Question about GTR in Perdix AI

[Hatul]

Does GTR include the time for safe ascent with all the stops or just the time at depth?

In other words if I'm at depth and GTR reads 0 do I still have enough gas to make it to surface safely with all the stops?

 

[BFRedrocks]

On my other AI computer, GTR is based on your actual gas usage as tracked by the computer down to some set psi value. Mine is set by default to "zero out" at 700 psi, so technically I have 700 psi gas remaining when it says my GTR is zero. I don't know how the Perdix AI handles that, but my guess is it's similarly set to a gas level above absolute zero in the tank.

 

[AJ]

My guess is you can set the limits yourself. In general SW assumes you know what you're doing

 

[Hatul]

On my other AI computer, GTR is based on your actual gas usage as tracked by the computer down to some set psi value. Mine is set by default to "zero out" at 700 psi, so technically I have 700 psi gas remaining when it says my GTR is zero. I don't know how the Perdix AI handles that, but my guess is it's similarly set to a gas level above absolute zero in the tank.

I think you misunderstood my question. There are two implementations of air time, The old Suuntos and Lynx do not include the ascent time but just the time at the depth.

The Oceanics, Cobalt and Uwatec include the total ascent time including stops. So when my Galileo reads zero RBT I still have enough gas to make a safe ascent and have a reserve at surface.

My question is how Sherwater defined the GTR.

 

[kelemvor]

It's a good question. Hopefully SW will answer, or they'll publish the manual soon.

 

[NAM001]

I think you misunderstood my question. There are two implementations of air time, The old Suuntos and Lynx do not include the ascent time but just the time at the depth.

The Oceanics, Cobalt and Uwatec include the total ascent time including stops. So when my Galileo reads zero RBT I still have enough gas to make a safe ascent and have a reserve at surface.

My question is how Sherwater defined the GTR.

That should be easy to tell. look at your psi ansd compare to to the GTR time If you have 700 psi and GTR is 2 then you have your answer. also if GTR increases as you get shallower you have your answer. Also you may want to look for a display called TTS. time to surface. that would be the time you have till you have to start your ascent to get to the surface with a designated psi remaining. That would probably make GTR the time remaining till MT. no matter what you can check it with say 700 psi at 60 ft and see what GTR and TTS and PSI reads If you have 750# then you have 1/4 tank for 19 cuft. at .5 sac rate that should be 38 minutes. GTR will probably say 38 min and TTS would say 10 min to get out of water with 500 psi. My aeris computer used a term called DTR dive time remaining. that was the time limit as governed buy PSI , gas consumptioin rate, the NBG/NDL or the OBG. the least value was DTR . TTS was based on consumption rate only to get to the surface with say 500 PSI.

Shearwater has a TTS and a TTS+5 reading if I remember right.

 

[KenGordon]

. Also you may want to look for a display called TTS. time to surface. that would be the time you have till you have to start your ascent to get to the surface with a designated psi remaining. That would probably make GTR the time remaining till MT.

Are you sure about TTS having anything to do with gas? Traditionally it is the time to surface given your depth, ascent rate, and stops given tissue saturation. So if you start an ascent NOW it is the minimum time to get out of the water assuming a proper ascent rate and doing all the stops.

 

[NAM001]

Are you sure about TTS having anything to do with gas? Traditionally it is the time to surface given your depth, ascent rate, and stops given tissue saturation. So if you start an ascent NOW it is the minimum time to get out of the water assuming a proper ascent rate and doing all the stops.

You may be right , i probably have it backwards., what you say makes sense.

 

[NAM001]

I pulled the manual and found this

Section 7.2 --------------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. This is calculated using the current SAC value. Safety stops and decompression stops are not considered by the GTR calculations. Why aren’t safety stops included? Safety stops aren’t included to simplify the meaning of GTR, and make it consistent across operating modes that do not include safety stops.

So if your reserve psi is 500 (on the boat at 500 psi). and you are at 60 ft (depth) and your measured air consumption is 100 psi per minute, and the computer calculates you will use 200 psi to get from 60 ft to the surface. Your GTR will say 0 minutes at 700 psi at 60 ft.. at 1000 psi GTR would say 3min. (100 psi per min)

In other words You have to do the safety stop using the reserve air of IE 500 psi. so if you want a 500 psi on the boat yo would have to set our reserve at 600 if you use 100 psi during the safety stop.

Using this GTR value you have to be confident that the xmitter is measuring psi correctly. Periodically verify the accuracy by checking the tank psi with another source when the tank is at around 500 psi. You have to do this when the tank psi is low. It may read correctly at 3000 but not at low presures. I have had to send 2 of these xmitters in for repair because of low end error in the past. both times they were 300 off. fortunately they were 300 low so when the tank had 500 in it it read 200. A full 3k fill read 2850. 150 off at fullo and 300 off at near empty.

I don't use AI any more because of the cost of sensor repair. If they ever read 300 high you have a problem. 500 on the display then is really 200 in the tank. Keep that in mind. When ever you get a fill ask for the tank psi before filling ( no heat issues from fast filling) or have your tank checked with a trusted mechanical source at end of dive day.

 

[NAM001]

Are you sure about TTS having anything to do with gas? Traditionally it is the time to surface given your depth, ascent rate, and stops given tissue saturation. So if you start an ascent NOW it is the minimum time to get out of the water assuming a proper ascent rate and doing all the stops.

You may be right , i probably have it backwards., what you say makes sense.

you are right good catch thanks.


From my petrel manual...

Time-to-Surface (TTS) The time-to-surface in minutes. This is the current time to ascend to the surface including the ascent plus all required deco stops. Assumes: • Ascent rate of 33 feet per minute (10 meters per minute). • Decompression stops will be followed. • Programmed gases will be used as appropriate.

This number is bad if you have configured gasses available that you are not actually using. IE you gas list has air 32 and 36 available. You make an air dive and the computer will assume you are going to do gas changing on the way up. bummer .. all you have is air for the dive. garbage in garbage out. You may also have to make sure you have configured the 20 ft stop active.