The accuracy of the SPG

[jhelmuth]

I recall a 1-2 minute discussion of this in my OW class, and I believe this was the gist of the information: SPGs are less accurate than most/all computers - especially at lower preasures (albeit the SPG was deemed "more reliable").
This is backed up by an article in Rodales...
In the dark ages of recreational diving it was difficult to convince divers to use a submersible pressure gauge. Now they are standard throughout the dive community. The effectiveness and reliability of SPGs has grown over the years, but the realities of dial- or needle-reading gauges have taught us to read them like a fuel gauge in a car. They provide only approximate air readings, usually +/- 300 psi, and tend to be less accurate as they approach zero and as they are subjected to more use.
Rodales article link
So I pose this question to you all (and hope to learn some finer points on the arguments for/against computers vs traditional guages...
What would you use if you could only choose one - SPG/Depth/timer vs Computer? I would ask that you focus on the pros/cons on the merit of the devices themselves. Please don't run off on tangents about the computer rotting the brain (in that case - consider the computer to be running in "guage" mode).

 

[Boogie711]

If you're forcing me to take away one area of redundancy, which is purely a hypothetical question anyway, then I'm going to stake my life on reliability.

 

[Drew Sailbum]

which is really no different than a air intergrated dive computer - digital pressure gauges.

These seem to not be popular at all, and the one LDS in Cayman that carries them is trying their best to sell remaining staock and never see them again. Too bad, I say.

It seems that it would be much easier to make a digital gauge more precise than a mechanical one. And it sure would provide some more accurate numbers for doing SAC calculations for gas planning purposes. And in a digital gauge, you've got the (supposedly more accurate) SPG without the computer to "rot your brain".

 

[jhelmuth]

Boogie,

Do you really feel that the reliability of the computer is that bad? Batteries are a "user" problem when not checked and/or replaced as required or recomended. How might this be different for a guage if the user/owner is that lazy. Might it be argued that they would have an equal aversion for keeping the SPG (or other guage) in good condition so as to decrease it's reliability? I've seen other post on this board which talk of the "relaibility" diffs in various mfg's SPGs. That sorta prompts me to think that there is a reasonable question on the puported reliability.
Don't get me wrong - I really have no stats to back any of this up. And I'm not against the analog guages. To the contrary, I'm now more inclined to "move" that way as my primary means of monitoring/adjusting my dive plan. I do observe that there seem to be somewhat conflicting notions on the advantages to traditional guages (no computers) from the "functional" perspective.
I wonder what the actual % of failures there are for computers which are used in guage mode and do not fail for lack of battery power?

 

[DiveTub]

Well based on my personal experience I will go digital over Analogue SPG.
Having had three HP hoses blow at depth and 2 SPG's giving me incorrect tank pressure (more than 500psi).

My AI computer has never once let me down it has always been spot on accurate nor has it ever lost a signal. It does not have a HP hose to blow, and is far more reliable. Besides if it ever did stop functioning all I lose is my tank information (not such a big deal, just end the dive) as oppose to SPG,s and HP hoses that tend to go BANG and lose allot more than information when they break

 

[Genesis]

I have a bunch of analog SPGs and a couple of AI computers, along with two digital pressure gauges on my mixing and filling gear.

My experience is:

1. An analog SPG is NOWHERE NEAR as accurate as ANY digital device that does the same job. I have in my possession one SPG that reads reasonably accurately at low and reasonable pressures, but is CONSISTENTLY 300psi low at 3500! I have another that reads consistently 200 psi HIGH at 500, which is NOT good. The former is an inconvenience, the latter could lead to an OOA. Worse, the one that is off at 500 psi does show a "true" zero AND its 3000 psi reading is ok - so there is no indication that its wrong on pre-dive check.

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.

Why aren't digital SPGs more popular? Primarily because the ones that have been available have been (1) expensive, and (2) unreliable!

If you want a digital SPG and bottom timer you're better off with something like a Suunto Cobra in gauge mode, assuming you can deal with it all in the SPG position.

If you want it on your wrist then the price goes up (wireless AI) but it can too be had (Vytec, etc)

Otherwise you're stuck with an analog SPG and whatever for a bottom timer.

BTW, I have a really nice analog depth gauge and digital bottom timer - an old US Divers unit that is probably 20 years old. A guy left it on my boat a few months ago and told me I could have it as the digital part was no longer working. I took it apart (even though it SAID there were no user servicable parts inside), found the problem fixed it, then reassembled and recalibrated the analog part. While its about 2-3' off on depth (as close as I could get it without a pressure pot to make repeated tests and adjustments) it works just fine and times both dives and surface intervals.

 

[DA Aquamaster]

My genesis Wisdom was about 200 psi on the conservative side in terms of accuracy at the low end and spot on at the high end.

Since it is in the process of being warrantied after dying on me (without loss of tank pressure reading) I was back in the water with my Matrix Master and Aqualung digital SPG. The SPG promptly quit on reaching the cold (mid 40's) water at depth. The Aqualung digital SPG has a habit of saying the battery is just fine at the surface at 90 degrees and then deciding it isn't and quitting at 45 degrees. Whoever designed it seems to have set the minimum voltage for the low battery warning based on the assumption it would be used in warm water. This was not unexpected as it was not the first time this has happened and I had another battery along. It eats about 2 per season at about 100 dives per year and eats most of another over the winter running the clock.

So on the one hand an analog SPG will (almost) never quit on you, and a digital SPG is more much more accurate and incorporates other nifty features (backlighting, clock, air consumption rate/air time remaining and temperature) but requires close attention to the battery condition.

 

[ElectricZombie]

I would go with analog. Digital SPGs are known to act up around magnetic fields. ScubaPro even states this in their manuals. I don't really feel that the technology of the wireless SPG units is perfected yet. I've checked my own SPG against several others, and they all appear to be accurate. I'll stick with the analog SPG for long term reliability.

 

[Boogie711]

Jhelmuth:

Do you really feel that the reliability of the computer is that bad? Batteries are a "user" problem when not checked and/or replaced as required or recomended. How might this be different for a guage if the user/owner is that lazy.

No, I don't think that at all. Again, it was a hypothetical answer to a hypothetical question. Personally, I dive with both.

This discussion seems to have, as would be expected I suppose, branched into a discussion on the merits of analog vs. digital accuracies and properties. Which is fine.

 

[jhelmuth]

Boogie711 once bubbled...
If you're forcing me to take away one area of redundancy, which is purely a hypothetical question anyway, then I'm going to stake my life on reliability.

Boogie,

I agree with the redundancy. Its hypothetical because I question the "reality" (maybe a perception in my opinion) that the analog guages are truely more reliable. More often than not, electrics are cited for failure due to battery death. From my experience, and those of whom I have direct contact with, this has been exclusively due to the maintenance of the batteries themselves. My issue with that is the user - you either maintain your gear, or you don't (there is no middle ground to me because maintenance is absolute - anything else is not maintenance).
As has been pointed out in this thread, analog SPGs are not so reliable. If an SPG is off by 300 PSI (high) at the end of a tank, then you are going to be faced with higher probablities of an OOA situation.
But if I am right, why is the argument always pro analog / anti-computer (at least the majority of those who I respect and/or who have a great deal of diving experience).