How you write deco plan on your slate?

[doctormike]

Computers can’t predict conditions or what a diver gets up to during a dive. Recently I spent 15 minutes cutting the bolts on a port hole at 67m, and sending it up to the boatman, so I did the stops for a longer bottom time. A computer couldn’t know that. If you lowered a computer into the water and brought it back at 30fpm it would record a dive, no need for a diver.

I don't understand. Diving a computer is a way of quickly, automaticaly and accurately generating an ascent plan that was based on what you got up do during a dive, right? Why wouldn't it know that?

 

[Celt]

I don't understand. Diving a computer is a way of quickly, automaticaly and accurately generating an ascent plan that was based on what you got up do during a dive, right? Why wouldn't it know that?

It will record your working time as leasure bottom time, it doesn’t know you’re working and have expended way more energy.

 

[doctormike]

It will record your working time as leasure bottom time, it doesn’t know you’re working and have expended way more energy.

Right, I see what you mean. I'm not saying that there isn't an effect from exertion, but this is also incompletely understood and there is at least some evidence that exercise during a dive enhances inert gas clearance.

However, assuming that you are going to assume that your exertion increases your decompression stress. If you are going to guesstimate padding your stops based on exertion, wouldn't you rather do that on top of the data from a device that actually knows what you did in terms of depth and time, which are the major determinants of inert gas loading? Don't see this as an argument against computers.

 

[Celt]

Right, I see what you mean. I'm not saying that there isn't an effect from exertion, but this is also incompletely understood and there is at least some evidence that exercise during a dive enhances inert gas clearance.

However, assuming that you are going to assume that your exertion increases your decompression stress. If you are going to guesstimate padding your stops based on exertion, wouldn't you rather do that on top of the data from a device that actually knows what you did in terms of depth and time, which are the major determinants of inert gas loading? Don't see this as an argument against computers.

I didn’t know you could manually add bottom time on the dive, I thought you had to set the computer at the surface and it followed a profile regardless of what you were doing.
When I was younger I done a lot of work deep and you’re really wasted at the stops. I wouldn’t get away with it today.

 

[Raphus]

I didn’t know you could manually add bottom time on the dive, I thought you had to set the computer at the surface and it followed a profile regardless of what you were doing.

With s shearwater you can adjust gf high, while diving.
So if you experience cold/hard work you can put gf high to a higher value. Or if you got a big problem and accept the risk, but a lower number in.
Decompression stops will automatically adjust

 

[doctormike]

I didn’t know you could manually add bottom time on the dive, I thought you had to set the computer at the surface and it followed a profile regardless of what you were doing.

I mean, it's sort of like what you said you were doing. You did something that isn't accounted for by the algorithm (exercise), so you do longer stops. Padding. While it's possible to adjust the algorithm in the DC itself during the dive, it's also possible to just do longer stops an guesstimate, which is what I thought you were saying that you did.

The problem with just guesstimating is that you may actually be increasing your decompression stress if you overdo it. See the controversy about "deep stops".

And as mentioned above, you can actually change GF hi, or just watch GF99 and SurfGF to get an idea what your leading compartment overpressure would be at any point in the dive or on surfacing, and use that to guide you to an ascent that you are comfortable with.

 

[Celt]

With s shearwater you can adjust gf high, while diving.
So if you experience cold/hard work you can put gf high to a higher value. Or if you got a big problem and accept the risk, but a lower number in.
Decompression stops will automatically adjust

That’s brilliant, I didn’t know that. I thought you’d have to ignore the computer. Just do longer stops.

 

[Wibble]

Just an expansion of the "adjust GF-Hi" on the Shearwater point... You could just add an arbitrary 5 mins to the end of the dive which will end up as dropping your final GF down by around 5% (depends how deep and how long your deco was).

Adjusting the Shearwater's GF-Hi will effect the slope of the gradient factor curves for all stops above where you changed it, so may add a "little" to every stop and longer to the last.

I like to see my SurfGF in the "NDL" space on the Shearwater(s). This tells me a lot about the dive and is a neat countdown as deco progresses. If necessary, such as a stressful dive or one of a series of dives over many days, I may adjust the last stop and run it for a while longer. Definitely do this for the last dive before driving home as need to be alert!

(Added...)
One thing that a pre-dive planned slate likes to assume is constant oxygen content (PPO2). If diving with open circuit, this is very simple as it only considers the backgas and deco gas(es).

However, diving with CCR does change this as you may manually increase the PPO2 after an oxygen flush and leave it higher than the planned PPO2. Similarly, if moving up from 6m/20ft to 4.5m/15ft, you may not be able to maintain your planned PPO2 (e.g. 1.3 will be impossible to use at 3m/10ft, and very difficult to maintain at 4.5m/15ft).

Therefore any pre-dive planning has to be conservative and won't consider the reality of the dive. Also, you'd have to be careful if diving in bottom scours that may exceed the slate plan, or running off the wreck to go lobster hunting. Computers are definitely more accurate and, within reason, a lot more safe than pre-dive "slate plans".

 

[b1gcountry]

My written turn pressure, depth, BT, etc is a way for surface me to sign a contract with underwater me about how much risk im willing to take on a given dive. When I decide to turn at 2400psi, and I write it out before the dive, it's a lot harder to lie to myself underwater, or decide to push it a little bit more...I can always go under those numbers, but no going over.

Im not a purist about dive tables, I just prefer to set my risk tolerance on the surface instead of increasing it as the dive progresses.

 

[b1gcountry]

Im also not speaking for anyone else here... no comments I've read indicate anyone here does otherwise