Computer differences?

[Seadogrun]

Hello, I am not sure if this is the right place for this type of question, but here goes.

I have had an Aqualung I550 for a while. I recently got a Garmin MK2i for redundancy and wanting the info on my wrist and also to use as a daily wear watch.

On a recent day with 2 dives(60'ish, ean31), on dive 2 the NDL was greatly different between the two computer with the I550 being much more limiting. At one point the NDL on the I550 showed 5minutes while the MK2 showed 25 minutes. The MK2 conservatism is set to high. My GF and dive buddy have the same setup and got the same discrepancy between the computers. We used the more limiting computer to end the dive, but wondering why these two computers would be so different.

So I am curious to know what others have experienced when using multiple computers.

 

[d^2b]

Different computers use different decompression models (also known as different algorithms). The Mk2i uses the same algorithm (Buhlmann ZH16C) as several other computers like shearwater and deep 6, so if you wanted matching computers (or at least closer computers), you could buy two with the same algorithm.

 

[The Chairman]

Yeah, I'd trust the Garmin over the Aqualung. If it's a Suunto clone, then it was developed in cold, cold seas and is quite conservative by nature. Deep 6 and Shearwater are much better companions for sub-tropical waters like Pompano.

 

[Seadogrun]

Thank you,

I figured there would be different algorithms, I am just surprised with how big the differences were between the two on this set of dives. Thank you for the info.

 

[The Chairman]

Thank you,

I figured there would be different algorithms, I am just surprised with how big the differences were between the two on this set of dives. Thank you for the info.

Henry's law applies. The colder a liquid, the more gas can be absorbed. As core temps decline, the total amount of gas that can be absorbed in a half time increases. While incredibly small at first, these are additive and can result in a major disparity over time. Core temps decline a lot more slowly in sub-tropical waters than in Scapa Flow. Unfortunately, there is no way at this time for your PDC to measure your core temp, so the algorithms vary depending on where the PDC was developed. FWIW, this was explained to me by Suunto's head engineer over a picnic table at Ginnie springs as I complained about the very same thing. By default, the Suunto PDCs are safer. Maybe a bit "too" safe.

 

[Pedro Burrito]

---

---

A ScubaBoard Staff Message...

Moved from Basic

---

 

[harvalen]

Also, since the big difference was noticed on the 2nd dive it might behave similar to Suunto Vyper Air (and likely other older Suunto computers, not sure about the newer ones), that gives a fairly big penalty on the 2nd dive, if the surface interval is shorter than 2 hours.

On the first dive, on default (least conservative) settings, the Suunto Vyper Air is roughly equivalent to GF High 85 (some more time at 30 m, some less in the 15-20 m range), on the 2nd dive it'll be more like GF High 70. With the middle setting it's roughly GF High 70 on the first dive and 50-55 on the second.

 

[scubadada]

Hi @Seadogrun

Your Aqua Lung computer runs a decompression algorithm called PZ+. The algorithm was developed by Pelagic Pressure Systems, purchased by Aqua Lung from American Underwater Products (Oceanic, Aeris, Hollis) in 2015. The algorithm is unrelated to Suunto, the Suunto comments do not apply to this discussion. PZ+ is moderate or middle of the road in the spectrum between conservative and liberal Your computer does have a conservatism setting that makes it even more conservative, you would want to make sure it is off.

Your Garmin computer runs Buhlmann ZH-L16C and has three presets and custom settings. The settings use gradient factors (a portion of the maximal exposure allowed by the algorithm). The settings have 2 numbers, GF low/GF high. The GF high dictates the length of NDL for no stop dives. The GF low only applies to decompression dives and dictates the depth of the 1st stop. Your computer has 3 presets, 35/75, 40/85, and 45/95. I assume you actually have your computer set at the least conservative setting (highest GF high), 45/95. If you were using a GF high of 85, the 2 computers would match more closely.

You are correct, the more conservative computer will always control your dive unless you are willing to satisfy any decompression obligation prior to surfacing. If you skip the deco stop on your Aqua Lung, it would lock you out for 24 hours.

Here is a table listing the NDLs for the 3 GF presets and PZ+ using 32%

Best of luck with your diving

 

[tursiops]

Agreed, all mentions of Suunto are Off Topic.

 

[azstinger11]

Hi @Seadogrun

Your Aqua Lung computer runs a decompression algorithm called PZ+. The algorithm was developed by Pelagic Pressure Systems, purchased by Aqua Lung from American Underwater Products (Oceanic, Aeris, Hollis) in 2015. The algorithm is unrelated to Suunto, the Suunto comments do not apply to the discussion. PZ+ is moderate or middle of the road in the spectrum between conservative and liberal Your computer does have a conservatism setting that makes it even more conservative, you would want to make sure it is off.

Your Garmin computer runs Buhlmann ZH-L16C and has three presets and custom settings. The settings use gradient factors (a portion of the maximal exposure allowed by the algorithm). The settings have 2 numbers, GF low/GF high. The GF high dictates the length of NDL for no stop dives. The GF low only applies to decompression dives and dictates the depth of the 1st stop. Your computer has 3 presets, 35/75, 40/85, and 45/95. I assume you actually have your computer set at the least conservative setting (highest GF high), 45/95. If you were using a GF high of 85, the 2 computers would match more closely.

You are correct, the more conservative computer will always control your dive unless you are willing to satisfy any decompression obligation prior to surfacing. If you skip the deco stop on your Aqua Lung, it would lock you out for 24 hours.

Here is a table listing the NDLs for the 3 GF presets and PZ+ using 32%

View attachment 786588

Best of luck with your diving

I like the chart! What’s the source of it?