Ever seen a computer malfunction in this way?

[Fastmarc]

Recently I began to suspect my Nemo Wide (1st gen) of giving me incorrect NDL on the conservative side. I have been more frequently diving with more experienced divers, so our bottom time are not limited by running low on air. That's when I started noticing that especially on the 2nd dive I have to get shallower much earlier than they do and they are not pushing their NDL.
It prompted me to pull the trigger on a Perdix AI and I finally got to dive with it yesterday. I carried the Nemo to see if my suspicions were correct and boy, were they.
I'm running 45/95 which in the planning mode closely matches the Mares for the 1st dive. On the dive itself, the Mares got to 1min NDL when the perdix was at 7min. That increased on ascent for them both with the perdix being more generous. My surfacing GF was 66%.
However, on the second dive the mares went into Deco 34mins into the dive when the perdix was showing me 42 mins NDL. I think at this time it was showing surfGF 70. Another diver was swimming past and I showed him. He said at that time he was at 50 mins NDL. I finished the dive as per normal and surfacing with GF56. The Mares got up to 12mins deco and never cleared before surfacing.
I was curious and downloaded the Mares dive. Below are the two.
What could have caused such a drift?
Should I be grateful that at least it did it in a conservative way?

 

[Fastmarc]

Oh, also saw that the temperature on the Mares is higher by 3 deg F.

 

[Oitzu]

What could have caused such a drift?
Should I be grateful that at least it did it in a conservative way?

Isn't the reason just a completly different algorithm between your 2 computers?

 

[JackD342]

There are several variables you have not defined in your original question:

• Is your Nemo by any chance set to a personal conservative factor? P0 is the least, there are also P1 and P2.
• Were you and the other divers all using the same air mix?
• Do you have any idea what computer(s) the other divers were using?
• Most importantly, you have not in any way described the profile for your first dive, or the surface interval between the two dives. Without that context, there is no way to consider whether or not the second dive was surprisingly restricted.

Also, 3° F is no big deal. There is usually a significant delay in reading the ambient temp as you move through thermoclines, and I am sure different units have different lag times. Also could just be one is more affected by proximity to your own warm body than the other.

 

[Centrals]

RGBM vs Buhlmann

As already suggested by "Jack", the first dive could have significant effect on the calculation of NDL for the 2nd dive especially Mares(RGBM). Short surface interval, reversed profile etc etc will reduce the NDL for the subsequent dive(s).

 

[Fastmarc]

Hi all,
No personal conservative factors. P0.
A couple of divers were using suunto and one an oceanic. They haven't adjusted theirs either.
We were all on air.
Surface interval was 1h 18mins.
1st dive was 42mins, max depth 91ft, average depth 56 ft.
I know Mares to be conservative, but this is quite a difference. I've only noticed this recently too.

 

[JackD342]

Perhaps this entirely explains why the issue is "recent":

I have been more frequently diving with more experienced divers, so our bottom time are not limited by running low on air.

 

[KenGordon]

Surface interval was 1h 18mins.
1st dive was 42mins, max depth 91ft, average depth 56 ft.

The various RGBM computers, certainly Suunto, really want a 2hr SI.

There is a thing in the first screenshot which looks like a representation of the saturation of each tissue group. One of the (presumably) slower groups is red, suggesting the earlier dive might be responsible. Also, did you complete a full 3 minutes between 10ft and 20ft on the first dive?

 

[Fastmarc]

Perhaps this entirely explains why the issue is "recent":

While my frequency with them has increased in the last few months, I've dived intermittently with them over the last year or so. Never noticed any difference with our bottom times in those dives.

¯\_(ツ)_/¯

 

[Fastmarc]

Also, did you complete a full 3 minutes between 10ft and 20ft on the first dive?

It was about 5mins.