has to be right at that depth. Otherwise, it penalizes by increasing the total deco time.

For example, if she must stop at 6m and up to 5.9m, the computer gives her an alarm signal and increases deco time.

I do not understand nor do I think this computer behavior is logical. My OSTC plus does not show any alarm if instead of stopping at 6m I do it at 5m. I understand that it will constantly recalculate the deco time as a function of depth.

I don't know if this is related to CF16.

I would like to know how the computer adjusts the real depth at which a deco stop is being made with the planned one.

E.G. if I have planned a 6m deco stop, how does it influence if I do it at 5.5 or 6.5 m

I would also like to understand how CF16 increases the forecast of the total decompression time.

Thaks in advance.

BR

mobu

OSTC plus #16088

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Re: Custom Fuction CF16
Posted by: jb2cool ()
Date: July 29, 2019 12:46PM

I've never seen anything like this on my OSTC mk2.

If it was me, I'd be inclined to take a note of anything you've customised and then reset all CFs back to their default value before reapplying your customisations, if you are not on the latest firmware then take this opportunity to update to this also.

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Re: Custom Fuction CF16
Posted by: mobu ()
Date: July 29, 2019 01:01PM

Thanks jb2cool


Firmware is V3.13. It is the last stable versión.

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Re: Custom Fuction CF16
Posted by: mobu ()
Date: July 29, 2019 01:52PM

I will try to explain the subject a little better.

For example. She has to make a 3' deco stop at 9 m.

If she stays fixed at 9 m and the 3 minutes go on, then the computer will show the next stop of X minutes at 6 m.

But if she stays fixed at 8.9 m then the numbers on the screen turn red and, even if she is longer than 1 minute at 8.9 m., deco time does not goes down. Only if she returns to the depth of 9 m. begins to discount deco time.

I hope I have explained myself better.

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Re: Custom Fuction CF16
Posted by: jb2cool ()
Date: July 29, 2019 02:53PM

I have not noticed that myself but I can kind of see some benefit in it. For example if you go above your deco ceiling then maybe it would be wise to pause the timer and only resume it once you are under the ceiling again to pad out the stops a little I don't think the computer is doing this though. I've occasionally been above the ceiling and the stops have still been counting down for me.

Hopefully someone more knowledgeable will reply for an authoritative answer

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Re: Custom Fuction CF16
Posted by: heinrichsweikamp ()
Date: July 29, 2019 03:08PM

mobu Wrote:
> But if she stays fixed at 8.9 m then the numbers
> on the screen turn red and, even if she is longer
> than 1 minute at 8.9 m., deco time does not goes
> down. Only if she returns to the depth of 9 m.
> begins to discount deco time.

That's not the case with the OSTC. Yes, the numbers turn red but there is no penalty or something like this added. Actually, the minutes will count down faster since difference between the ambient pressure and tissue pressure is bigger then at 9.0m. And the OSTC "just" applies this to it's deco model resulting in shorter deco times.

Regards,
Matthias

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Re: Custom Fuction CF16
Posted by: mobu ()
Date: July 29, 2019 06:41PM

Thanks Matthias,


we will check that there is no penalty via simulation.

Anyway, could you tell me how how CF16 value increases the forecast of the total decompression time?

BR

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Re: Custom Fuction CF16
Posted by: mobu ()
Date: July 31, 2019 07:06AM

Hi Matthias,


you can see in Custom Functions Description Manual this:


"CF16: Estimated distance to decompression stop
This value sets the estimated distance to the current decompression in meters. Usually, the stop is
done below the shown value. This value increases the forecast of the total decompression time.
Example: CF16=0.5m - The decompression stops are made 0.5m below the required depth.
Default value: 1.0m
Suggested range: 0.0m...2.0m"


I would like to know what this function is for and how that value increases the forecast of the total deco time.


Thanks in advance.


Regards

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Re: Custom Fuction CF16
Posted by: Ralph ()
Date: July 31, 2019 07:48AM

Hi Mobu,

the existence of this function dates back to the times when quite a number of divers used straigth Bühlmann without the GF extension. With straight Bühlmann, each ascent to the next deco stop brings the leading tissue to 100% super-saturation. So to put in some safety margin, people stayed a little bit below the stop depth, reducing the supersaturation a bit. The CF16 function tells the deco calculation how much deeper you intend to stay below all the stop depths. As staying below stop depth increases the deco time, the calculations can now figure this in and deliver corrected results, that is longer stop times.

As using GFs became common as of now, and with resonable GF values chosen, stopping at the exact stop depth (and even oscillating a bit around it) is ok to do. So it is ok to set the CF16 value to 0 if you want. Actually, in the latest hwos Tech firmware, this parameter has recently been removed from the code for exactly this reason.

BR
Ralph

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Re: Custom Fuction CF16
Posted by: mobu ()
Date: July 31, 2019 09:38AM

Hi Ralph,



thanks for the explanation. Fast, clear and concise as usual.

Now I understand it.


BR

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