Air Time Remaining Runs Out Early VT4.0

[Charles2]

Didn't see where the poster said he had HP steel, just steel. If it's LP the calcs are different.

You are right, of course, I am just not familiar with an LP 80 steel. If you have the specs on one, I can repeat the calcs?

 

[Bob DBF]

You are right, of course, I am just not familiar with an LP 80 steel. If you have the specs on one, I can repeat the calcs?

2400# service +10%, 80.6 cuft or 78 cuft @2640#, depending on manufacturer. And you would have to do the calculation twice because he may or may not be able to fill above service pressure if no current + stamp.

And then there is the Faber "mid pressure" 3180# service +10%, 80 cuft @3498#, which might be considered high or low depending on the + stamp.

Have fun!


Bob
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The most important thing to plan when solo diving is to make sure that you are not diving with an idiot. Dsix36

 

[Charles2]

2400# service +10%, 80.6 cuft or 78 cuft @2640#, depending on manufacturer. And you would have to do the calculation twice because he may or may not be able to fill above service pressure if no current + stamp.

And then there is the Faber "mid pressure" 3180# service +10%, 80 cuft @3498#, which might be considered high or low depending on the + stamp.

Have fun!


Bob
-------------------------------------
The most important thing to plan when solo diving is to make sure that you are not diving with an idiot. Dsix36

OK, Bob, just for fun, here are the calculated SAC rates for the OP's gas consumption from 800 psi to 500 psi for every nominal 80 cu.ft. tank that I could find. By the way, for the purposes of this exercise, I don't think it matters whether or not the tank has the "+" stamp - the capacity from 800 psi to 500 psi is all that matters.

Manufacturer	Service Pressure	Final Capacity (+10%)	Available Gas 800 to 500 psi	Calculated SAC rate
Faber FX-80	3442	80	6.97 cu.ft.	1.07 scfm
Faber LP-80	2400 +10%	78	8.86 cu.ft.	1.36 scfm
Faber HP-80	3180 +10%	80	6.86 cu.ft.	1.05 scfm
PST E7-80	3442	80	6.97 cu.ft.	1.07 scfm
PST LP-80	2400 +10%	80.6	9.16 cu.ft.	1.41 scfm
PST 80	3500	82	7.03 cu.ft	1.08 scfm
Worthington X-7 80	3442	80	6.97 cu.ft.	1.07 scfm

Cheers,
Charles

 

[kelemvor]

I bet it's just the confusing (at first) nature of how AI works with reserve pressure. Basically, you set a reserve value -it sounds like he's got it set to 500. The computer then totally ignores the 500, and calculates things based on "the rest". So he had effectively 5 minutes at 700 and 0 minutes remaining at 300psi. Add in the depth and whatever stop he needed, it makes sense.


The manual for your computer is here.


It covers Air Time Remaining on pp22-23

Air Time Remaining (AIR)
ATR is calculated using a patented algorithm that is based on a diver's individual Air Consumption Rate and Current Depth. Tank Pressure is measured once each second and an average rate of Consumption is calculated over a 90 second period. This Rate of Consumption is then used in conjunction with the Depth to predict the Air required for the diver to make a safe controlled Ascent including the No Deco Deep and Safety Stops and any required Deco Stops.

Air Consumption and Depth are continuously monitored and ATR reflects any change in circumstances. For example, when you suddenly find yourself swimming against a strong current and begin breathing more rapidly, the VT 4.0 will recognize the change and adjust ATR accordingly.

ATR is the time you can remain at the present Depth and still safely surface with the Tank Pressure reserve that you selected during setup (End Pressure Alarm Setting).

ATR is displayed on the Dive Main screens (Fig. 42a).

ATR Alarm
When ATR decreases to 5 minutes, the Audible will sound and the ATR time digits and graphic AIR TIME will Rash (Fig. 43). If it decreases to 0, the Audible will sound again. The time digits and graphic will continue to flash until ATR becomes greater than 5 minutes.

You should initiate a controlled Ascent while monitoring Tank Pressure. However, there is no reason to panic, the VT 4.0 has allowed for the Air necessary for a safe Ascent including the No Deco Deep and Safety Stops, if set On, and any Deco Stops required.

So that means when you hit "0 minutes" (the computer thinks) you have enough air to safely ascend, do your safety stop, and find yourself on the boat with 500psi.

 

[lukemil]

Wow, thank you guys for your in-depth analysis of my issue. The tank is a Faber HP-80. The computer was set with a 500 psi warning. My air consumption has been between .80 and 1.0 cuft/s depending on how strenuous the swimming has been. I definitely suffer from paranoia about keeping air in my lungs even if not ascending, but my buoyancy control is improving. Hopefully, my breathing will improve as well.

The majority of the dives I have done are in New England waters between 30 and 60 feet, so there is a safety margin, but I would like to get the hang of the computer before I go much deeper.