Recreational Scuba Deco Diving

[Blackcrusader]

I don't believe you understand the "Last Stop" function. This does not set the depth your safety stop. OC Tec Mode does not have a safety stop feature. The "Last Stop" function sets the depth where you will perform your last decompression obligation. Hence, this is going to be shallowest depth of your decompression ceiling. I'll venture a guess and say that the most of us have set this between either 20 or 10 feet.

The Perdix does provide a safety stop function but per its owner's manual, only in OC Rec, which Shearwater also calls Recreational Nitrox Mode. Page 14 of the Recreational Nitrox Mode manual states the diver can set the length of the safety stop between 3-5 minutes. The Perdix will not automatically require a safety stop. It will add the safety stop if the diver exceeds 35ft of depth. Furthermore, unlike other dive computers which have you hoover at 15feet. The Perdix initiates the safety stop countdown once the diver ascends above 20ft but must remain below 7ft.

I'm hoping you are not confusing a safety stop for a decompression stop. They are vastly different, where omitting a mandatory decompression stop carries far greater risks than omitting an optional safety stop.

Yes sorry I set mine to 6m. So it shows DECO CLEAR if clearing a deco stop obligation and CLEAR if safety stop time. Sorry worked all nighter a bit tired Yes you have to manage your own safety stop in OC TEC mode I was not confused between a deco stop and safety stop. I often spend the last 10 - 15 mins of my dive at 6m and clear pops up plus the extra time I am there. I guess I forget that I do that all the time I think it's the Perdix managing me when it is me managing myself. oops...If using AIR mode my safety stop time I set to 5 mins.

Certainly if you missed a Deco stop you are going get get a nice warning for that.

 

[lermontov]

You can set a safety stop at different depths and time in OC TEC mode. You do not need to start and stop a timer. Once you reach the safety stop depth the clock starts and once you are CLEAR you get plus minutes. So in this picture I have cleared the safety stop and have another 8 mins at the safety stop depth. Often I am hanging around taking photos on the reef at that depth or shallower.


View attachment 632776
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im a little confused ? you mentioned that you can set a safety stop when in Tec mode and I assumed the pics were of your perdix in tec mode, so is the first picture set to REC or TEC mode? it shows clear and TTS is 1 min which is what id expect
cant recall ever seeing a time display -once im clear im clear - are you sure this pic isnt set in rec mode?

 

[Blackcrusader]

im a little confused ? you mentioned that you can set a safety stop when in Tec mode and I assumed the pics were of your perdix in tec mode, so is the first picture set to REC or TEC mode? it shows clear and TTS is 1 min which is what id expect
cant recall ever seeing a time display -once im clear im clear - are you sure this pic isnt set in rec mode?

This applies to the OC TEC mode. You need to have Firmware version V65 or later. I made an error describing the safety stop from being tired. You set last stop say at 6m or 3m. In TEC mode you have to manage your own safety stop time there is no automatic timer for this as in Air Mode. However CLEAR will display in Green Colour when you have completed the safety stop obligation and add time showing how much time past the safety stop you have added.

If you have done a DECO dive then DECO CLEAR shows up in the top right hand corner when you have cleared that.

 

[Blackcrusader]

im a little confused ? you mentioned that you can set a safety stop when in Tec mode and I assumed the pics were of your perdix in tec mode, so is the first picture set to REC or TEC mode? it shows clear and TTS is 1 min which is what id expect cant recall ever seeing a time display -once im clear im clear - are you sure this pic isnt set in rec mode?

Hope this helps. Version 65 release notes.

 

[Angelo Farina]

@Angelo Farina,

For me personally, I decided on two separate cylinders for RSDD diving because I realized that events can happen that will compromise my entire air supply if my air supply is not separated into two separate cylinders.

In the case of putting all my air supply in a single cylinder, some possible compromising events are (1) a tank-to-valve O-ring failure, (2) a burst disk failure, and (3) the inability to shut down a free-flowing regulator (in the case of two regulators connected to a Y-valve).

In the case of putting all my air supply in a set of manifolded doubles, a possible compromising event is any event that could be solved by isolating the cylinders, but there is an inability to isolate them.

Keep in mind, I am doing solo dives, so I have no dive buddy there carrying an emergency, back-up gas supply for me.

rx7diver

OK, I see.
I agree with you, for solo diving I would definitely add a pony tank to my single back-mounted cylinder. However, I tendentially never dive solo, I always dive with a very reliable buddy such as my wife (she is an instructor with even more dives than me), or one of our two sons, which we trained personally since when they were 2 years old, and so now we can thrust them entirely, being aged 25 and 30 and with hundredths of dives on their back.
Regarding the twin tank: my own is old, so the valve system did not allow true separation, except for the reserve mechanism which traps air in just one of the two cylinders. If coming back to twins, I would prefer a modern manifolded valve system, of course starting the dive with the manifold CLOSED (so the tanks are tendentially always separated), and using the manifold valve for connecting the tank only in case one of the two regs is malfunctioning, so I need to use the other for getting the air trapped in the cylinder with the malfunctioning reg.
But I am speculating, as our two twin sets are old Aralu systems, and I have no money for buying new ones. And the single tanks are much more comfortable and lighter to carry, whilst providing almost the same volume of air, so I have no plan to come back to twins.

The point, for me, is that the need for a true independent air source (separated twins or back cylinder plus pony) comes from the requirement of diving solo, not from the requirement to need a couple of mandatory short deco stops to be done on back gas...
If diving with a buddy, a single cylinder (of proper size, definitely NOT al AL80, with double valves and two independent regs) is safe enough for me, even with deco obligations. This had been the standard setup available here in the Mediterranean sea (Italy, France, Spain, Greece, etc.) since the eighties, when recreational deco diving with tables was the norm, being taught since first-level certification.

 

[EFX]

So, based on a previous post you are diving a GFHi of 95. Whether you are doing an NDL or deco dive the computer will calculate stop depths and times or an NDL to bring you to the surface at or just below your set GFHi. By waiting at your last stop or SS you are artificially changing the GFHi to 85 by using the SurfGF. You can get the same result by just setting your computer to a GFHi of 85.

No, not exactly. I spend more time at depth and then spend the time at my safety stop to lower my SurfGF

I think we're in agreement. I want to be clear for the sake of others reading this. When you say "spend more time at depth" you can't mean spending more time beyond an NDL time remaining of 0 minutes. If you meant that, you would be in deco. Deco dives are fairly straightforward. You either follow your computer or a pre-cut ascent schedule all the way to the surface which will bring you at or just under your GFHi. You could spend extra time in the shallows to bring SurfGF lower but that is a separate discussion.

There are four scenarios for a dive planned with a GFHi of 95 that could include extra time at a SS to bring the SurfGF down to 85 or planned with a GFHi of 85:

1. Stay at depth until NDL is 0 and ascend all the way to the surface to finish at a SurfGF of 95. (Note: I'm making an assumption that leaving at NDL = 0 will not incur any deco obligation during a deep ascent.)

2. Stay at depth until NDL is greater than 0 and make a direct ascent to the surface so that SurfGF equals 85.

3. Stay at depth until NDL equals 0 (or closer to 0) and make a SS for x minutes until SurfGF equals 85.

4. Set the computer for a GFHi of 85. Stay at depth until NDL equals 0 and ascend to the surface where SurfGF equals 85.

You are advocating scenario 3. I suggested scenario 4 to accomplish the same SurfGF. When you say you spend more time at depth it is from an initial NDL greater than 0 to a lesser time closer to the NDL that requires the SS. Just to be clear, I'm not arguing with you or suggesting a particular ascent strategy. Whatever works for you is fine. I'm providing this information because there has been some discussion on computer conservatism settings and some divers (ex. @boulderjohn) are using SurfGF to control their final ascent as opposed to the standard and time-worn safety stop of 3 or 5 minutes at 15 ft (5 m).

 

[lermontov]

OK, I see.
I agree with you, for solo diving I would definitely add a pony tank to my single back-mounted cylinder. However, I tendentially never dive solo,

If coming back to twins, I would prefer a modern manifolded valve system, of course starting the dive with the manifold CLOSED (so the tanks are tendentially always separated), and using the manifold valve for connecting the tank only in case one of the two regs is malfunctioning, so I need to use the other for getting the air trapped in the cylinder with the malfunctioning reg.

i hardly ever dive a single - but interestingly i feel somewhat vulnerable when i do- i too like to take a pony (as 99% of my diving is wreck diving and my view is if your going to spend X dollars to drive to a venue get on a boat and pay your fees i want as long as possible in the water, a single in my mind is poor economy)

agree with the twins i would dive as independents but until I went to ccr i normally dived sidemount and most of the diving incurred a decompression obligation, id quite often work backwards and decide how much deco i was prepared to do depending on water temp and what gear i had on, time etc usually water temp is around 12º so once i hit my schedule or completed my goal (20 min of deco) id ascend and spend some time shallower until it was time to pay the piper

 

[Angelo Farina]

i hardly ever dive a single - but interestingly i feel somewhat vulnerable when i do- i too like to take a pony (as 99% of my diving is wreck diving and my view is if your going to spend X dollars to drive to a venue get on a boat and pay your fees i want as long as possible in the water, a single in my mind is poor economy)

agree with the twins i would dive as independents but until I went to ccr i normally dived sidemount and most of the diving incurred a decompression obligation, id quite often work backwards and decide how much deco i was prepared to do depending on water temp and what gear i had on, time etc usually water temp is around 12º so once i hit my schedule or completed my goal (20 min of deco) id ascend and spend some time shallower until it was time to pay the piper

Problem with two independent tanks in sidemount is that you do not have a manifold. So if one reg fails, it becomes quite tricky to recover the gas trapped in that tank...
And if it is the tank connected with your BCD you have to orally inflate it. A manifolded twin (with manifold normally closed) remains the safer OC system in my opinion.
However I think that a single tank of proper size and with a proper double valve is just marginally less safe, and can be used for these low-risk recreational deco dives.
I you do wrecks, caves or extended deco, this is quite different and requires more redundancy.
But the topic here is open water recreational dives, at rec depths (50m max according to my Cmas cert) where you simply stay at bottom a few minutes more than the standard NDL time.
Which was the standard way of diving when I started; no BCD, no computers, no SPG...
Just a twin tank with reserve (having same air volume as my current single) and two independent regs.

 

[tursiops]

Which was the standard way of diving when I started; no BCD, no computers, no SPG...

But it is NOT the standard way of diving today. We now dive safer.

 

[rx7diver]

@Angelo Farina,

This just happened! As I was moving this cylinder just now, one of the knobs (Delrin?) on its Y-valve simply split apart! So, this knob does NOT work! If this had happened during a deco dive when the attached reg was free-flowing, a solo diver who had all of his gas supply in a single cylinder would be in deep poo!

rx7diver