Necessity of a back up computer/watch for NDL diving

[caruso]

This question has been asked and answered countless times on this forum, and yours is probably not the first poll. What's the point of a poll anyway? Are you going to do what most people do? Hopefully not. Do what works for you even if it's only 15% of the diving population or whatever.

It all comes down to whether or not you're ok with rising losing one or more dives and up to a weeks worth of Nitrogen loading data as well as logbook data in exchange for not spending the money on a replacement computer which can be found for under $100 used on Ebay. To me it makes no sense to hold onto the money which is about the same as a 2 tank dive trip in most places.

 

[dmaziuk]

Now that I own only one non-diver watch for the benefit of daily wear, I won't take it diving due not having a dive bezel and so as not to deteriorate the seals with a lot of salt water exposure, but I sure wish I did not sell the divers in hindsight. Fact is, unless they have a depth complication via bourdon tubes or membranes (IWC Aquatimer Deep Three comes to mind) to be worked with using max depth and assuming a square profile on tables for the next dive, they are of limited use as a backup. The problem with most of these watches is that you can stack a bunch of Terics on each limb and have a drawer full of transmitters for the price of otherwise fairly unattractive (to me) watches.

Gotta wonder how bad these are:

https://www.amazon.com/Pyle-Waterproof-Multi-Function-Chronograph-Temperature/dp/B0068NYFCC

US $84.9 50% OFF|NORTH EDGE Men Sport Watch Altimeter Barometer Compass Thermometer Pedometer Calorie Depth Gauge Digital Watch Diving Climbing-in Quartz Watches from Watches on Aliexpress.com | Alibaba Group

At $80 US they're not quite down to "just buy one to find out" level, but they're getting there... Not that I care to have any more redundancy than my buddy, or wear watches. But I did recently discover that my 20-yo casio tough solar no longer works at night.

 

[MaxBottomtime]

My ex-wife used a Shark watch she bought for $5 at a swap meet for her first 300 dives before getting her first computer.

I used this gear for several years, including a couple of years ago just for fun.

 

[Diver-Drex]

The spares become the backup for continued diving. They don't have the tissue loading, but the backup from the dive where the computer failed does. The backup now becomes the primary. I can also dive with a watch, depth gauge and tables, but if I have a backup computer, I don't have to. The spare computers save the dive trip, not the day of diving. I don't dive with four computers, but you have given me an idea. I did dive with 3 at one point when I was testing some for personal use.

Have a great day! Dive more, read less!

Cheers -

I fully I understand what you say you are doing. My point is that the backup computer that was on the boat during your previous dives isn’t worth anything more than a bottom timer. Without the tissue loading from your previous dives it will not predict accurate NDL or assent profiles. If your first backup, now your primary, fails you cannot rely on the information from your second backup.
You may know this. But, as this is the Basic forum, there are likely some who do not. You need to either saturate at your surface pressure and start over with your two computers. Or jump to tables, going back to your first dive in the series, and figure your NDL for your next dive.
The only computer that is valid is one that has been with you on every dive of your current series.

 

[caruso]

I used this gear for several years, including a couple of years ago just for fun

I gotta ask. What's fun about watching a needle move around the face of a vintage depth gauge?

 

[dmaziuk]

I fully I understand what you say you are doing. My point is that the backup computer that was on the boat during your previous dives isn’t worth anything more than a bottom timer. Without the tissue loading from your previous dives it will not predict accurate NDL or assent profiles.

There are no accurate ascent profiles on a no-stop dive, there's just 30 fpm and an optional-not-decompression stop. "Accurate NDL" is of course an oxymoron: a very accurate line drawn through a fuzzy grey area.

It takes some experience, but with that it's not that hard to guesstimate your loading from prior dives and decide whether you want to sit out the next 24 hours or dive a clean computer with conservatism factor upped to +3, or 0: it is mainly about your risk aversion and not much else. But what it really means is that the main difference between a backup computer you can rent if your primary packs it and the one you own and keep in your dive bag, is the price.

 

[Bigbella]

I have been diving long enough to realize the limitations of electronics; that and our increasing dependence upon it. When I began diving, most had, at least, an SPG and a cheap watch; and carried dive tables to the site. An older friend of mine simply ended his dives and activated a J-valve when his breathing became labored! He dives to his day and has full motor functions.

My first three dive computers failed almost out of the box, all due to manufacturer's defects -- two, at over thirty meters. One was an early air-integrated model, the size of a brick; and were it not for my post HAL-9000 distrust of all things snarky and silicone, and the paranoid possession of an SPG, it would have been a very short dive.

I would be far more concerned with actual air and depth management than some false assurance of safety, based upon dive computer ascent rate calculations; theoretical tissue loading; or generic algorithms. I always carry at least some analog gear, as a hedge . . .

 

[Diver-Drex]

There are no accurate ascent profiles on a no-stop dive, there's just 30 fpm and an optional-not-decompression stop. "Accurate NDL" is of course an oxymoron: a very accurate line drawn through a fuzzy grey area.

It takes some experience, but with that it's not that hard to guesstimate your loading from prior dives and decide whether you want to sit out the next 24 hours or dive a clean computer with conservatism factor upped to +3, or 0: it is mainly about your risk aversion and not much else. But what it really means is that the main difference between a backup computer you can rent if your primary packs it and the one you own and keep in your dive bag, is the price.

Ughh... I loath the pedantic tone replies always seem to take on.

If you’re four days into no stop diving at or near NDLs on a live aboard and then day five decide to use a new computer you could very violate your actual NDL because the computer will give you an NDL and ascent profile that isn’t based on your actual tissue loading.

Yes, yes... it’s all theoretical and everyone is different and they’re all just models.... Ughh

 

[MaxBottomtime]

I gotta ask. What's fun about watching a needle move around the face of a vintage depth gauge?

It's not a needle. It's a bubble, and very accurate. I was able to dive without a computer for years using these tools. The fun part was making a dive again without the use of batteries.

 

[scubadada]

I'm perfectly happy with my choice, hope you are too. If push comes to shove, I'm ready, are you? Necessity of a back up computer/watch for NDL diving