New Shearwater AI transmitter - the Swift

[ATJ]

Does knowing your gas volumes to ten decimal places add any value? Unless you’re a Vulcan of course ;-)

Point being that approximation is more than sufficient as your gas consumption varies by so many factors, not least the accuracy of the sensor, depth, amount of BCD gas, breathing rate, workload, temperature, surface testing/pre breathe, etc, etc.

You need safety margins when you’re diving. Especially with your planning SAC. Cavers use the rule of thirds as the absolute maximum; mostly they’ll use much less; quarters, sixths, simply to cater for unknown issues.

Do you ever get a full 300bar fill? Are your gauges calibrated? (mine are probably 10% out). I’d sooner plan based on 230bar and fill them to 260bar.

Your questions and comments are not relevant to the issue I am discussing!

I'm talking about two different dive computers side by side on the same dive providing vastly different estimates for gas time remaining early in the dive.

The two dive computers (Shearwater Perdix AI and Oceanic VT 4.0 - and an Atom 3.0 before that) receive and display the exact same pressure data from a single transmitter on my first stage.

Five minutes into a two hour dive when I'm at "average depth" for the dive and my buoyancy and breathing is fully settled, the Oceanic usually estimates GTR to be around the 2 hours. The Perdix can be as low as 60 minutes and I don't think I have ever seen it over 90 minutes. Throughout the dive the Oceanic estimate makes much more sense than the Perdix but the Perdix estimate gets closer to reality towards the end of the dive.

Yes, perhaps the Perdix is being conservative to add a margin of safety, but it is so far out at the start of a dive that I basically ignore it.

 

[Wibble]

Different algorithms, different implementations.

Estimating gas durations that far out in a dive isn’t prudent, especially if someone may actually rely on that info to push beyond the bounds of safety.

Glad to see the Shearwater algorithm errs towards conservatism.

 

[jborg]

As noted the Shearwater doesn't take compressibility into account, and it appears to base the GTR on something like the max consumption in the last x minutes, or at least something weighted towards some recent maximum. With a 300 bar tank (and sometimes cold water) the initial GTR estimates are indeed very pessimistic.

I'm fine with that though. To the extent that I ever care about GTR it's usually towards the latter half of the dive.

 

[stuartv]

Different algorithms, different implementations.

Estimating gas durations that far out in a dive isn’t prudent, especially if someone may actually rely on that info to push beyond the bounds of safety.

Glad to see the Shearwater algorithm errs towards conservatism.

I think that is, well, ridiculous. If you have a measuring tool, it should give as accurate a measurement as it can. Definitely not put in some undocumented and unknown "fudge" to (allegedly) make it safer for the user.

Oceanic gives a more accurate GTR measurement. That seems to have been reported by multiple users and (as far as I know) undisputed.

We should not make excuses for Shearwater. We should report that their measurement is not very accurate, and that another manufacturer is much more accurate - so we know that is in fact possible to be more accurate - and request that Shearwater undertake to be as equally accurate as Oceanic.

 

[Diving Dubai]

I think that is, well, ridiculous. If you have a measuring tool, it should give as accurate a measurement as it can. Definitely not put in some undocumented and unknown "fudge" to (allegedly) make it safer for the user.

I don't get this

GTR is a guess at best by the computer. It doesn't know if your going to inflate a dsmb or a lift bag or about to hit current and need to work harder, all which will affect the calculation after the event. Its based on only averaged consumption of the last 2 minutes dive time, to give you a direct ascent to surface maintaining reserve.

I don't know nor care what fudge factors they put in as it's only a reference guide not an absolute

The most accurate GTR is a combination of using the remaining gas contents and the divers brain, and being able to calculate GTR in your head is a simple exercise anyhow

 

[stuartv]

I don't get this

GTR is a guess at best by the computer. It doesn't know if your going to inflate a dsmb or a lift bag or about to hit current and need to work harder, all which will affect the calculation after the event. Its based on only averaged consumption of the last 2 minutes dive time, to give you a direct ascent to surface maintaining reserve.

I don't know nor care what fudge factors they put in as it's only a reference guide not an absolute

The most accurate GTR is a combination of using the remaining gas contents and the divers brain, and being able to calculate GTR in your head is a simple exercise anyhow

I think that is intentional.

I think everyone (involved in this conversation) understands what GTR is, including its limitations and assumptions.

That does not change the fact that experience suggests that the Oceanic GTR/ATR calculation in the early part of a dive is more accurate than the Shearwater calculation in the same (early) part of the same dive.

For example: A diver splashes for a drift dive at a fairly constant depth. 5 minutes into the dive, the Shearwater says GTR is 60 and the Oceanic says GTR is 90. The diver drifts along for 60 minutes at more or less the same depth and not working at all, the whole time. After 60 minutes, both the computers say GTR is 30. It seems to me that the Oceanic is more accurate - and therefore, more useful - in this scenario.

 

[Diving Dubai]

That does not change the fact that experience suggests that the Oceanic GTR/ATR calculation in the early part of a dive is more accurate than the Shearwater calculation in the same (early) part of the same dive.

The Oceanic is probably more accurate than my Eon too at the beginning of the dive, since the consumption data and Gas time is compete nonsense for the first 2 mins (just looking back at some logs, I've seen consumption rates over 30l/m at depths less than 20m and then it drops right down to 12-13l/m across the rest of the dive.

I've always assumed its down to the reasons stated above - but none of it is important since I know my depth, dive time and gas pressure

I'm not challenging that the Oceanic has a more accurate early dive calculation, I'm just saying who cares

 

[Wibble]

I think that is, well, ridiculous. If you have a measuring tool, it should give as accurate a measurement as it can. Definitely not put in some undocumented and unknown "fudge" to (allegedly) make it safer for the user.

Oceanic gives a more accurate GTR measurement. That seems to have been reported by multiple users and (as far as I know) undisputed.

We should not make excuses for Shearwater. We should report that their measurement is not very accurate, and that another manufacturer is much more accurate - so we know that is in fact possible to be more accurate - and request that Shearwater undertake to be as equally accurate as Oceanic.

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

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.

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.

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

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.

 

[stuartv]

The Oceanic is probably more accurate than my Eon too at the beginning of the dive, since the consumption data and Gas time is compete nonsense for the first 2 mins (just looking back at some logs, I've seen consumption rates over 30l/m at depths less than 20m and then it drops right down to 12-13l/m across the rest of the dive.

I've always assumed its down to the reasons stated above - but none of it is important since I know my depth, dive time and gas pressure

I'm not challenging that the Oceanic has a more accurate early dive calculation, I'm just saying who cares

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

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.

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.

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

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.

Okay, Negative Nancy and Negative Nelly.

Who cares? Well, there's obviously a few people who have posted in this thread who care. They probably represent an even larger number in the real world. I care. I want my instruments to show me the most accurate numbers they can. Oceanic is showing a more accurate number than Shearwater. I care and I would like Shearwater to do better. Side note: Nobody said anything about high precision. Knowing it to 9 significant digits is nothing ANYONE has suggested. Knowing that it's 3 and not 30 is accuracy, not precision, and is what would be good (and Oceanic proves is possible).

It absolutely IS something to do with Shearwater. Shearwater is providing a calculation that is not very accurate. Oceanic is an actual example proving that the calculation can be done more accurately than Shearwater are doing it. If that's nothing to do with Shearwater, then who IS responsible for the calculation being displayed on a Perdix AI or Teric for GTR?

Oceanic is "just plain wrong to assume"? Does that mean that Shearwater is "just plain wrong to assume" a 30 foot per minute ascent rate when they display a TTS calculation? That is pure bollocks. Oceanic isn't making an assumption about what you are going to do. They are measuring your consumption (in psi/min) and measuring your current cylinder pressure, and then telling you how long it will last IF you stay at the same depth and continue to breathe at the same rate. What the diver does with that information is up to them. Same as TTS. And NDL, for that matter.

 

[Wibble]

Okay, Negative Nancy and Negative Nelly.

Who cares? Well, there's obviously a few people who have posted in this thread who care. They probably represent an even larger number in the real world. I care. I want my instruments to show me the most accurate numbers they can. Oceanic is showing a more accurate number than Shearwater. I care and I would like Shearwater to do better. Side note: Nobody said anything about high precision. Knowing it to 9 significant digits is nothing ANYONE has suggested. Knowing that it's 3 and not 30 is accuracy, not precision, and is what would be good (and Oceanic proves is possible).

It absolutely IS something to do with Shearwater. Shearwater is providing a calculation that is not very accurate. Oceanic is an actual example proving that the calculation can be done more accurately than Shearwater are doing it. If that's nothing to do with Shearwater, then who IS responsible for the calculation being displayed on a Perdix AI or Teric for GTR?

Oceanic is "just plain wrong to assume"? Does that mean that Shearwater is "just plain wrong to assume" a 30 foot per minute ascent rate when they display a TTS calculation? That is pure bollocks. Oceanic isn't making an assumption about what you are going to do. They are measuring your consumption (in psi/min) and measuring your current cylinder pressure, and then telling you how long it will last IF you stay at the same depth and continue to breathe at the same rate. What the diver does with that information is up to them. Same as TTS. And NDL, for that matter.

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.