What's the difference between a dive computer and a bottom timer?

[simmonsjr]

I've seen several posts praising bottom timers over dive computers, but what's the difference between the two? I'm assuming that a dive computer has more functions so why would you want a b.t. instead?

I'm especially interested because I'm looking at getting a computer in the near future, most likely an AI Oceanic VT3 or ATOM 2.0. And yes I do realize that I'm sticking my neck out there by admitting it. Thanks.

Jason

[H2Andy]

a computer measures your time of dive and depth, and keeps a running calculation of your remaining dive time (the no-decompression limit, or NDL); computers also can offer safety stop depths and times, various kinds of alarms (too deep, too fast ascent, etc).

a bottom timer (most commonly a bottom timer/depth gauge) just measures depth and dive time

most (if not all) computers these days can be set to act as bottom timers/depth gauges

there are divers who prefer to plan their dives ahead of time and do not like to rely on a computer while diving. instead, they plan their dives ahead of time and follow those plans using a bottom timer/depth gauge. the DIR school of thought spouses this method.

most recreational divers and a good number of tech divers still use a computer rather than a bottom timer

personally, i dive with my computer in computer mode but i turn off all bells and whistles and i do not follow the computer's ascent profile (mine is much slower)

[Walter]

A bottom timer only keeps track of time underwater. A dive computer also keeps track of depth, temperature and decompression status. Bottom timers were fairly common (most divers just used a watch) in the mid 80s. At the time, dive computers were rare. I bought my first dive computer in 1987 (I've never owned a bottom timer). Today, almost all divers use dive computers and have never seen a bottom timer. In my opinion, dive computers are the best improvement in diving since the Jet Fin.

[ptyx]

It's not just more functions - it takes some decisions for you.

Computers have the nasty habit of locking themselves and beeping madly if you go beyond the parameters they decide are safe for you, "for your protection". "For your protection" as well, they are usually very conservative. Sometime conservative enough that for long, deep dives, following their recommendations is just not practical (blame liability lawsuits).

Also, like car GPS, it is tempting to become lazy and blindly trust the computer. Even if its sends you over a cliff or in a flooded river (or the diving equivalent).

Is that good or bad? Up to you... You can always turn them to bottom timer mode anyway.

[DevonDiver]

Dive computer reads depth and bottom time and then calculates your exact no-decompression limit, based on an algorithm (similar to how dive tables are calculated). After dives, it applies an algorithm to calculate your off-gassing.

Most of the algorithms are developed using statistical medians - meaning that if you are within that profile, then you are statistically safe.

Bottom Timers simply give you a reading of depth and time. Some also provide information such as water temperature and keep a basic log of your dive.

I've used dive computers for 12 years and never had one "lock themselves". For that to happen, you would have to ignore the basic information that the computer was giving to you...by exceeding safe ascent rates and/or going into decompression and then failing to complete the required off-gassing stops (that the computer calculates for you).

If ptyx consistently has problems with his dive computer locking up, then I suggest he needs further training.... or attempting to complete dives that are outside of the parameters of his computer (i.e. technical dives).

[gcbryan]

It's my experience that very few people use a dedicated bottom timer. Many tech divers use computers in gauge mode which turns their computer in essence into a bottom timer with perhaps a few more features...downloading capability for example.

Dedicated bottom timers usually are sealed (won't flood) but have batteries that aren't changeable as well. They are usually lithium and are supposed to last around 10 years. They usually don't have buttons as failure points and use metal contacts instead.

They cost about the same as a cheap air computer. To me, and apparently most others, they aren't worth it even if you only care about gauge mode most of the time.

However, not all computers have gauge mode but most do. You can use any computer that way if you just ignore NDL info and look only at depth and time but that can be annoying if the time figures are small and the NDL numbers that you don't care about are large.

[Lee Taylor]

I cannot imagine anyone wanting a bottom timer over a dive computer unless they have an issue with the price of the computer. They may be trying to conserve their monies by using a bottom timer instead.

[Riger]

Most of the differences between bottom timers and computers are all listed above.

Additionally, computers continually track your theoretical nitrogen loading in your tissue and give you credit for not remaining at the maximum depth for the duration of the dive. Of course you can plan for multilevel dives manually, the computer just makes it a whole lot easier.

Another advantage of computers is that many of them track lots of other data continuously and this can be uploaded to your desktop/laptop computer and presented graphically for inclusion in an electronic logbook.

IMO computers are pretty reliable, however I carry an relatively cheap backup anyway so that on a liveaboard I can revert to it easily if the primary fails. In this regard, my backup is more conservative (Suunto) than my Primary (Uwatec).

Hope this helps.

Best Regards
Richard

[ptyx]

If ptyx consistently has problems with his dive computer locking up, then I suggest he needs further training.... or attempting to complete dives that are outside of the parameters of his computer (i.e. technical dives).

I do not, I never locked mine (although I've seen people locking theirs), and happily dive with my Vyper (which is even more conservative than the average). However, most of my diving is shallow (most operators limit themselves to PADI stuff), with a single tank - so I'm air limited more than deco limited.

I fully appreciate the capability my computer has to adapt to whatever profile I do (usually triangular), especially since I seldom know in advance exactly what I'll do. It is also very helpful computing lots of dives over a week - something which would be really painful with table or software. It does keep track of seesaw profiles/slow ascents better than I would be able to as well (yes, I'm evil and do those sometimes).

That said, I'm trained for deco/nitrox (and trimix will follow I find find something I want to dive requiring it), know my computer limitations and realize that if I were doing deep square dives (typical wreck), my Vyper would give me deco times which - even compared to tables - are ridiculous.

I'm not against or for computers - they're a tool - and a damn convenient one. But as all tools, they have limitations - and blindly ignoring them ("computers rock, those DIR guys are retarded!") or considering them an abomination ("real divers plan everything, computer users are unsafe slaves to their machines!") won't help the OP much...

[ptyx]

An example: a 'camping on the Yukon' dive. I'm rusty (and new to iDeco), so feel free to verify my numbers/correct me.

100 ft, double AL80, 3000 PSI - on air (I know... see below).
40 minutes at depth - wreck is big and there is plenty to see (rule of third with iDeco seems to be 44' - I'm lazy and will trust it for that post). It's a square dive unless you really want to do a thesis on the mooring line.
Nothing especially technical there (actually, tech divers would probably be smarter and use trimix to avoid that long deco nevermind, can't do much here - thanks to gcbryan for the correction).

Total ascent time:

• US Navy tables: 20'
• iDeco: 29'
• Vyper: 39' - and you're breaking thirds (and it takes forever to pry that number out of the computer)

Isn't the bottom timer option more tempting now?
(And yes, I still love my Vyper - in computer mode - thank you very much).