diving with two algorithms

[scubadada]

I will continue to dive with 2 computers running the same algorithm. One is AI, the other has a SPG backup. I'm pretty likely to be able to finish any dive I start without interruption. Thus far, 4 1/2 years, 480 dives, have never needed the backup

 

[EFX]

You sure ARE overthinking it needlessly. Use ONE computer, and obey the NDL it computes for you. Simple. Presuming that a computed NDL is just a somewhat arbitrary line the computer attempts to draw through a fuzzy gray area is a poor way for a rec-only diver to think about it. Get more training, and THEN let yourself start thinking deeper and more independently. I think for us rec divers, it's GOOD to be a little dogmatic about obeying our computer's NDL.

Most new divers uneducated on deco theory "blindly" following their computer will keep them safe because of the inherent conservatism of the algorithms. I think problems can occur when divers don't think of the NDL as a point (line) drawn through a fuzzy gray area. Certain precursors can stack the decompression stress deck against our favor so that diving on the NDL side can get you bent. The lack of these precursors may also explain why some divers can get away with violating their computer and not getting bent. This is the gray fuzzies surrounding that point or line your computer calculates and displayes as time remaining to NDL. Precursors such as dehydration, fatigue, over-exertion, cold, stress, etc. can make you more prone to DCS. The best computer is the one between your ears and when you use it to consider the precursors you can adjust your dive plan accordingly and stay out of trouble. This might mean going shallow sooner or extending your surface interval to be more conservative.

I do not suggest diving with just one computer if he has two; if the one fails, he is likely going to have to just stay out of the water for the rest of the day. Use both computers, but follow the most conservative, whichever one it is on that dive.

Using one computer and it fails, why would you have to stay out of the water the rest of the day? If diving with a buddy get his max depth and bottom time and log the dive as a square profile with tables. This is plenty conservative. Use tables on the rest of the repetitive dives. If diving solo estimate max depth and BT based on the last computer reading and calculate with tables. Then continue diving with tables.

I always dive with two computers: Mares Puck and my brain.

 

[boulderjohn]

Tables won't address the concern. If you have two different tables, they can produce different results. The PADI table say you can do 55 minutes at 60 feet and Navy tables say 60 minutes at 60 feet. Over a series of repet dives the difference do become more pronounced but not as pronounced as differences seen in a series generated by multilevel computer profiles.

This just shows why you can't switch tables when you do repetitive dives.

The Navy tables were built with the 120 minute compartment controlling repetitive dives, resulting in very long surface intervals that didn't matter too much to Navy divers who were usually doing one dive a day. It was not so good for recreational sport divers trying to do repetitive dives. PADI researched the kind of diving done by recreational sport divers and found that the 40 minute compartment could safely control surface intervals. They added some conservancy by building their tables on the 60 minute compartment and by reducing the times for the first dive. This ensured that recreational divers could have reasonably short surface intervals and still be safe.

 

[Centrals]

I am diving with two 18yrs old Aladin and I hate to think what should I do if one of them dies eventually. I would have to buy a new replacement and follow the one which is the most conservative!! Since I do not dive more than couple times a day, conservatism shouldn't be too much of a problem. Five dives a day is a distant memory.

 

[Steve_C]

If diving with a buddy get his max depth and bottom time and log the dive as a square profile with tables. This is plenty conservative. Use tables on the rest of the repetitive dives.

I have had a number of buddy dives where one of us went 10-15 ft deeper than the other one.

On a typical multilevel dive with some time at 85 -90 but more at 60 etc. The square profile could pretty much kill the remaining NDL.

 

[EFX]

Just occasionally going deeper isn't going to make any difference. If he consistantly stays deeper then subtract the difference from his max. To get more BT split the dive into two depths and approximate the BT for each depth and work it through the tables. I dove a year on tables and did this on a lot of dives especially on days where I knew we were doing 3 dives. The point is do you want to dive or sit out until you can get another computer. I always carry dive tables in my log book in case the computer dies.

If you're doing so many dives in a day (ex. live-aboard) where tables limit you then stay close to your buddy and dive on his computer. If he sits out a day or stays topside when you want to dive, have him run a dive plan on his computer to your next dive planned depth. Work it through the tables to get a starting group and then dive using the tables from then on. If you have a spare computer with you on the boat use it the next day starting with the most conservative setting. After a few dives back off one click on the conservative setting. The following day back off one more setting. The next day back off again. Repeat until you reach the no conservative setting. I don't expect newbie divers to do this but they're probably not doing live-aboards anyway. Seasoned and tech divers should be able to do this.

This is not how I was trained. I was taught (PADI) to abort the dive doing a mandatory safety stop. Then sitting out the day and starting the next day on tables or with a new computer. So, turisops is not wrong. The standard advice is geared toward new divers who don't have the understanding more experienced divers have. Use your built-in computer and dive within your training.

 

[KeithG]

Using one computer and it fails, why would you have to stay out of the water the rest of the day? If diving with a buddy get his max depth and bottom time and log the dive as a square profile with tables. This is plenty conservative. Use tables on the rest of the repetitive dives.

this only keeps you in the water if you really were doing square profiles. multi level dives makes it impractical.

my typical dive is 90 feet for 90 minutes, on air. the majority of the dive is shallow. by computer i never get close to NDLs. trying to translate that back to tables has me heading for the bar right after the morning dive.

i have 2 take aways from this thread
- make sure all of your computers use the same algorithm (this eliminates a decision point and a source of confusion)
- make sure you really do understand NDL concepts

 

[scubadada]

I will continue to dive with 2 computers running the same algorithm. One is AI, the other has a SPG backup. I'm pretty likely to be able to finish any dive I start without interruption. Thus far, 4 1/2 years, 480 dives, have never needed the backup

I appreciate the comments about switching to tables to continue diving. If you have a spare computer, I don't know why you would leave it on the boat rather than wear it. I nearly always dive multiple dives per day, often 4, over multiple days, whether I'm at my home in Florida or on dive vacation. I'm not interested in interrupting a dive or a dive sequence because of a computer malfunction.

 

[Centrals]

I appreciate the comments about switching to tables to continue diving. If you have a spare computer, I don't know why you would leave it on the boat rather than wear it. I nearly always dive multiple dives per day, often 4, over multiple days, whether I'm at my home in Florida or on dive vacation. I'm not interested in interrupting a dive or a dive sequence because of a computer malfunction.

I dive with two but came across plenty divers who would rather keep the spare one in their rooms. It doesn't make sense at all.

 

[Basking Ridge Diver]

this only keeps you in the water if you really were doing square profiles. multi level dives makes it impractical.

my typical dive is 90 feet for 90 minutes, on air. the majority of the dive is shallow. by computer i never get close to NDLs. trying to translate that back to tables has me heading for the bar right after the morning dive.

i have 2 take aways from this thread
- make sure all of your computers use the same algorithm (this eliminates a decision point and a source of confusion)
- make sure you really do understand NDL concepts

Not sure I understand - are you doing random multi level dives? In other words you dive without knowing what depth you would be diving?

If you know your profile before hand you can do this dive as a multi-level single dive - dive at 90, 70 and 50 feet on the same dive multi-level using the tables.
You know you can use the tables to cut a 90 foot dive for 10 mins you are at group C use a 10 minute SIT Time and then 70 foot dive for 15 mins you are at group F use a 10 minute SIT Time and then finish the dive 50 feet for 20 minutes you are at group I when you finish. No deco all within the NDLs and you dont miss any diving - sure if you had a computer you could stay longer but hey at least you are diving...