Question What's your safety stop depth?

[Dan]

The real issue is on what dives should you do a safety stop, and on what dives can you skip it. The practice nowadays is to do a safety stop on all dives, but newer computer technology may play a role in that.

When I do an NDL dive with my Shearwater computers, I leave them in tech mode, and I have the SurfGF mode open in one of them. That tells me how close I would be in percentage to the Buhlmann limits if I were to surface right away. What a lot of people are doing these days is ignoring the standard time of a safety stop and instead ending it when they see a SurfGF they like. In theory, anything under 100 should be safe to surface. An 85 should be good, and 75 should be very safe. Some will stay until it drops to 70.

When you start thinking along those lines, you look at the safety stop in a whole new way. I was just diving for 2 weeks in Roatán, and we spent a lot of time in pretty shallow water while using nitrox. I was the only one on those dives with a SurfGF feature, and everyone else did standard safety stops every time. I stayed with them regardless of what my computer told me. On nearly every dive, my final SurfGF was below 20, and it was sometimes below 10. On one dive it never got above 19 throughout the entire dive, but, by golly, we all did a safety stop. I do not believe a safety stop was necessary on any of the dives we did.

You don't have to set your SW computers in tech mode to monitor your SurfGF, do you?

 

[Dan]

99 feet/10 meters

Typo: 99 feet/30 meters

 

[Dan]

The higher the ambient pressure, the more gas in the liquid!

I think you meant:
The higher the ambient pressure, the more dissolved gas in the liquid (Henry's Law).

 

[dmaziuk]

Saturation divers would, though.

There's a rumour they don't use ZH-L, though.

 

[dmaziuk]

What are examples of the fastest & slowest tissues?

Fastest (edit: nitrogen) tissue compartments have half-times of 2.5 (Cressi RGBM), 2.65 (ZH-L12), 4 or 5 (pick one) in ZH-L16, and more commonly: 5 minutes. They are not tissues, they are half times that approximate gas kinetics at the "fast" side of the model.

Typically the "safe ascent rate" takes care of them, e.g. at 9 metres/minute ascent from 30 to 3 msw will take 3 minutes, during which time the 2.5-minute TC will lose (oversimplifying for clarity) more than half of its gas loading. What this means is if you can trust your user base to stick to safe ascent rate, a 2.5-minute TC will never matter and so is not worth having in your model; you might as well start somewhere slower.

 

[boulderjohn]

You don't have to set your SW computers in tech mode to monitor your SurfGF, do you?

No--the vagueness of y response was due to the fat that I have never dived with a SW in recreational mode and ma not sure what it looks like.

 

[EFX]

Typically the fast tissue compartments (1-3) and the slow TC's (14-16) do not control the dive. It is usually the intermediate compartments (4-13) that control the dive in terms of keeping the diver out of DCS.

Here is a screen shot of a typical rec dive to 66 ft that shows the tissue compartment pressures in all 16 compartments and their affect on the dive:

If you want a clearer picture of this get my Excel spreadsheet version 34.1 which can be downloaded from:

HOME | My Site

ecfehlau.wixsite.com

Make sure to read the information found on the DIVE_HELP tab at the bottom of the spreadsheet for instructions and other useful information.

 

[DiveProKoko]

My Eon Core gives me two options, 10 feet and 20 feet. I set my to 20 feet so the timer starts. If I set it to 10 feet, it won't give me "credit" for a stop and start the timer unless I come up to 10 feet, but I hang out at 15 feet as recommended. You do bring up a good point about the computer offerings. I've wondered the same thing.

 

[EFX]

You don't have to set your SW computers in tech mode to monitor your SurfGF, do you?

No you don't. I typically run my Perdix in rec mode because the safety stop timer is automatic once the safety stop depth is reached. In tec mode, which I do use occasionally when running VPM-B +2, the timer must be started, stopped, and reset manually which for me is a PITA. Here is a screen shot of the main screen on my Perdix. I'm running the latest version of the software (84). SfGF is the surface GF, GF99 is the GF at the current depth, COMP is compass:

 

[boulderjohn]

I did two recreational dives this morning, and I ended each safety stop when the SurfGF went to 70. By the time I surfaced, it was probably in the 66-67 range. I don't know how much time that all took, because I didn't care. Having a countdown timer would have been a needless distraction.