New Shearwater AI transmitter - the Swift

[ATJ]

When I can go diving again (Sydney is in a strict lockdown that looks like it is going to last months), I will do a dive with a 15L 232bar tank.

A 15L tank actually holds more air at 232bar than a 12L at 300bar (3,250L vs 3,197L) but ignoring the physics suggests the 12L 300bar holds more (12 x 300 = 3,600L vs 15 x 232 = 3,480L).

It will be interesting to see the differences between the Perdix and the Oceanic for GTR. I would expect the Perdix to be less inaccurate, and tell me I have more gas time remaining early in the dive.

 

[Wibble]

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.

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

My point is also a very simple concept too: having too much information means that you're overloaded and may come to over rely on that information when it's clearly inaccurate.

I cannot remember a single dive where the bottom phase is constant depth, stress and workload. It's different for an hour of deco at 6m/20' where nothing changes.

As has been said before; this is a philosophical discussion. You like more info that has variable quality, I like more relevant information that is useful to me. TTS is pretty accurate, certainly within 20%. GTR is highly inaccurate until the deco phase. My planning is accurate enough for risk aversion and is backed up by appropriate contingency.

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

I live in the UK. We have very unpredictable weather, especially for longer term forecasting.

As we say over here: What's the difference between a UK summer and winter?

The rain's warmer.

 

[Diving Dubai]

Not sure what Shearwater does, they don't say

They do, it's just buried on pages 17 & 18

SAC is calculate over the previous 2 mins (excluding the first 30 seconds of a dive) (pg 17)

GTR is calculated from the SAC and depth (pg 18)



For @stuartv they have this explanation also on page 18

 

[RayfromTX]

Bravo! RTFM wins again. Thanks!

One other thing is that the gas consumption at the beginning of a dive can be off based on the initial inflation of the BC while descending. It takes awhile for that to average out of my ongoing consumption monitoring(a little calculation that I do in the background of my thoughts, stupid restless brain syndrome) but I have a pretty good idea at this point of how much it matters and for how long.

 

[inquis]

For various reasons SAC is often higher at the beginning of the dive. This tends to offset the error induced by the ideal gas assumption for real dives. The result is the GTR prediction may be more accurate than it would seem at first glance.

 

[scubadada]

They do, it's just buried on pages 17 & 18

SAC is calculate over the previous 2 mins (excluding the first 30 seconds of a dive) (pg 17)

GTR is calculated from the SAC and depth (pg 18)

Thanks, pp 52-53 for the Teric.

So, PPS uses a 90 second rolling average and Shearwater uses a120 second rolling average for the SAC and the calculation of air or gas time remaining. Shearwater does not correct for deviations from the ideal gas law. I have not received a reply to my query to PPS on the topic. The PPS algorithm for ATR has been used since the mid to late 1990's, I would be surprised if they corrected for deviations from the ideal gas law, but you never know.

I do most of my diving with AL80s filled to around 3000 psi. The deviation from the ideal gas law and the error at this pressure is small. I do not have a good explanation for the significant differences observed in ATR/GTR during the earlier phases of my dives. I will try to capture some data on my next dives, perhaps others could do the same.

 

[rob.mwpropane]

They do, it's just buried on pages 17 & 18

SAC is calculate over the previous 2 mins (excluding the first 30 seconds of a dive) (pg 17)

GTR is calculated from the SAC and depth (pg 18)



For @stuartv they have this explanation also on page 18

View attachment 674353

Stupid question. Why not compensate for ideal gas law deviations? Why even have a feature that's not usable or accurate until later in the dive?

I don't use GTR, just wondering?

 

[inquis]

@rob.mwpropane more accurate models require the volume. Shearwater has stated this to be a likely source of error, which is why they report SAC in psi/min rather than cuft/min (aka RMV).

Edit: I wouldn't say it's unusable. In fact, in light of the typically inflated SAC early in the dive, using a more exact model for the gas quantity may yield LESS accurate prediction for the overall dive.

 

[lexvil]

Interesting that people can obsess over the smallest detail yet ignore the accuracy of the tank pressure reading, my tanks sit in my shop which can get to 100°+, drive to the ocean and jump into 50ish ° water, as the tank cools pressure drops which have a pretty dramatic effect on SAC and GTR, I pay attention to psi more than gtr. I see gtr as a handy guesstimate nothing more, no computer, at the beginning of a dive can know if I’ll hit a current later, drop through a thermocline into extra cold water (I breath more when I get cold) at the middle of a dive, gtr is a guess at a constantly moving target.

 

[AJ]

I don't use GTR, just wondering?

Nope. Unless the computer becomes clairvoyant and understand the whole dive profile, GTR is nonsense.