SS at 10ft or 20ft -- Why not standard 15ft?

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Basheirt

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Location
York, PA
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On all my guided boat dives, the dive master/guide has always done a 3 minute SS at 15 feet. Always.

I just bought the Perdix AI yesterday and found that it only allows SS settings at 10ft or 20ft. I'm curious about the rationale for this and whether one is "more right" than the other (computer options vs dive masters).

Regardless of which is "more right," I will presume that future dives will all do a SS at 15ft. Therefore:

If I set it for 20ft and hover at 15ft, will it think I blew through my SS?
If I set it for 10ft and hover at 15ft, will it think I never made it to my SS?

I'm diving next week and would love to know how to set this properly so I don't look like an idiot or create computer problems.
 
My oceanic DCs start the count down at 20 feet and will end it if you go above 10. Target 15.

Edit,
Also if you drop back under 20 ft by a little and not too long it will resume from the time you dropped under.
 
If I set it for 20ft and hover at 15ft, will it think I blew through my SS?
If I set it for 10ft and hover at 15ft, will it think I never made it to my SS?
I am not familiar with the recreational mode for Shearwaters. This sounds more like technical mode, where you can set your final deco stop for 20 feet or 10 feet. On a technical dive, I set it for 10 feet and do the final stop at about 15. No problem. On a technical dive, you are probably using oxygen for your final decompression stops, so your actual depth doesn't matter. By setting it to 10 feet and doing the stop a few feet lower, you don't have to be careful about your depth.

I would imagine the same thing would be true for a safety stop. If you set it for 10 feet and do it at 15, there should not be a problem, even if you are just breathing air. Doing the stop at 15 feet is in fact better than doing it at 10, IMO, so do it at 15 no matter what.

Now, for the same reason that 15 feet is better than 10, 15 feet is better than 20. That is why I am having trouble understanding a computer in recreational mode having you do a stop just below 20 feet. You really want to do it above 20 feet.
 
That's odd: I don't remember my Perdix AI doing this. I'll have to scroll through and look. (Not handy right now.)

However, as it is set, once I cross 20' it starts a 3 or 5 minute countdown timer, and as long as I stay below 10' it's happy. It seems to be tracking anything between 10 and 20' as the required safety stop. (The longer safety stop is dives that push closer to the NDL.) To be honest I don't recall if I set this way back when or not.

I learned in an era when safety stops were not as universally done and, if they were done at all, they were at 10'. Times change, improvements are made. Now it's something rec. divers all do unless in an emergency situation and it's 15'. Or 10-20', depending on who you ask.
 
I think I have an old plastic NAUI card for calculating pressure groups and bottom times. It had a bright red recommendation for a three minute safety stop at 15' for any repetitive dive or single dive greater than some threshold depth.
 
On all my guided boat dives, the dive master/guide has always done a 3 minute SS at 15 feet. Always.

I just bought the Perdix AI yesterday and found that it only allows SS settings at 10ft or 20ft. I'm curious about the rationale for this and whether one is "more right" than the other (computer options vs dive masters).

Regardless of which is "more right," I will presume that future dives will all do a SS at 15ft. Therefore:

If I set it for 20ft and hover at 15ft, will it think I blew through my SS?
If I set it for 10ft and hover at 15ft, will it think I never made it to my SS?

I'm diving next week and would love to know how to set this properly so I don't look like an idiot or create computer problems.
As noted above, the stops at 10 and 20 feet are user defined final deco stops.
Also as noted above, the safety stop timer starts when you ascend above 20ft (and will stop if you go past 7ft). A stop timer will not be included unless your dive goes below 35 feet. Choose OC Rec mode or 3-gas Nitrox mode for a display that may make more sense.
Screenshot_20210709-112311_Drive.jpg
You can have the Perdix automatically increase your timer to 5 minutes if your dive exceeds 100 feet. It's called the adaptive safety stop option.
This stuff is discussed in the Nitrox Manual, but not in the general Perdix manual or the AI manual.
And some of this (3-gas mode) may only be on the release notes downloadable from their site, depending upon which Manual edition you have.
 
On all my guided boat dives, the dive master/guide has always done a 3 minute SS at 15 feet. Always.

I just bought the Perdix AI yesterday and found that it only allows SS settings at 10ft or 20ft. I'm curious about the rationale for this and whether one is "more right" than the other (computer options vs dive masters).

Regardless of which is "more right," I will presume that future dives will all do a SS at 15ft. Therefore:

If I set it for 20ft and hover at 15ft, will it think I blew through my SS?
If I set it for 10ft and hover at 15ft, will it think I never made it to my SS?

I'm diving next week and would love to know how to set this properly so I don't look like an idiot or create computer problems.

Hi Basheirt,

I dive a Perdix AI in rec mode (or three gas nitrox, actually). I follow what my Tech 40 instructor told me, which is choose 20 feet in case it is rough. At 10 feet, it can be hard holding depth when it is rough topside. I have never had a problem with a DM who states 15 ft. for the SS and you do it at 20 feet, or visa versa. They usually add a caveat for you to follow your computer, no matter what.

My Perdix AI does not care if I am at 15 feet or 20 feet. It does stop the count-down when I am below 20 feet. My Oceanic starts the SS countdown long before the Perdix and is usually finishing the SS about a minute ahead of my Perdix.

Learn to read your tissue bar graph. That tells the story. If none of those black lines are approaching the red portion of the graph, you should be OK. Get a basic understanding for what your GF 99, Surf GF, and Ceil are telling you. ∆+5, @+5 and of course TTS is nice to understand. The boredom of the SS is alleviated by checking all of those screens out. For rec diving will they be critical? No, it is confirmation though.

During the last minute of my Perdix's countdown I drift up to about 12 or 15 feet of depth. I then wait for the boat, or other divers to board the boat at about 10 feet or a little less. I am usually one of the last aboard.

I think your DMs are splitting the difference between 10 and 20 feet as people have their computers set differently. The SS is not mandatory, and theoretically, you SHOULD be fine if you omitted it. I'll watch you test that theory from between 10 and 20 feet. Your Perdix will not lock you out if you don't complete the SS.

Take the SS at the depth your DC is set to and forget about what the DM says. Follow your computer.

cheers,
m
 
Thankfully the recreational "no deco" people do their "safety stop" (no, definitely not a decompression stop, oh no) at 5m/16' whist hanging on the line and flapping their legs around. This gives people hovering flat who are doing a decompression stop below them a chance to not be kicked in the head. (better add a :) )

Decompression stops are traditionally done in 3m/10' increments. The final stop frequently being 6m/20' so there's enough "headroom" to avoid the effect of waves and difficulties if 'light'. It's possible to set the last stop to 3m/10' and then do the best depth you can achieve in the conditions. If you set your dive computer to use 3m as the final stop, you can of course keep to a deeper final stop if conditions demand. There's no material difference in the overall decompression time.

I'd guess that PADI et al chose their final decompression safety stop at 5m/16' simply to give some headroom too.
 

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