New Shearwater AI transmitter - the Swift

[tridacna]

 

[Bigbella]

@tridacna : so it's three; I knew it!

Regardless of the benefits or drawbacks of a given algorithm -- whether Shearwater, Suunto or Oceanic -- it still strikes me as an embarrassment of riches, coming from someone who was among the first generation to enjoy a submersible pressure gauge; and whose slightly older business partner only thought of ending his dives when the breathing resistance became too great, on a J-valve . . .

 

[tridacna]

@tridacna : so it's three; I knew it!

Regardless of the benefits or drawbacks of a given algorithm -- whether Shearwater, Suunto or Oceanic -- it all still strikes me as an embarrassment of riches, coming from someone who was among the first generation to enjoy a submersible pressure gauge; and whose older business partner only thought of ending his dives when breathing resistance became too great, on a J-valve . . .

I started with a pink ribbon. GTR harumph!

 

[stuartv]

Feel that this is like Zen And The Art Of Motorcycle Maintenance -- we see things in different ways. When diving I don't care for accuracy when there's a long time to go; absolutely need accuracy when there's not a lot left. I'd rather rely on my diving plan and reserves.

I agree that a high degree of accuracy is not all that important early in a dive. But, why would anyone in their right mind prefer low accuracy when they could have much higher accuracy??

Would you be satisfied with an SPG that was +/- 500 psi when it was greater than 2000 as long as it was only +/- 50 when it was less than 1000?

I wouldn't! We know that an SPG could be +/-50 at 3000 AND at 300. Why would you not prefer that over +/-500 at 3000???

Especially when, in this case, it's just a firmware update away? It's not like they would need to change any hardware. Oceanic is giving more accurate results from the same transmitters.

 

[stuartv]

The Cult of Shearwater has worn me down today. I'm done with this little subplot.

 

[tridacna]

The Cult of Shearwater has worn me down today. I'm done with this little subplot.

Promises, promises, promises.

 

[Hoag]

I have seen my DC (an Oceanic PP3) tell me at the beginning of the dive that I only had 5-10 minutes left and I hadn't even descended past 20 feet yet.

There can be a LOT happening at the beginning of a dive that would affect the pressure in the tank and therefore the calculated ATR. If your tank has been sitting in the sun on a day when the temperature is in the 90s and then jump into water where the temps are in the 70s, the pressure will drop significantly and rapidly. The part of the DC that calculates ATR doesn't really factor temp into it's calculations. All it "cares about" are that in the last 30 seconds, you went through 300psi (as a hypothetical) based on that rate, you will have less than 10 minutes before the tank is OOA, and you are SOL. Gradually, as the dive progresses, this initial rate becomes less and less of a factor and your DC is able to have a far more accurate approximation of the ATR.

I tend to not trust ATR values for the first few minutes of any dive.
__________________________________________

Edited to change "DTR" to "ATR".

 

[Wibble]

In the red corner we have apparent accuracy…

In the blue corner we have too much information and a scepticism of over-reliance on computer projections, especially those made on huge assumptions using simplistic algorithms.

Whichever; the good thing is we love diving and we’re all aware of our equipment's limitations.

BTW I’ve several gauges all reading different values. My gas pressure transducers read the lowest of them all but because they are digital readouts, they have more weighting; they appear more accurate than the analogue gauges, but they’re just as inaccurate as all the others.

 

[scubadada]

I have seen my DC (an Oceanic PP3) tell me at the beginning of the dive that I only had 5-10 minutes left and I hadn't even descended past 20 feet yet.

There can be a LOT happening at the beginning of a dive that would affect the pressure in the tank and therefore the calculated DTR. If your tank has been sitting in the sun on a day when the temperature is in the 90s and then jump into water where the temps are in the 70s, the pressure will drop significantly and rapidly. The part of the DC that calculates DTR doesn't really factor temp into it's calculations. All it "cares about" are that in the last 30 seconds, you went through 300psi (as a hypothetical) based on that rate, you will have less than 10 minutes before the tank is OOA, and you are SOL. Gradually, as the dive progresses, this initial rate becomes less and less of a factor and your DC is able to have a far more accurate approximation of the DTR.

I tend to not trust DTR values for the first few minutes of any dive.

Oceanic/PPS ATR uses a 90 sec moving average for gas consumption. Not sure what Shearwater does, they don't say

 

[ATJ]

So you're the one who believes the weather forecast? (Obviously talking about a temperate climate, not Florida)

It sounds like you don't trust weather forecasts. Why would that be? Because you have found them to be inaccurate?

Hmmm... same goes for GTR on a Shearwater computer. I have found it to be inaccurate and so I don't trust it and it is of little value.

It's absolutely nothing to do with Shearwater. It's everything to do with common sense: there's so many variables that will affect your Gas Time Remaining during a dive that a long duration at the beginning is completely meaningless. A simple >1h is the best you could expect.

Yes. Common sense says you should make decisions based on the most accurate information you have available. Shearwater GTR is not accurate and so should not be used as a basis for decisions.

I fear that some Muppet with a reverse profile dive (shallow, then deeper) or must return to shot in a current would then "sue" the manufacturers for obeying the rules of physics because they disobeyed the rules of planning and common sense. God help someone who's really relying on that data in an overhead/deco rather than proper planning.

Actually, Shearwater are NOT obeying the rues of physics. Physics tells us that non-ideal gases do no compress linearly. Shearwater's GTR assumes they do compress linearly!

Aside from Mr Spok, knowing information to 3.14159265 decimal places is missing the point. You leave margins for unknowns, changes, contingency and error.

You are the one that is missing the point. No one is asking for extra precision. We are asking for accuracy: not extra accuracy, just accuracy.

In this case, IMHO, Shearwater could enhance their UI (User Interface) by setting it to >60mins and Oceanic are just plain wrong to assume the dive will continue with the current parameters.

Again you are missing the point on GTR. GTR is specifically assuming that the dive will continue on the current parameters. That's the whole point of it. It is not trying to guess what may or may not happen.

As I already posted:

Gas Time Remaining (GTR) is the time in minutes that can be spent at the current depth and SAC rate until a direct ascent to the surface at a rate of 33 feet/min (10 m/min) would result in surfacing with the reserve pressure.

This is the starting point. As a diver, if you do maintain your current breathing rate and stay around the current depth, you will know how long your gas should last. Obviously if you go deeper you gas will last for less time. If you breathe more heavily your gas won't last as long. If you go shallower, your gas will last longer.

It is actually a very simple concept. I don't understand why you can't grasp it.