Can a computer fully replace an SPG?

[Cosmographer]

I seem to recall my instructor telling me something along the lines of "when preparing your gear, breathe through your 1st stage and watch the SPG - if the needle starts swinging a lot, something's wrong." I don't recall what that "something" was, but I think it might have been whether the valve was sufficiently opened or not.

Anyway, can a computer (especially an AI unit) replicate this test?

 

[Bubbletrubble]

Your instructor was trying to show you a check for whether the tank valve is open fully.

If the tank valve is closed but the reg is still pressurized, as you take a breath, the analog SPG reading will go down. Take another breath and the needle will go down even more. With successive breaths, the needle will eventually read zero. The solution is to open the tank valve fully.

If the tank valve is only partially open (e.g., quarter turn open from the fully closed position), as you take a breath, the needle on the analog SPG will bounce down momentarily and recover to the original level. The solution is to open the tank valve fully.

If the tank valve is open fully, as you take a breath, the needle on the analog SPG will not move at all. No need to do anything.

Many air-integrated computers will not reveal the momentary needle bounce that a partially open tank valve might show with an analog SPG.

FYI, the way to avoid having a partially open tank valve is to keep the tank valve in one of two positions: fully open or fully closed.

 

[driftwood]

Anyway, can a computer (especially an AI unit) replicate this test?

First only an AI computer monitors your air pressure. Second, the answer to your question depends on how often your computer updates the data. That varies a lot between different computers and how they are configured.

You can set a Suunto at either a 3s or 5s refresh rate (I think factory setting is 5s). If you are diving on a Suunto both refresh rates are too slow to reliably replicate the swings an analog SPG would display.

 

[Wayne at DiveSeekers]

I know people like to get lose a hose by going to an AI computer but I always recommend that you still use a single SPG (short 24" hose) and keep it clipped of back behind you just incase your sending unit fails or in case you want to check the accuracy.

 

[drbill]

The reason has been pointed out above, and the fact that only an AI computer would allow you to see air pressure has too so I won't address those. I never fully trust a single computer. They do fail. I had one go out on me (premature battery failure) with a ton of deco remaining. I dive with an SPG and TWO computers for safety (but then most of my diving is solo).

 

[TSandM]

Air-integrated computers CAN show you the same variations -- I know, because I have seen it happen. They will CERTAINLY show the fall if you are breathing off a tank which isn't open.

 

[VooDooGasMan]

The SPG cost nothing to maintain for years besides a good rinse. I will continue with it as Dr bill said above computers can and will fail, mine only fails when the ones I have turn on at the surface and it turns off cause of to long prepping to get in and shuts off, I am only in 5' when I check so it really is no big deal. I suppose others have failed during dive.

The reason analog SPG is my buddy is I can tell my depth from monitoring sac, and know when i need to head for deco in case comp failure. When I spearfish it sometimes is all I take on the scuba kit and attached where I can just glance at it.