Overshooting NDL and mandatory deco stops

[Mike1967]

default setting for the Shearwater Perdix A.I.

Med 40/85

explanation

Exclamation

 

[almostDIR]

zoop novo is perfectly fine for normal rec diving I think (assuming it is working correctly of course) .
the backlight is pretty crappy though so I tend to not use it at all and instead will shine diving light to the display every now and then to charge the luminescent background if needing to see the display in the dark (just like I do with the SPG and compass). will work way better than trying to use those tiny buttons with gloves on and trying to get the backlight on by feel. no need to try to find the buttons in the dark and the display is always clearly visible

 

[Mike1967]

That being said, in tech mode, the defaults are actually more conservative than the Rec mode, until you go fiddling with GF etc

They're actually the same by default on the Perdix, 40/85.

 

[Edward3c]

Caveat this has not happened to me and im not going to do it. But if someone else has in the dive what would be the time for a mandatory deco stop? And im only asking the question as im sure it does happen to divers I just want to know the process.

If a diver were at 100ft depth and for whatever reason lost track and overshot the NDL by say 2 minutes and started up then at what depth would the mandatory stop be and for how long? On air. I assume if you hit the NDL on 32 nitrox it would end up being the same thing you were just at 100ft longer to get there.

There is no single answer to your question as there are too many unknown variances. However, assuming a square profile, the computer (dependant on make/model) will show, either:
• a stop depth and the time required there, or
• the total time required to safely reach the surface, including stops.

I often plan to do up to 10 minutes of stops (it’s in my gas plan). My previous computer would jump from ‘zero’ to a 5 minute stop required. And when I found a body at 30m it quickly ramped up to 15 minutes in the 3 minutes we spent confirming we had found a diver. [This was the incident that convinced me to dive with a pony.]

Also do all modern dive computers automatically warn you at NDL and then tell you when and how long to stop?

Warning; No. Display a get out of the water plan: Yes.

Obviously if theres not enough air left you are going to avoid a mandatory stop of course.

No, I would switch to my pony; which isn’t included in my gas planning, even when planning deco stops.
---
To me the terms ‘recreational’ and ‘technical’ diving are meaningless, it’s all just diving.

 

[seeker242]

by the way why would i even want to change my GF settings if im just diving recreationally? I trust shearwaters settings.

You can't directly change the GF setting in a Perdix in rec mode anyway. They are preset and unchangeable, determined by the low, medium or high conservatism settings. The idea that you need to know what GF are, using Perdix in rec mode, is nonsensical. Even when you have it on lowest conservatism setting, or most aggressive, it's basically equivalent to padi table NDL times. It's called recreational mode for a reason!

 

[RainPilot]

They're actually the same by default on the Perdix, 40/85.

Mine came with 30/70 as the default

 

[taimen]

Mine came with 30/70 as the default

Mine as well. I think sw changed the tec mode default gf:s to more conservative 30/70 some time ago.

 

[uncfnp]

You can't directly change the GF setting in a Perdix in rec mode anyway. They are preset and unchangeable, determined by the low, medium or high conservatism settings. The idea that you need to know what GF are, using Perdix in rec mode, is nonsensical. Even when you have it on lowest conservatism setting, or most aggressive, it's basically equivalent to padi table NDL times. It's called recreational mode for a reason!

Respectfully disagree. Owning a Perdix and not having a basic understanding of GFs, even the preset Recreation mode options, is kinda like owning a Bugatti and never taking it out of idle.

 

[doctormike]

and you took that totally out of context. you OMITTED the next sentence. Care to add in my next sentence?

Sure!

My dad was a NAUI scuba instructor at the YMCA in 1962 and his whole life he said all you need to know about scuba is to go slower than your slowest bubbles. In fact now we know to go much slower but the point remains the same.

Maybe you meant "..all you need to know about a safe ascent rate.." which would be a bit more reasonable. But since new divers read these threads, it's worth correcting this statement.

New divers, please understand that even though an instructor is quoted as saying that "all you need to know about scuba is to go slower than your slowest bubbles", this is inaccurate. Even if this rule of thumb is approximately equal to a reasonable ascent rate, there are many things other than ascent rate that you need to be aware of. Here are a few, for example:

1) You need to be aware of your no-deco time, and not enter into a staged decompression obligation without the appropriate gear, gas reserve and training.

2) You need to be situationally aware and monitor your gas so as not to run out underwater.

3) You need to be aware of the maximum safe breathing depth for whatever you are breathing, especially if you are breathing enriched air.

4) A safety stop, while not mandatory, is still a good idea, and shouldn't be omitted. The reason for this is that many divers will inadvertently violate an ascent rate or NDL to a small degree during the dive, increasing decompression stress without their computer necessarily giving them a mandatory stop. The safety stop is an extra safety buffer in this instance. Also, people tend to ascend faster when they are almost at the surface, figuring that the dangerous part of the dive is done. By always doing a safety stop, you make this less likely.

5) You should avoid a sawtooth dive profile, with multiple ascents within a dive. This is how I ended up taking a chamber ride despite my computer saying that I was "safe".

 

[doctormike]

Respectfully disagree. Owning a Perdix and not having a basic understanding of GFs, even the preset Recreation mode options, is kinda like owning a Bugatti and never taking it out of idle.

Respectfully disagree. Many recreational divers do not understand GFs, I didn't before tech training, and I don't think that means that they shouldn't be diving.

Shearwater computers are awesome for many reasons. They have rock solid construction, they have incredibly visible and well organized displays, you can replace the battery easily in the field, and the company provides amazing technical support. Why wouldn't a recreational diver want one of these if they could afford it?

There are no advantages of a Bugatti other than looks, speed and handling, right? It's not like they are incredibly reliable, or cheap, or maintenance free, or get great gas mileage, or have a lot of trunk or passenger space. If you are going to buy one with all the downsides, you probably want to use it for it's intended purpose (driving fast!).

A Shearwater, on the other hand, is a terrific dive computer for anyone, even someone who never does more than shallow reef diving within NDLs.