NDL at very shallow depth (6.5m)

[wotje]

Just finished two dives. First one lasted 70 minutes and the second dive my Suunto Zoop gave me quite little time to dive deep (which makes sense) but also at 6.5m the NDL's kept counting down untill 1 minute. I felt I was pushed out of the water.

Normally above 9m I get a stripe meaning I am safe and have infinite NDL. Now it has happened two times I am only safe above 6m. I even got a Decompression minute the other time because I descended to 6.5m. It does not make any sense. Me being the only one of my group with the computer pushing me out. Any idea what happens? My interval on the surface was 1 hour.

 

[Tassie_Rohan]

What was your maximum and average depth, your bottom time and your total time for both dives?

A screenshot of your dive profile (both dives) would be helpful.

 

[wotje]

Dive 1:
Start time: 08:50
Duration: 75 min
Max. Depth: 27.5m
Average: 14.6m

Surface Interval: 1h7m

Dive 2:
Start time: 11:13
Duration: 76 min
Max. Depth: 21.9m
Average: 11.8m

I am afraid Suunto only gives me a dive profile to scroll through and no complete overview.

 

[inquis]

Suunto's RGBM algorithm is known to be restrictive on repetitive dives. It is proprietary (so details are not known for certain), but anecdotally, short intervals, rapid accents, and up & down profiles seem to penalize you. Check the conservatism setting, but it's a known issue for some whose dives are not gas-limited.

Be careful that you adhere to all it's guidance if it goes into mandatory deco mode. Stay above a stop too long and it will lock you out for 48 hrs.

The long term solution may be to switch to a computer with a different algorithm like Buhlmann ZHL-16C. Shearwater and Garmin are two popular manufacturers of such products.

 

[Tassie_Rohan]

The simple answer to your question of why you were penalized relative to your friends: they were probably diving computers with algorithms developed by a scientist, while you were diving a computer with an algorithm that has been developed by a legal department.

If they had Garmins or Shearwaters then they would be using an algorithm that has the principle aim to avoid nitrogen saturation (bubble formation) in various tissue compartments. Your Suunto has a highly modified bubble-modelling algorithm that has the principle aim of making sure you do not sue Suunto for decompression injuries.

To achieve this, as Inquis mentioned above, Suunto have made many modifications to the original model, to protect you from yourself. Only the folks inside the company really know what many of these changes are.

Suunto seems to dislike repetitive dives like yours where you seem to riding close to the NDL limit, (particularly if you have a sawtooth profile), so I'm not surprised that their legal department, as you stated, started 'pushing you out of the water'.

The behavior of Suunto computers is a very common topic here. A quick search should enlighten you on your computers behavior - here's last months thread on the subject, there are 100's more:

Dive Computer No Deco Computations Question

While we are not new to diving we are new to the dive computer world. We started out with basic Suunto Vypers for the first couple years. We just upgraded to Apple Watch Ultra 2/Ocenaic+ this year. We are running both computers together on our dives. We are noticing that the No Deco times...

scubaboard.com

Given that 99% of divers just want a safe number to follow, and that decompression limits are a slightly grey zone that can vary between individuals and conditions, I can understand Suuntos approach.

Cheers
Rohan.

 

[NothingClever]

They make excellent compasses, I'll give Suunto that much.

 

[Landau]

Be careful that you adhere to all it's guidance if it goes into mandatory deco mode. Stay above a stop too long and it will lock you out for 48 hrs.

Actually, the reason to adhere to the deco guidance is to prevent injury or death. This is still the case even on computers that will not lock you out if you happen to survive.

That said showing an NDL at 6.5m, assuming the OP had been there some time or at least ascended slowly to that depth, is strange. I’ve never seen this on my Suunto or on any divers I’ve guided. The NDL is usually > 99 just before we hit safety stop depth..

 

[inquis]

Actually, the reason to adhere to the deco guidance is to prevent injury or death. This is still the case even on computers that will not lock you out if you happen to survive.

My thought was someone could easily follow their buddy who is well within their NDL to safety stop depth or even the surface, without realizing their Suunto wants a stop. I agree with you that they should obey the computer, but sometimes they have no idea what it is telling them.