Hypothetical - 2 AI transmitters instead of backup SPG?

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Oh, thanks for the clarification. Before I read your post I had no clue that batteries are always going to fail at exactly the same moment.

In my case, as I would expect with lots of other divers, my dive computer indicates low battery so I'd have more than enough warning to change one or both batteries, as the low battery indicator will illuminate at a point where there is still sufficient battery power remaining for a few more dives.

You have a point and a good one, My counter for that would be if it were not for the programming on some computers that prevents gong into dive mode on low battery how many would push the envelope and end up with dead puters and xmitters. You have to admit that there is probably more threat of aleaky oring on the battery cap than a dead battery. battery change effects the other. And yes i have had a computer battery go dead with out warning. As long as there is no deco involved this is not a biggy. expecially in the rec world. tech world is a different thing.
 
Try again - both of my computers and transmitters don’t just spontaneously drain their batteries with no warnings! It’s actually quite easy to see the state of charge on the computers and the transmitters warn you when the batteries are low - they won’t just die mid-dive (actually good for a few dives after a warning). So your concerns are really baseless...
you are entitled to your point of view
 
Yeah sure if both batteries last 40 dives then both batteries will fail on the 40th dive. No one can argue that.
That is not what I said I said ABOUT. and I have put new batteries in a xmitter and have them dead the next day. that happens when you put the reg water plug in the reg on a yolk regulator. remaining gas kept the xmitter energized because pf presure build up in the reg. The point being you have no guarantee that a 40 dive batter will last you 40 dives. Yes there are to many variables in play. And knowing that , then using two devices with the same vulnerabilities is not a good plan.

I learned that lesson on my first night dive. I had 2 lights. both tha same light just different colored cases, both failed at depth both leaky gasket.
 
I'm having serious problems with your logic which I consider faulty. In the event of a failure of the first transmitter, the diver continues their dive on their backup transmitter. The odds of the second transmitter failing at some point during the remainder of that dive is astronomically small as compared to the primary transmitter failing at some point during the course of many, possibly hundreds or even thousands of dives.

No. The odds of the backup transmitter failure are exactly the same: they are not dependent events. If your probability of backup failure is "astronomical", then so is the probability of primary failure. The failure is in ass-u-me-ing the failures are dependent and that 2nd failure is somehow less probable than the first.

:shrug: It's your money, if you want to pay twice for everything, far be it from me to discourage you.
 
OK, since we are on the hypothetical world.
Transmitter #1 interfering with #2. Both activated at the same time since they are on the same regulator, There are plenty of cases where this has been reported. Recently Shearwater recalled the yellow transmitters as they were found to be the same as the gray ones even though they were suppose to be different (to avoid interference).

Also good luck finding a regulator with 3 high pressure ports. (this was 2 transmitters + 1 SPG, I forget it was a few pages back now). So now you need a splitter adaptor on one of the HP ports. How many more O-ring seals are you now adding to the system?

Ever seen that guy that has 2 octos? Why? Because the first octo may not work. But in reality they are more likely to have a failure (freeflow) that will cause more of a problem compared to what the solution is trying to prevent.
 
... using two devices with the same vulnerabilities is not a good plan.

That too, but I'm having a problem with the concept just in general: I have a feeling when the primary system fails on a sub, they don't just continue on the backup business as usual. They are working frantically to restore the primary system.

Whereas here we have a backup device because presumably the dive was unsafe to execute on a single pressure sensor, and then the primary fails, and our diver continues on the single (backup) pressure sensor happily ever after. Because what? It's safe now?
 
No. The odds of the backup transmitter failure are exactly the same: they are not dependent events. If your probability of backup failure is "astronomical", then so is the probability of primary failure. .

I suppose you carry a lot of spare tires in your car? Of course not, no one does. Not because we are all defying logic and common sense. It's because the odds are certainly NOT the same as here we are talking about the probability of failure over time.

Over 100s of dives, 100s of hours of dive time, the odds of a computer, or any piece of gear- failing, are astronomically higher than the probability of failure during what might be at most a 30 minute window- the remainder of a dive following failure of a primary piece of dive gear, in this case a dive computer.

Here's another way of looking at the probabilities, for those of us who are statistically challenged.

There's a woman who works for me, and when she goes on vacation she expresses her fear of flying and says "I know it sounds morbid but I almost wish there was a plane crash the day before my trip because what are the odds of TWO plane crashes in the same week?"

Now of course her logic is flawed, the odds of a second plane crash do not change just because there's a first (unless of course there are safety bulletins issued, aircraft taken out of service, maintenance issues that are addressed, etc, that do in fact decrease the odds of another failure but lets not go there as it is not relevant to this particular scope of discussion).

The odds of 2 transmitters failing are astronomically smaller than one, (just like the odds of two major plane crashes occurring at the same time are astronomically small), even though a failure of the first does not change the odds of the second, just like throwing a coin that lands heads doesn't make it any more likely that the next toss will be tails, even though OVER TIME the odds approach 50/50.

I hope this clears up the misconception on this thread regarding the odds of 2 failures being the same as one.
 
That too, but I'm having a problem with the concept just in general: I have a feeling when the primary system fails on a sub, they don't just continue on the backup business as usual. They are working frantically to restore the primary system.

Whereas here we have a backup device because presumably the dive was unsafe to execute on a single pressure sensor, and then the primary fails, and our diver continues on the single (backup) pressure sensor happily ever after. Because what? It's safe now?

OUr syttems are full stand alone systems because of the primary reason that you can not abort the mission. Lets take depth sensor for example in a hypothetical system. there is a hole in the hull connected by piping to a D/A converter to convert pressure to 1' and 0's and is sent to a number of processing systems as digital data. form start to finish there are pripng cables software and displays involved in getting the info to the desired locations. One blown fuse or a smoked router or switch and a branch or the whole automated depth system thing goes down. In that instance the long term is not the question is t is the short term so with out doing anything but looking at a manual gage which uses a different hole in the hull and piping and perhaps a half dozen manual valves you have depth information. iF NECESSARY YOU CAN DO THE NEXT YEAR OPERATING IN THAT MODE. while waiting for parts or the chance to surface to get the parts or canabalize another system of less priority to restore the function. Your diving issue is differrent from the start. In rec diving you have no mandatory mission to complete the dive. As such you dont have level of backkups that were meant to complete a mandatory dive. You have backups that tasked with getting you to the surface again. Yes large enough, high wuality enough backups will allow you to complete the dive , but that is because the backup is over kill. In the case IMO in pressure sensors there is a primary need and that is to provide tank pressure top know if youare going to run out of air. any purpose beyond that is not a life necessary need. Yes a xmitter is cool because of the data that can be generated like SAC (after dive) and what is determining the limitation of remaining dive time. tank PSI or NDL or CNS or what. Those combined are not vital processes needed to sustain life on a rec dive when a dive is being called. Yes Most dives or divers do not implement redundant SPG functions. So A. If you have a redundancy it is probably overkill considering the reliability of the SPG it self. Even cave and tech divers use one SPG that I know of. B. Use of 2 computers is a plan to overload the diver by having to answer to 2 devices that are subject to teh same failure to given conditions. such as gas switches on both computers when deco is involved. Which in rec diving deco is not involved. You are not diving RB so you don't need a backup RB controller. REC diving and its very basic survival needs is the key to the answer about this. \You ask your self, why the need of a backup,,,, and you aquire a form of backup that is void of the fundimentals that created the need for a backup. In the submarine you agree to forfiet auto data input to related subsystems. ON a sub the orders would then be given to manual input depth to connected systems when ever depth varies beyond the most critical system tollerance and you train for those situations. Example If weapons launching must be with in 20 ft of actual programmed launch depth to get a good shot the CO will say,,,,, if ship deviates over 10 feet from last manual or auto received depth input to the weapons firing system, depth shall be manually updated and the CO informed. That may take 5-10 people to do this to cover all systems throughout the ship. IN diving there is only one person to accomplish it all. You can not count on a buddy to do this for you AND HIMSELF, There fore in rec diving the answer to say a backup for a computer may not be another computer but a watch and depth gage in your pocket may be what is best. I never thought I would be saying this but Tech divers are doing 2-300 foot dives on a watch and depth gage and all is well. If they can do it safely then a rec diver diving to 60-100 ft can do that to with out costly,, failure prone,, backup computers. If you want to use 2 computers then have one shut off and turn it on if the one fails to regain clock and depth info to accomodate a safe surface.
 
both xmitters are subject to joint failure because you probably changed both batteries at the same time. same device technology same flaws and probably set up to both fail at the same time.

I was at a somewhat boring graduation party this afternoon and found myself thinking about this thread and this one point of yours stuck out as something that needs to be addressed because it too is false, for all practical purposes.

If a diver is using two transmitters and two receivers it's highly unlikely that they purchased and put them both into service at the same time. Odds are they bought one, used it for some time and then decided to purchase a second transmitter and receiver. In this case the batteries obviously would not be replaced nor would they go dead at the same time.

The other scenario is that a diver decided from day one that he or she wanted two sets of backup computers and transmitters. In this highly unlikely scenario, even then you don't know the status of the batteries that were shipped with the units by the manufacturer or sitting on a storage shelf in a local dive shop - they could be completely different lot numbers, brands, expiration dates, etc. If one or both transmitters were purchased from different suppliers, or used, as often happens, then clearly the battery status will differ between the units as well.

To introduces even MORE variability to the equation: A diver is more likely to turn on their primary to do a pre-dive gas check while the other dive computer might not activate and turn on for 10 minutes or more- on every single dive.

Other variables (that affects the receiver) would be In the case where a diver uploads their dives, or manually goes through the menus to copy the dive information, they'll only be using one of their two computers which will drain that computer faster than the other one. The use of the backlight on the primary computer when reading it at depth or in dark conditions such as inside a wreck. Or changing menus during the dive to obtain different information, again only on the primary.

So to say "both xmitters [or systems] are subject to joint failure at the same time because the batteries were replaced at the same time" is simply not correct (again for most practical applications).
 
I was at a somewhat boring graduation party this afternoon and found myself thinking about this thread and this one point of yours stuck out as something that needs to be addressed because it too is false, for all practical purposes.

If a diver is using two transmitters and two receivers it's highly unlikely that they purchased and put them both into service at the same time. Odds are they bought one, used it for some time and then decided to purchase a second transmitter and receiver. In this case the batteries obviously would not be replaced nor would they go dead at the same time.

The other scenario is that a diver decided from day one that he or she wanted two sets of backup computers and transmitters. In this highly unlikely scenario, even then you don't know the status of the batteries that were shipped with the units by the manufacturer or sitting on a storage shelf in a local dive shop - they could be completely different lot numbers, brands, expiration dates, etc. If one or both transmitters were purchased from different suppliers, or used, as often happens, then clearly the battery status will differ between the units as well.

To introduces even MORE variability to the equation: A diver is more likely to turn on their primary to do a pre-dive gas check while the other dive computer might not activate and turn on for 10 minutes or more- on every single dive.

Other variables (that affects the receiver) would be In the case where a diver uploads their dives, or manually goes through the menus to copy the dive information, they'll only be using one of their two computers which will drain that computer faster than the other one. The use of the backlight on the primary computer when reading it at depth or in dark conditions such as inside a wreck. Or changing menus during the dive to obtain different information, again only on the primary.

So to say "both xmitters [or systems] are subject to joint failure at the same time because the batteries were replaced at the same time" is simply not correct (again for most practical applications).

I dont think you have any concept of what constitutes failure points or behaviors at all. You premis of diffferent time of purchase making the argument moot is also moot. If you have 2 reg sets irregardless of age, do you get one rebuilt in jan at SHOP A and and the other in aug at SHOP B. Or do you eventually get them done together during off season at the same shop to simplify things. Same goes with computers if you and your wife send in your gear for annuals you quite often send the puters in also for fresh batteries to dive the year on. The only way to avoid the effects of people habits is to have a backup that is not subject to peoples habits or subject to a different set of habits. that is what valildates a reliable backup system. The very nature of the backup being different than the primary system with minumum reliance on common factors is what makes it work.

In this thread there have been m any comments such as MY COMPUTER WARNS ME OF A LOW BATTERY. Yes it does because that was instituted because of the failure point a battery going dead. Our processes have become simpler not because they are perhaps but possibly because they are not being taught which starts a domino effect of not having backups for failures. I think computers are probably the greatest advancement to diving i know of. but for it to be a an asset you have to know how to use them. Majority do not know crap about their puter beyond the main screen. They dont know the concepts of a dive table or how to use one so you buy a puter to do it for you, and you still have no clue if the info is right or not. The solution is to buy a backup puter for that???? Now you have 2 puters that you dont know how to use. you cant remember how to get to the menus to shut off the audio alarms on one let alone 2 puters. Many cant tell if they are set for nitrox or air and compound the problem even more. My point being that some forms of backup is more a complication than a fix for a problem. And then you add to the mix the unknown of if the seal leaks or not when you tried to change your own battery on both our computers. What i just describes is a bad behavior that leads to possible failure with the potential ( not guarantee) of loss of all info you are accustomed to using and with out the logic or hardware to shift to something basic to get you home. Im not saying that there is no 100% solution to any problem but their are answers to problems that are better than others. Of course BETTER is a relative word and is often driven by opinion more than facts.
 
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