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 (http://www.scubadiving.com/gear/computers_ai/compuspg.shtml)
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
August 4th, 2003, 09:27 PM
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
August 4th, 2003, 09:28 PM
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
August 4th, 2003, 10:09 PM
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
August 4th, 2003, 10:28 PM
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
August 4th, 2003, 11:15 PM
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
August 4th, 2003, 11:49 PM
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
August 4th, 2003, 11:55 PM
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
August 5th, 2003, 08:34 AM
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
August 5th, 2003, 10:52 AM
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).
DA Aquamaster
August 5th, 2003, 06:02 PM
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
August 5th, 2003, 06:33 PM
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
August 5th, 2003, 08:02 PM
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
August 5th, 2003, 09:54 PM
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
August 9th, 2003, 07:01 AM
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
August 9th, 2003, 07:06 AM
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
August 17th, 2003, 12:57 PM
....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
August 18th, 2003, 07:31 AM
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
August 18th, 2003, 08:00 AM
I am interested in the references to digital pressure guages. Who makes these and where can they be purchased?
Thanks
roakey
August 18th, 2003, 08:48 AM
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
scubafanatic
August 18th, 2003, 08:47 PM
...well cornfed......that level of accuracy is kinda cool for SAC experiments/measurements.......plus I just like it, there's no reason not to have the most accurate spg......just a personal preference......and, apparently I'm not the only one that appreciates it...otherwise these products wouldn't exist.
Karl
cornfed
August 19th, 2003, 08:25 AM
scubafanatic once bubbled...
...well cornfed......that level of accuracy is kinda cool for SAC experiments/measurements.......plus I just like it, there's no reason not to have the most accurate spg......just a personal preference......
Well Karl... I just played around with my log book and a 200 psi change in my ending pressure resulted change in my SAC rate of about 0.05 cfm... so I would say it doesn't matter.
and, apparently I'm not the only one that appreciates it...otherwise these products wouldn't exist.
There are a lot of things I appericate but that doesn't make them worth the extra money.
How much difference does 10psi make in your SAC calculations? Is it worth the extra money?
Cornfed
JohnStrr
August 19th, 2003, 10:05 AM
Here's a digital pressure guage that can actually go underwater, hence the S in SPG:
<sarcasm>
And it's only $350!! What a deal!!!
</sarcasm>
nickjb
August 20th, 2003, 02:36 AM
Uwatec make a digital SPG for about $100.
I'm not sure where analogue SPGs got their great reputation for reliability. There are countless stories of them going wrong and it is one of the few pieces of kit that I have had fail.
No moving parts or a bent bit of tube turning a tiny plastic gear wheel. Digital for me.
Nick
Inkypoo
August 20th, 2003, 04:12 AM
DA Aquamaster once bubbled...
Not to sound too barnacle encrusted, but I did not even own an SPG for the first couple of years that I dove.
Wow....that must have been some time back.
Also how did people gauge the volume of air remaining in those days?
:confused:
DA Aquamaster
August 20th, 2003, 07:43 AM
Inkypoo once bubbled...
Wow....that must have been some time back.
Also how did people gauge the volume of air remaining in those days?
:confused:
SPG's were recommended but using J-valves only was still pretty common. And with a well tuned unbalanced reg you could feel the subtle change in breathing resistance increase as the tank pressure dropped. With a J-valve you would feel the breathing resistance increase substantially as the valve started to close to conserve your reserve air supply.
In the worst case scenario the reserve lever got unknowingly pulled during the dive meaning the reg was breathing hard because you had already used the reserve and were just plain almost out of air. But even in that case, with an unbalanced reg you still had enough air remaining to surface (assuming no deco stops.)
