Matching Dive Computer Algorithms

[uncfnp]

To confirm, my Giotto has SF 0,1 or 2.

To the op, FWIW set the Cressi to SF 0 and leave it. Too complicated to switch between dives and too prone to human error.

@CandiveOz I use Divelog on my ipad forthe Petrel and the Cressi program on my iMac for the Cressi.

So I found the cable, uninstalled and then reinstalled the Cressi interface program and now it works. And it does show NDL times throughout the dive.

A quick review tonight shows that for first dives of the day, on nitrox, the Cressi is more liberal then the Petrel on 50/80, on 100% of my dives. It will take some time to compare repetitive dives given the many variables.

 

[CandiveOz]

Hi @uncfnp
Thanks for your response. I apologise if I sound pedantic but you are running both computers on nitrox; not the Cressi on nitrox and the Petrel on air with a GF 50/80.

 

[divinh]

It's interesting that the NDL times are slightly different than from the Cressi Leonardo.

Cressi Giotto SF0, SF1, SF2 (There's no SF3, but mentioned in the manual.)

<depth in meters> <NDL time in min>

SF0

09 99
12 99
15 68
18 50
21 36
24 28
27 22
30 18
33 13
36 11
39 09
42 07
45 06
48 05

SF1

09 99
12 89
15 57
18 42
21 31
24 25
27 19
30 14
33 11
36 09
39 07
42 06
45 05
48 05

SF2

09 99
12 68
15 47
18 34
21 26
24 20
27 15
30 12
33 09
36 08
39 06
42 05
45 05
48 04

 

[divinh]

The reason I picked up the Sherwood Vision is because it was on sale during Black Friday and it is air integrated. Cressi doesn't make an air integrated dive computer.

The goal was to have two dive computers, one to serve as a backup:

1. A backup in case one fails during a trip
2. A backup to pick up where the other one left off, which ideally would use the same algorithm

With this combo, I can definitely satisfy 1. However, since the algorithms don't match, I'm not picking up exactly where I left off for 2.

So...

Would the best course of action be to use the Giotto as my main dive computer since it's more conservative and leave the Vision in its most liberal mode so that I don't trip its NDL? And should the Giotto fail and I need to rely upon the Vision alone, I would use the Vision knowing that it's much more liberal with shallow dives and slightly more liberal with deep dives? Switching to DSAT +C seems too conservative, as it seems to match SF1 and SF2 of the Giotto?

My diving, so far, is vacation diving with typically 2-4 dives per day and 5 if there's a night dive. Surface intervals are usually at least an hour, but sometimes if I'm diving from a RIB, the surface intervals are shorter (not by my choice, but due to the dive op).

Most diving is within NDL. I've only touched mandatory deco stops twice in 188 dives with the first due to depth (43m) and the second due to depth and it being the third deep dive of the day (26m, 26m, 27m). In both instances, the instructor/guide had planned for the deco, i.e. we knew we were going into deco and would have mandatory stops, so enough air was factored in. For 43m, we had two stops. For 27m, we went up to 4-5m and spent 15 min there.

Thanks to everyone who's participated. It's really enlightening.

 

[atdotde]

GFlow is only applicable for dives requiring decompression. It affects the depth of the first deco stop. So it shouldn't make any difference for the NDL/no-deco dives you are looking at. I think.

It's true that GFlow determines the depth fo the first deco stop but of course this means that in particular it determines if a dive has a first deco stop, that is if it is a dive within NDL or not. So indeed, it does influence the NDL time.

Note that there are other factors and NDL is not really such a well defined concept as one might think: For example the ascent rate is quite important: When it is slow enough, there could well be a ceiling at the end of your bottom time but there is never a mandatory stop as the tissues have already released the extra gas during the ascent to that depth if the ascent is slow enough. This phenomenon can easily be studied using the recreational mode (which is the somewhat pimped version of NDL) in Subsurface.

 

[Jay]

It's true that GFlow determines the depth fo the first deco stop but of course this means that in particular it determines if a dive has a first deco stop, that is if it is a dive within NDL or not. So indeed, it does influence the NDL time.

On a NDL dive GFLo is completely ignored and doesn't affect NDL ... at least for SW it doesn't.

 

[uncfnp]

Hi @uncfnp
Thanks for your response. I apologise if I sound pedantic but you are running both computers on nitrox; not the Cressi on nitrox and the Petrel on air with a GF 50/80.

No problem. For the dives I compared last night both computers were set for nitrox 32.

 

[uncfnp]

The reason I picked up the Sherwood Vision is because it was on sale during Black Friday and it is air integrated. Cressi doesn't make an air integrated dive computer.

The goal was to have two dive computers, one to serve as a backup:

1. A backup in case one fails during a trip
2. A backup to pick up where the other one left off, which ideally would use the same algorithm

With this combo, I can definitely satisfy 1. However, since the algorithms don't match, I'm not picking up exactly where I left off for 2.

So...

Would the best course of action be to use the Giotto as my main dive computer since it's more conservative and leave the Vision in its most liberal mode so that I don't trip its NDL? And should the Giotto fail and I need to rely upon the Vision alone, I would use the Vision knowing that it's much more liberal with shallow dives and slightly more liberal with deep dives? Switching to DSAT +C seems too conservative, as it seems to match SF1 and SF2 of the Giotto?

My diving, so far, is vacation diving with typically 2-4 dives per day and 5 if there's a night dive. Surface intervals are usually at least an hour, but sometimes if I'm diving from a RIB, the surface intervals are shorter (not by my choice, but due to the dive op).

Most diving is within NDL. I've only touched mandatory deco stops twice in 188 dives with the first due to depth (43m) and the second due to depth and it being the third deep dive of the day (26m, 26m, 27m). In both instances, the instructor/guide had planned for the deco, i.e. we knew we were going into deco and would have mandatory stops, so enough air was factored in. For 43m, we had two stops. For 27m, we went up to 4-5m and spent 15 min there.

Thanks to everyone who's participated. It's really enlightening.

That’s a tough question. Here is what I would do first then adjust if needed.

You bought the Vision for the AI so obviously you want to use it as primary and pretty much ignore the Cressi unless needed and you want to dive both computers. So you need to keep the Cressi unlocked. Plus you have had no problems diving conservative and you know how to recognize and handle “lite deco.”

As said, I would leave the Cressi in SF 0. I would start with the Vision in the conservative mode. Go diving and and watch both computers. If the variation is small between computers I would not worry overly about it since it will not matter unless you bump the NDL limits. If this happens, the more conservative will clear as you ascend and do your safety stop. If the NDL time varies greatly with the Cressi more conservative, you will have to dive the Cressi as primary, otherwise you risk getting locked out.

Or just leave the Cressi in your gear bag. If the Vision fails you will have the Cressi for the next day dives.

If you find the Vision more conservative, then you can dive it as primary and the Cressi as backup. But while testing them out make sure the Cressi is good throughout the day’s dives.

A third option if needed to keep the Cressi out of lockout as backup on repetitive dive days, and I hesitate to suggest it, is to program the nitrox one or to points higher on the Cressi, for all dives or just the 3 and 4 dives of the day.

 

[CandiveOz]

@divinh ultimately the decision is yours. I think members of this forum have given you a number of good ideas to try and in return we have learned a bit more about the Sherwood and Cressi computers due to your original post for which I am personally grateful. Members like @uncfnp and @dmaziuk who actually dive with the Cressi like the computer and don't find it restrictive as others perceived. Therefore, I don't expect you'll have any issue diving with both computers set the way you want; now that you have a better understanding of them. Again, you always have the option of swapping one in the second hand market. You mentioned that the Vision has air integration so I assume, like @uncfnp, you probably favour it over the Cressi. Keep your eyes open for a used Sherwood or Oceanic on Ebay or Craigslist and I'm sure the price differential between it and selling your Cressi would be minimal. Good luck with your diving.

 

[scubadada]

Hi @divinh

If I were in your current position, I would dive the Sherwood in DSAT and the Cressi in SF0. The Cressi will control your dives and you will need to stay under the NDL or satisfy the deco stops to keep from being locked out. Diving with 2 different deco algorithms has been discussed many times on SB. If you failed to satisfy the deco stops on the Cressi, but the Sherwood still had NDL, this would still be a "safe" dive, just like diving DSAT alone.

You could still use the Sherwood for the AI pressure reading and the air time remaining (ATR), as well as your backup NDL

In the meantime, you can keep your eyes open for a good deal on an Oceanic Geo 2 or a Sherwood Amphos. Their cost new is a little more than your Cressi. They both periodically go on sale and are certainly available on the used market. You could probably sell your Cressi fairly easily.

Best of luck and good diving, Craig