New Shearwater AI transmitter - the Swift

[ATJ]

Hitting the left button will take you straight back to the main screen.

There you go! Thanks!

 

[ATJ]

I love the compass on my Perdix and I use it regularly.

I have two shore dive sites where I use the compass immediately after descending to get me where I want to be as quickly as possible. It gets me there perfectly (or within a few metres) every time.

I also love the AI on my Perdix. I have my Perdix mounted on my camera rig immediately above the viewfinder which literally makes it a "heads up display". It is easy to glance at it while taking photographs or while swimming. Having the Perdix mounted there also makes using the compass easier and more accurate - no bending of my arm and holding it steady to get a reading.

The only thing I don't like about the AI on my Perdix is the accuracy of the GTR especially earlier in the dive. It is ridiculously conservative - I don't know if this is by design or that they aren't taking the compressibility of gas into consideration. It gets more accurate as the dive progresses. I also carry an Oceanic and it pretty much nails it for the whole dive.

 

[Wibble]

GTR?

 

[ATJ]

GTR?

Gas Time Remaining.

From the manual:

GTR Mode

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. The SAC rate is averaged over the last two minutes of diving for calculating GTR.

GTR can only be based on one tank. The Surface Air Consumption (SAC) measurements are also based on the tank selected for GTR calculations.

 

[scubadada]

I love the compass on my Perdix and I use it regularly.

I have two shore dive sites where I use the compass immediately after descending to get me where I want to be as quickly as possible. It gets me there perfectly (or within a few metres) every time.

I also love the AI on my Perdix. I have my Perdix mounted on my camera rig immediately above the viewfinder which literally makes it a "heads up display". It is easy to glance at it while taking photographs or while swimming. Having the Perdix mounted there also makes using the compass easier and more accurate - no bending of my arm and holding it steady to get a reading.

The only thing I don't like about the AI on my Perdix is the accuracy of the GTR especially earlier in the dive. It is ridiculously conservative - I don't know if this is by design or that they aren't taking the compressibility of gas into consideration. It gets more accurate as the dive progresses. I also carry an Oceanic and it pretty much nails it for the whole dive.

GTR on my Teric is not very accurate at the start of a dive. My Oceanic VT3 is quite accurate over more than 1500 dives. The Teric is better toward the end of the dive. No idea what the explanation is.

 

[ATJ]

GTR on my Teric is not very accurate at the start of a dive. My Oceanic VT3 is quite accurate over more than 1500 dives. The Teric is better toward the end of the dive. No idea what the explanation is.

Yeah, we've had this discussion before. I still think that Oceanic consider the compression of gasses better.

The overall SAC for most of my dives with a 12L tank is around 1.0 (which can be anywhere between 0.95 and 1.05) as displayed on the Perdix. I typically do 2 hour dives. At the start of the dive, once I've reached steady state, my Perdix displays my SAC as 1.5-2.0, and GTR of around an hour to 90 minutes. So it appears it just does a straight calculation from the SAC.

Now, I typically use a 300bar tank. Using the data from Subsurface, @300bar I have 3,197L of gas, @270bar it is 2,945L., @80bar it is 967L and @60 it is 717L. So the first 30 bar I use at the start of the dive is around the same amount of gas as the last 20bar. i.e. the SAC will appear to be higher at the start of the dive due to the differential compression of gas. In this example, it is 50% higher, which explains the SAC of 1.5 and the shorter GTR.

Does the Oceanic take this into account? I don't know, but it might be one explanation. My Oceanic doesn't display SAC during the dive so I can't check that.

 

[Wibble]

Now, I typically use a 300bar tank. Using the data from Subsurface, @300bar I have 3,197L of gas, @270bar it is 2,945L., @80bar it is 967L and @60 it is 717L. So the first 30 bar I use at the start of the dive is around the same amount of gas as the last 20bar. i.e. the SAC will appear to be higher at the start of the dive due to the differential compression of gas. In this example, it is 50% higher, which explains the SAC of 1.5 and the shorter GTR.

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.

 

[Diving Dubai]

Does knowing your gas volumes to ten decimal places add any value?

Quite.

Because the metric system is easier (better) I round up my SAC on a 15l tank to 15l/m which is also 1 bar/m at the surface

Thus at 40m its 5 bar/m I don't bother with reducing the consumption for acsent panning, so 40m to SS requires 20bar, another 10 bars for the 5 min stop, rounded up to a nice round 50bar + reserve and that's my calc from a direct ascent with no deco stops.

I've never used GTR. It got my wife worried once when she inflated her DSMB from the reg, and the instantaneous rate put it into the red

And for me, once stuck in a down current, I didn't need my computer flashing red to tell me I was in a very bad position

 

[scubadada]

Hi @ATJ

Yes, the effect of deviations from the ideal gas law on the calculation of RMV has been previously discussed on SB. We know that Shearwater does not take this into account. I communicated with Aqua Lung/Pelagic Pressure Systems, but, did not get an answer.

I generally use AL80s filled to around 3000 psi, the error would be considerably less than the error you get with your 300 bar cylinders. The difference I see between my Oceanic VT3 and my Shearwater Teric seems larger than the few percent I should see. I have not looked into it systematically,

 

[Manatee Diver]

Go my Swift from @DiveTucson today.

I mounted it on my rec reg set for now. It was as easy to setup as my prior transmitters.

Since they didn't bring the configurable mini displays to the Teric, for now I'm not entirely sure how I will run the Swift, as my goal was to get even my stage tanks on my Teric's home screen, but unless I get a mini-display so I can combine PPO2 an my Delta 5, that isn't happening.

Also in order to get GTR on T3, the one on my rec reg, I had to disable sidemount mode. It would be nice if they moved GTR or even the entire AI setup setup under the OC Tec vs Rec setup screens similar to how you can have different screen setup for each mode. Although it isn't a huge deal for me, I'm experienced enough with sidemount that I don't need reminders to switch regs, and for the dives that I do in sidemount I don't need GTR, and IIRC GTR mode shuts off when you go into deco anyways.

I also see why Shearwater chose the 1.5mm screen for the transmitter, this is the same tool as they include with the Teric. Though I still would've preferred 2mm screws, as for those that don't have Teric they can use a common metric sets.

Depending how I decide to go with my stage tanks I might order a fourth transmitters.