Discrepancy between my computers during the dive

[pisauron]

Hi @pisauron

View attachment 839043 View attachment 839044

At just under 30 minutes of your second dive of the day, your Ratio computer had 1 min of NDL left and an average depth of 23.3 meters. Your Garmin computer had 13 min of NDL left with an almost identical average depth of 23.2 meters. I find it hard to believe that the difference in NDL is due to a difference in the depth.

I wonder why the CNS O2 are different

You are correct. The depth shown here is a result of me hovering the mouse not good enough. Underwater i notice a depth difference on 0.01m at most.

There is something defenetly wrong with the Ratio algorithem implementation. I will be messaging them too.

 

[stepfen]

I think i am running the latest update.
I tried updating and it's stays the same.

Can you check which one you got?

I got the 5.2.4.5/016

Indeed, we've got the same (and latest) firmware. Hmmm strange indeed.

 

[boulderjohn]

I think you should contact Ratio. I hope they will be helpful. You may help them discover a problem they can fix.

Quite a few years ago, when the first Shearwater computers came out, a friend of mine was using one and thought the depths were wrong. He contacted Shearwater, and their discussions led Shearwater to realize they were not considering altitude. They changed that accordingly.

Not long after that, another diver and I discovered that our Shearwater computers would sometimes give different depth readings, but sometimes they would be the same. After discussions, they realized that the problem was that when you turned on the computer manually during predive checking, it calculated the altitude. If you descended after too long, it would shut off on its own, as designed. If it turned on automatically because of your descent, it assumed you were at sea level. They changed that accordingly.

 

[pisauron]

I think you should contact Ratio. I hope they will be helpful. You may help them discover a problem they can fix.

Quite a few years ago, when the first Shearwater computers came out, a friend of mine was using one and thought the depths were wrong. He contacted Shearwater, and their discussions led Shearwater to realize they were not considering altitude. They changed that accordingly.

Not long after that, another diver and I discovered that our Shearwater computers would sometimes give different depth readings, but sometimes they would be the same. After discussions, they realized that the problem was that when you turned on the computer manually during predive checking, it calculated the altitude. If you descended after too long, it would shut off on its own, as designed. If it turned on automatically because of your descent, it assumed you were at sea level. They changed that accordingly.

Hopefully tommorow i would be able to produce video proof of this and later send a detailed mail to Ratio.

I doubt anything is wrong with my specific computer as today i re-updated software (to the same one), and still got major differences.

Tommorow dives would be easier and i will be able to film.

I will update here.

 

[Belzelbub]

AFAIK, subsurface is displaying its own GF setting, not the one of the dive computer.

Yes. This is correct. However, in the preferences, you can set it to match what your computer is displaying.

only by dragging the mouse along the dive profile and reading the data from the floating menu for the point on the dive selected

Yep. The floating menu will display two NDLs. I'm not in front of Subsurface on my computer but I believe one is just simply "NDL" and one is "NDL (calc)". IIRC, the first will be from the DC, the second will be from what Subsurface calculates on it's own based on whatever data the DC provides.

 

[Seaweed Doc]

This whole thread is giving me the heebie-jeebies. I'm glad pisauron is aware of the problem and, if the situation is as we're assuming, the errant computer is more conservative.

I'm old enough to remember the first generation of dive computers and hearing stories of divers who got bent trusting computers. (I think some of the training material I've read through over the years acknowledges the problem of those early computers.) For this and cost reasons, I was a late adopter of using a computer.

But now I trust them, perhaps too much. In fact I really trust mine: When teaching AOW, I have students compare their computer's reported depth to their buddy's and mine. I have two computers (Perdix and AL i330R) which have always agreed as to depth. (They differ for NDL because they use different algorithms.) I figured mine were right since they agreed and I knew I'd cared for them reasonably well. Students using rental computers, though....

The only failure I had was when the pressure sensor on the Perdix failed during a drive (yes, typed correctly, not a dive). Coming home from a dive, while in a slightly below sea level tunnel, it turned itself on and started reading several hundred feet deep. Thankfully it was a catastrophic failure that was immediately obvious.

I'm curious about others' experience with failed computers: Do they seem to consistently fail by reading deep or requiring deco when it's not required? That is, do they "fail safe?" Anybody had them read depth clearly shallower or underestimate NDL?

 

[pisauron]

This whole thread is giving me the heebie-jeebies. I'm glad pisauron is aware of the problem and, if the situation is as we're assuming, the errant computer is more conservative.

I'm old enough to remember the first generation of dive computers and hearing stories of divers who got bent trusting computers. (I think some of the training material I've read through over the years acknowledges the problem of those early computers.) For this and cost reasons, I was a late adopter of using a computer.

But now I trust them, perhaps too much. In fact I really trust mine: When teaching AOW, I have students compare their computer's reported depth to their buddy's and mine. I have two computers (Perdix and AL i330R) which have always agreed as to depth. (They differ for NDL because they use different algorithms.) I figured mine were right since they agreed and I knew I'd cared for them reasonably well. Students using rental computers, though....

The only failure I had was when the pressure sensor on the Perdix failed during a drive (yes, typed correctly, not a dive). Coming home from a dive, while in a slightly below sea level tunnel, it turned itself on and started reading several hundred feet deep. Thankfully it was a catastrophic failure that was immediately obvious.

I'm curious about others' experience with failed computers: Do they seem to consistently fail by reading deep or requiring deco when it's not required? That is, do they "fail safe?" Anybody had them read depth clearly shallower or underestimate NDL?

Indeed the Ratio computer is greatly more conservative then the Garmin. But i trust the Garmin more.
Anyways the Ratio cleared the deco by itself since i ascend slowly anyways and the deco was 1-2 min max. Also my GF is 70/85 that pushed the first stop to 6meters anyways.

But moving forward with longer deco dives i suspect the Ratio will demand crazy deco times that i simply don't have enough gas for. Meaning i might skip some, and might cause a lockout. We'll see.

Sadly i couldn't video today so i will be sending Ratio the info from the logs as it is and wait for the reply.

 

[Belzelbub]

Sadly i couldn't video today so i will be sending Ratio the info from the logs as it is and wait for the reply.

Please share what you find. The discrepancy is definitely concerning.

I also dive with two different branded dive computers running the same Algorithm. A Garmin on my left wrist and a Shearwater on my right. I've never seen much of a difference between the two. NDL, Depth, and Surf GF are pretty much spot on. Occasionally, I may see a slight discrepancy in Depth, NDL, or Surf GF, but I'm only talking about 1', 1', or 1%. And that could be attributed just to differences in where my arms are / have been, as well as sensor tolerances. Certainly nowhere near what you are seeing. Mine are close enough that I'm confident that they are right enough.

 

[scubadada]

...I'm curious about others' experience with failed computers: Do they seem to consistently fail by reading deep or requiring deco when it's not required? That is, do they "fail safe?" Anybody had them read depth clearly shallower or underestimate NDL?

My son and I have been diving computers since 2002, my wife and daughter since 2004. Altogether we have a little over 3000 dives on computers. We've used around 10 or so computers during this time.

We've had only had two pressure sensor failures. Both were in very old Oceanic Pro Plus 2s that started out in 2002 and were passed down. In both, they started reading as if they were in the next highest altitude group, 3001-4000 feet rather than 0-3000 feet. Like diving at altitude, the NDLs were decreased, this was quite obvious as the NDLs were much shorter. Both were replaced by Oceanic/Huish in their service program for $175 and are still in use today.

 

[ahereng]

Did you check if altitude settings on the Ratio is not set to an elevated altitude?
This would shorten ndl and prolong deco.