Safety stop feature request

[tbone1004]

Sorry, I've lost the bubble, starting at Tom's post #61. I thought the idea was to stay OUT of deco, not to compare how long you are IN deco with various GF pairs. 30 mins at 100 ft is only an NDL dive for a GFhi of 99, maybe 95. So what is the discussion about doing that dive with a GFhi of 70, for which it IS a deco dive and therefore the 30/70 and 70/70 bottom times WILL be different. Seems like a bunch of apples and oranges.

@KenGordon summed it up pretty well.
I have three examples of "NDL" diving at 100ft on EAN32 using different gradient factors to show how changing the GF hi would adjust the NDL, but also how GF lo would adjust it. I put the first NDL in quotes because I'm using GUE's deco planner and NDL isn't an option so you have to play with it and the minutes may be off by one or two, but it's all relative using the same variables so your planners may give 28 minutes instead of 30, but it's close enough.

What I understood the OP to be asking is how to maximize NDL, so I did that with GF99 and a 5 minute safety stop to show what the GF would be when you got to that stop at 15ft and then what it would be at the surface.
I then added a "safety stop" *which is actually planning a multi level dive* to get the surface GF ratio's to around 70 which is what he said he was doing. I then took the same bottom time and just ran it on 70/70 which is what I think they should be doing.

All of that to say that one profile yields more decompression stress on the ascent, and only serves to prevent the computer from showing a mandatory deco stop. I have said for years that if you are going to try to manipulate the computers because of fear mongering and ignorance about NDL, which is what this is, then you should just take a decompression procedures course and dive the profiles that you want.

 

[CWK]

It sounds like you don’t think pure Buhlmann is too aggressive but then it sounds like you don’t want to actually dive it.

When I dive tech, I use a gf of xx/75. In the dive, I go past m-value, say to surface gf of 300, then I offgas till my surface gf goes back down to 75 before I surface.

All I want to do in a recreational dive is to dive to m-value, gf 99 is close enough, then offgas till my surface gf goes back down to 75 before I surface.

I trust Buhlmann equally in my tech dives and my recreational dives. In tech diving, the stops for offgassing are called deco stops. In recreational diving, the offgassing stop is called a safety stop. Call the offgassing stop anything you want. Irrespective of whether I am diving tech or rec, I would like my computer to guide me back to xx/75 before I surface. So I am asking Shearwater to give me a safety stop option to be able to do this.

 

[CWK]

Shearwater already allows a gf 99/99 setting in rec mode. This is recognition that no deco diving includes setting NDL to gf 99.

I am asking to be able to do my recreational offgassing stop based on gradient factors and to call that offgassing stop a safety stop because that what stops are called in recreational diving.

 

[rsingler]

At any time during the dive, look at the difference between your SurGF and your desired ending GF.
EDIT: DIVIDE that difference by 1.5 to 2.
That's your "Safety Stop" duration in minutes.
If you are mosey'ing up the reef like me, that time will improve as you ascend and offgas. If you're doing a straight blue water ascent, then what you see is what you get.
YMMV.

 

[CWK]

At any time during the dive, look at the difference between your SurGF and your desired ending GF. Multiply that difference by 1.5.
That's your "Safety Stop" duration in minutes.
If you are mosey'ing up the reef like me, that time will improve as you ascend and offgas. If you're doing a straight blue water ascent, then what you see is what you get.
YMMV.

Yes. There are ways of getting around. But I would prefer having a SS setting that is based on gf.

 

[CWK]

At any time during the dive, look at the difference between your SurGF and your desired ending GF. Multiply that difference by 1.5.
That's your "Safety Stop" duration in minutes.
If you are mosey'ing up the reef like me, that time will improve as you ascend and offgas. If you're doing a straight blue water ascent, then what you see is what you get.
YMMV.

BTW,
Surf GF 99
Desired ending GF 75

99 – 75 = 24
24 x 1.5 = 36

This is more than a little off the mark. But as you said, "YMMV".

 

[NAM001]

Using deco planner.
100ft on EAN32, all plus or minus a minute, so don't be pedantic
With 30/70, you get 10 mins on the bottom
With 70/70, you get 20 mins on the bottom
With 99/99, you get 30 mins on the bottom

The gf-lo sets the threshold not to exceed for the first stop which is how it controls the stop depth. That will absolutely change your bottom time when that "first stop depth" becomes shallower than the surface.
Now what is worth noting and why I don't agree with what you guys are doing is this. If you do what you're doing when you say I want to surface at GF70, you are still exceeding GF70 during your ascend. If you're ok with ascending at GF99, then you may as well get the rest of the way to the surface since you offgas faster at the surface than you do at 15ft since the partial pressure of inert gas is still higher than 0.8.

Isnt this only if you go into deco. with out going into deco (keeping the dive less than NDL) the gf lo should not be in play. perhaps the use of gf70 should not have been a criteria. how about if it was 60/90 or 20/90 and never entering deco. The 90 should determine NDL and the game should be over as far as direct ascent goes. I am not sure I see the issue with x/70 the NDL sould be shortened and you should still be able to do a direct ascent to the surface. I think the root question here is why not use actual metrics to determine the time at a safety stop instead of the hard rule of thumb of 3 or 5 minutes. And cwk was asking why not have teh SS time governed by actual GFsurface metric. That the computer should be able to know that if you arrive at SS with a gf80 and you have a hit the surface criteria of 85 that the computer should be able to tell you once you arrive at SS it will take 2 `1/2 minutes at SS prior to surface. I think the 70/70 examples were used with out any real thought. this has always been a conflict in the idea of NDL is a time that allows direct ascent to the surface. I am viewing the SS parameter as adding time to the ascent to reduce the surface GF below 100 whether it is needed or not. That if you turn off SS you will not get flagged for the SS and all should be ok if the ascent rate was in copmliance. I and I think others are thinking that you can remove the 3 or 5 minute set times all together with no consequence as the NDL time is prior reduced to accomodate what ever gf hi you enter making a SS never needed. That the use of SS of 3 or 5 minutes is there as a antiquated carry over form the dive tables when no computers were available. So you either do the max bottom time with a GF99 NDL and do a safety stop to cover any unforseen abnormalities or do a reduced bottom time via conservatism level and need no SS because you have created a buffer of sorts when you bottom time is such that your surf GF is 90 or 80 or 70. I do agree with you that teh lower the GF hi is, perhaps teh slower you have to ascend because that number is the max limit for he dive and not actually the planned surf GF. If you can start a clock of 5 minutes and stay at SS untill it reaches 0 or you can reference the surf GF and stay at SS until it drops below the GF hi value, which the computer can say it would be 2 minutes at the calculated off gas rate.

 

[tbone1004]

@KWS
the point is that GF-lo is a variable in the NDL calculations. It's explained poorly in most areas as it determines the depth of the first stop. Most people assume that when you are doing NDL diving, that the computer assumes the surface is as high as you can go and if I can go straight to the surface, then the first stop is irrelevant. The issue though is that when GF-lo is lower than GF-hi, the gf-lo becomes the leading factor for NDL diving. This is poorly explained, but it is how it works.
The problem with Shearwater implementing a new SS paradigm is liability and I highly doubt they are going to buck industry establish standards for recreational diving. You are asking them to implement true decompression into their recreational computers that claim the dive is NDL. That defeats the purpose of having a recreational mode, and if that's what you want to do, you should actually plan the dive and have a rough idea of what that stop should be. It sounds like no one is actually planning their dives and are riding their computers which is why they are wanting the actual TTS based on surfGF. I also don't see why just watching the surfGF is so difficult when the difference is only 5 or so minutes more than the safety stop. Use your brains and be proactive and plan. Use the computer to tighten the parameters and find out if your planned 10 minutes is actually 8 or 11

 

[rsingler]

BTW,
Surf GF 99
Desired ending GF 75

99 – 75 = 24
24 x 1.5 = 36

This is more than a little off the mark. But as you said, "YMMV".

Your reply got me thinking, how did he get a 36 minute safety stop?? My kludge works for me! I do this every Rec dive!

Well, I inverted my ratio when replying in this thread.
What I SHOULD have said is that SurGF declines at ~2% per minute.
That means you should take the difference between your current SurGF and your desired ending GF and divide by two, not multiply. My error, my fault.
I have edited my previous posts.

So using your example of a SurGF of 99 and a desired ending GF of 75, 99-75 = 24, which divided by two means a 12-minute safety stop.
In practice, you need to include part of that blue water ascent in the total time, so you're probably really looking at a 10 minute stop to finish at 75.
It works! It's easy to compute. And as suggested in a post above, you can use TTS data from the computer to gas plan on the fly as well.
Whether or not Shearwater chooses to add this extended safety stop as a feature, the data is right at your fingertips with the existing display, albeit with a little math.
Math which, like me, you might screw up.
At which point, like @scubadada, you just hang out until your SurGF is where you want it!