The accuracy of the SPG

[DA Aquamaster]

A 300 psi error at the low end of the scale is unlikely to cause an OOA situation as with proper air management you won't let the pressure get that low in the first place.

However, if a digital SPG fails an alert diver will still know basically what he had when it failed while an experienced diver will know what to expect in terms of the air consumption for a given dive profile with or without it. If you always have left what you planned to have left at the end of the dive, and plan for an ample reserve, there should be no real surprise involved. An immediate abort and emergency ascent is not always required.

So in my opinion, a failuire of an SPG is not what I would call life threatening and if you feel that diving requires you to stake your life on the reliability of an SPG, you really ought to find another sport.

Not to sound too barnacle encrusted, but I did not even own an SPG for the first couple of years that I dove.

 

[jhelmuth]

That's a really good point DA! Of course, most of us who have not had the beneift of your years of experience have been taught to abort the dive upon known equipment failure (in most instances).
I have some reservation on your contra argument with respect to the 300 psi error. You correctly state that if proper air management is used, the OOA will not happen. True enough in most cases. There can be times when, due to unplanned emergengies (like having to share air with another OOA diver toward the end) that this can still happen. Not likely a real big issue since you'd be close enough to the surface in that instance (but are there other circumstances which might not be so favourable).
Maybe there is another thought here that we should consider.
In general, the reliability and accuracy of most equipment is safe (or at least manageable) under normal conditions - even in failure modes - assuming the diver has taken proper care and maintenance of their equipment and the diver has sufficent knowledge of planning their dive - then diving their plan.
This sounds reasonable - so why do some people make such a fuss over the reliability of their equipment choices (computer vs. no computer)? Maybe I'm way off base here and I have misunderstood some of the past arguments regarding this topic.

 

[DA Aquamaster]

I guess my thought is that if you have an emergency situation where you are having to share air with a real hoover, then you are either going to have adequate air or you aren't and what the SPG says will have little bearing on it. If your reserve is that low to start with (under 500 psi) you are probably already in trouble.

I think a mid range error in an SPG is potentially more significant because it can affect your decision as to when to turn the dive around and start your ascent. A 300 psi error in the middle of the gauge range could put you in a situation where you do not have the reserve you need in an emergency and could lead to an OOA situation.

I agree that with proper planning a computer or SPG failure should not automatically consititute an emergency. In the last two weeks I have had a computer failure (my first non-battery related failure ever) and an SPG failure and neither dive required an abort as the dives were well planned.

In the case of the computer failure, I knew the contingency plan for the dive in the event the computer failed so finishing the dive more or less normally only required a watch and an occasional glance at my buddy's depth gauge to confirm what the ambient light and thermoclines were telling me regarding depth.

I never under any circumstances start a computer dive without knowing the remaining NDL limit for the planned depth of the dive and I constantly keep track of this number on the computer, so that if i have a failure, I know where I stand.

On deep dives I will write the worst case depth/time/deco schedule (figured from a table or from the sim mode on the computer just before the dive) on a piece of 2" vinyl tape or duct tape and stick it to the top of my fin for use in the event of a computer failure. I also have a reel with the line marked for 10, 20 and 30 ft deco stops in the event I have a computer failure while solo. Shooting a lift bag with the reel takes the guesswork out of finding the correct stop depths absent a depth gauge or functional computer.

Diving with a computer is not unsafe and a computer failure is not a saftey issue as long as you take proper precautions to always plan for the inevitable failure.

With the SPG failure I knew the air pressure just before the failure and knew the dive could be completed with the air available without cutting into a 1/3 reserve (without even counting the air in the right hand independent double with it's working SPG.) based on a lot of recent experience diving to the same depths under the same conditions.

It is however a call you need to make based on accurate information with proper regard for an honest assessment of your skills and limitations. For a new diver or a diver with little idea of their air consumption and/or who had not checked the SPG recently before the failure and/or has high air consumption under normal circumstances anyway, aborting the dive and starting a normal ascent immediately is a really good idea.

 

[jhelmuth]

Thanks for the information DA! Your insight has certainly made me think more of my pre-dive planning. I've got something additional now to aim for in my on-going lessons.
I wonder why UP, WreckWriter, CJ, PadiPro, Mike F, or some of the others have not posted anything here. Seems like they always have some good things to consider...

 

[cornfed]

ElectricZombie once bubbled...
I would go with analog. Digital SPGs are known to act up around magnetic fields. ScubaPro even states this in their manuals.

And where do you expect to find a large magnetic field while I diving?

 

[cornfed]

Genesis once bubbled...

2. My digital gauges are far more accurate. Within 50 psi, and they're the cheap ones (on my fill whips.)

3. My AI computers are damn near spot-on, checked against a 1/4% digital gauge.

OUtside of mixing, I don't understand why you need this level of accuracy.

Cornfed

 

[scubafanatic]

....my experience has been that analog pressure guages are actually more accurate than my AI computers. My Oceanic DataMax Pro computers typically read 150--200 psi lower than my DiveRite guages.....and I've found the guages really do indicate 0 psi when the tank is empty.

I dive both simultaneously, and have found the AI computers are more conservative......they read that I have 150--200 psi less than I really do have.......I don't know if they are designed that way or if I need to have them recalibrated.

The guages are in 100 psi incriments....the AI computers read in 10 psi increments......I prefer the smallest possible increments.

Karl

 

[cornfed]

scubafanatic once bubbled...

The guages are in 100 psi incriments....the AI computers read in 10 psi increments......I prefer the smallest possible increments.

Outside of mixing, I don't understand why you need this level of accuracy

 

[scywin]

I am interested in the references to digital pressure guages. Who makes these and where can they be purchased?

Thanks

 

[roakey]

scywin once bubbled...
I am interested in the references to digital pressure guages. Who makes these and where can they be purchased?

http://www.automationsensors.com/

Products->Pressure Products->PG-5000

This is what used to be known as the PSI-Tronix PG-5000 pressure gauge which is kinda the standard gas mixing gauge.

There's a discussion of it, where to get it as well as some alternative gauges at:

http://www.scubaboard.com/t11290/s.html

Roak