Air Time Remaining Runs Out Early VT4.0

[lukemil]

I have an Oceanic VT4.0 that I am pretty new to, but I seem to be having a problem. The last two dives I made, I got the "5 minutes of air time remaining" alarm with about 1200 pounds left in the tank. I am still a heavy breather, but not quite that bad. I got the second "0 minutes of air time remaining" with about 800 pounds in the tank. Am I missing something? Did I set some alarm perimeter that I didn't mean to?

 

[FlyinV]

I think there is a setting for reserve pressure -- the pressure you want to have when you end your dive.
I believe it will take that into account so you could adjust it down a bit.

 

[luscioman]

Goto the set menu then alarms and change the end pressure. What was yours set to?

 

[lukemil]

The end pressure alarm is set for 500 pounds. Today it suggested that I was out of air time with 1200 pounds. I must be missing something.

 

[Charles2]

I think it takes into consideration the ascent time and the safety stop gas consumption requirements so that you can arrive at the surface at your end pressure selection.

How deep were you? What was your tank size?

 

[lukemil]

I was at about 40 feet with a steel 80cf tank. I do use air rather enthusiastically, but that still seems off.

 

[Insta-Gator]

Better safe than sorry. Your computer is obviously set up for safety. You paid for an AI computer that monitors your entire dive and profile, why argue with it?

 

[Charles2]

I was at about 40 feet with a steel 80cf tank. I do use air rather enthusiastically, but that still seems off.

According to my calculations, the amount of gas available in an 80 cu.ft steel (service pressure 3442 psi) from 800 psi (0 air time remaining) to 500 psi (end of dive pressure) is 6.97 cu.ft.

Let's assume a SAC of 1.0 cu/min

An ascent from 40 ft will take 1.33 minutes at 30 ft/min and will consume 2.14 cu.ft. of gas.

A 3 minute safety stop at 15 ft will use 4.36 cu.ft.

Total gas consumed is 6.50 cu.ft. ---- Sounds about right. ------ Trust your computer and then verify your results with common sense and arithmetic.

A SAC of 1.0 is a little high, but usually that will come down with experience. A very good post.

 

[eelnoraa]

You should read the manual in a bit more detail. In essense, most AI computer works like this. You set our reserved tank pressure to 500psi. At ATR=0, if you start your ascent according to the computer's algorithm, I think that is ascent rate 60ft/min below 60ft, 30ft/min from 60ft to surface, 3 minutes safety stop at 15 ft. Then you should arrive at surface with 500psi (your reserve setting).

What you described is inline with the computer's algorithm. You have ATR=0 with 800psi at 40ft. If you ascent from 40 to 15 with 30ft/min rate (15 seconds here), stay at 15 for 3 minutes, then from 15-0 with 30ft/min (15s here). You will have 500psi at surface.

 

[Kharon]

According to my calculations, the amount of gas available in an 80 cu.ft steel (service pressure 3442 psi) >>>

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