Conservative vs aggressive GF Algorithms

[Nomad-Diver]

Hi
I recently did 6 recreational dives (2 per day over 3 days max depth in mid 20m usually) and had two computers
1. Shearwater Petrel (GF 50/75), and
2, Liquivision X1 (GF 50/90)

I keep the Liquivision more aggressive in case a quick exit is ever needed, but I usually dive to the Shearwater times and stops. Both computers were set to Nitrox 32%.

Usually on deco dives the X1 clears before the Shearwater (as I thought one would expect it to)

My dilemma is - In two of the dives, the X1 added a mandatory 3m stop yet the Shearwater didn't !?

• On one dive the 3m stop appeared at the end of the dive just before ascending and cleared before I got to my safety stop. No problems (apart from the fact the Shearwater didn't add a deco stop at all)
• On the other dive, the mandatory stop appeared after 25 mins (half way through the dive) and held me after my usual 5 minute safety stop until it had cleared its mandatory 3m deco stop. Again, the Shearwater didn't impose a mandatory stop at all.

Why would, what I thought was a more aggressive setting, add a mandatory deco stop, when the more conservative computer didn't.

Thanks for your help

.

 

[Trace Malinowski]

Different algorithms for calculating the dive are being used in two different computers. I can run a VPM +2 in Multi-Deco and VPM +2 through my Shearwater and get two different ascent schedules based upon the exact same ascent philosophy.

 

[stuartv]

Some computers show a mandatory deco stop as soon as it determines that you have a virtual ceiling. Shearwater (and maybe others) are smarter than that. Shearwater factors in the deco that will happen during your ascent. If the Shearwater determines that your ceiling will clear by the time you ascend at 30 feet per minute to arrive at the ceiling, then it does not display a mandatory stop.

I suspect that that is the explanation behind your first bullet point. Maybe your second too, but you didn't give enough details about times and stops to really say.