why hate safety devices?

[BoltSnap]

By George! He finally got it! Took 'im long enough! There might be hope for the boy yet!

Chocolate is better. Vanilla is no good!!

 

[BoltSnap]

You need to retake basic dive physics.

Cannot say about the common sense part.

Yep, "Physics in Wu's Universe." That should be a short course.

 

[ronbeau]

Forgetting to replace the battery is pretty common, in my experience. I started carrying a second computer (in gauge mode) to lend to buddies whose battery had expired.

The limited sampling of dive computers that my family and I have used tell you before you get into the water that there is insufficient battery life for the dive.

This happened to my daughter a few years ago in Curacao. We simply replaced the battery and went on the dive.

The spare battery is always on my dive checklist that I go through before leaving the house.

I don't like and won't buy a dive computer that does not have user-replaceable batteries.

 

[4sak3n]

Often the low battery warning on computers is unreliable. I have personally had a computer which told me it had "4 bars" (the maximum number) left of battery and then on the next dive a week later only 1 and gave me the low battery warning. The technician at my local dive shop tells me that batteries fail all the time with no warning. Thus I have anecdotal and personal experience with this.

 

[Kingpatzer]

If you make a good, slow, ascent, safety stops are irrelevant.

I really don't disagree with most of your points, but I'd question this one on two points. First, you can ascend too slowly. Second, safety stops have a huge impact on micro-bubble formation. At least so sayeth "Deco For Divers," which is where I tend to go for answers to such questions.

 

[Bob DBF]

Often the low battery warning on computers is unreliable. I have personally had a computer which told me it had "4 bars" (the maximum number) left of battery and then on the next dive a week later only 1 and gave me the low battery warning. The technician at my local dive shop tells me that batteries fail all the time with no warning. Thus I have anecdotal and personal experience with this.

Two rules of batteries:
1. Batteries Die
2. Batteries die at the worst possible moment.

Due to these rules I replace my dive batteries before I go on any major trip. $5 for a computer battery is a small price. And still carry a spare.

My second point is that Lithium batteries die differently than lead acid/alkaline which gradually decrease in power and voltage as they expire. Lithium batteries hold their power characteristics (needed to run the computer properly) until they are spent and then they deteriorate very quickly and do not rebound. Your manual probably has specs for battery life, use that rather than the indicator for replacement.

When you purchase the battery buy from a retailer that turns over their stock. You may not like Walmart but if they carry your battery it's because they make money turning over a lot of them. Li batteries have a long shelf life but they have a shelf life.


Bob
----------------------------------
I may be old, but I’m not dead yet.

 

[Crush]

Often the low battery warning on computers is unreliable. I have personally had a computer which told me it had "4 bars" (the maximum number) left of battery and then on the next dive a week later only 1 and gave me the low battery warning. The technician at my local dive shop tells me that batteries fail all the time with no warning. Thus I have anecdotal and personal experience with this.

Many batteries fail in that manner - it is not the fault of the computer, but how the battery is designed. Trusty, old, bulky alkaline batteries can be counted to fail slowly. Many mercury and hydride batteries begin to fail very slowly, then drop off precipitously.

Edit: I missed Bob DBF's post, above - he said the same thing, but better.

I always have spare batteries in my "save a dive" kit on the boat.

 

[DevonDiver]

I really don't disagree with most of your points, but I'd question this one on two points. First, you can ascend too slowly. Second, safety stops have a huge impact on micro-bubble formation. At least so sayeth "Deco For Divers," which is where I tend to go for answers to such questions.

Safety stops? (I've not read the book yet).

Here is my understanding (correct me if I am wrong):

1) Recreational diving time & depth limits prevent tissue saturation reaching a level where so much nitrogen exists in the tissues that it cannot be eliminated by an ascent within prescribed speed limits. This is calculated using known data about the absorbtion and release of inert gases from varying speed tissues in the body, coupled with the known tolerance for super-saturation before DCI symptoms occur.

2) Ascent rate is based upon a model that accounts for the decreasing ambient pressure super-saturating the loaded nitrogen, but allowing that nitrogen to be off-gassed at a rate that prevents bubble expansion and/or congregation. This is based upon a linear relationship between ambient pressure and tolerated inert gas pressure within tissues of different absorbtion and release speed. If the ascent speed is too slow, then further loading can occur in the slower tissues, even whilst faster tissues off-gas.

3) The super-saturation of nitrogen on ascent causes the inevitable formation of non-symptomatic 'micro-bubbles' (bubble seeds or nuclei) within body tissues and fluids. Providing these micro-bubbles are not permited to coalesce or grow, they will not result in DCI symptoms. However, the existence of micro-bubbles is increasingly believed to trigger an inflamatory/immune reaction (platelet activation and aggregation) within the body, in addition to micro-infarction of tissues, leading to symptoms of post-dive fatigue, malaise, headache etc. The potential damage caused by sub-clinical micro-bubbles is only now being investigated - but there is speculation that they can lead to long term issues such as cerebral lesions, retinal damage.

4) It is believed that micro-bubbles can exist in the body for up to 48hrs, post-dive. This has severe implications for multi-day, multi-dive activities. It is also the primary danger of 'saw-tooth' profiles, as more micro-bubbles are formed and able to accumulate with each individual ascent phase within the dive.

5) The concept of deep stops was introduced in the belief that it helped prevent initial micro-bubble formation. The model used to predict the effectiveness of deep stops accounts for a higher level of ambient pressure contributing to the stabilization and growth-prevention of those bubbles at an early stage, when the over-pressure gradient on fast tissues is highest.

6) Safety stops were originally introduced to account for variances in inert gas de-saturation caused by individual pre-disposing factors (body fat, body temperature, circulation irregularities through injury, dehydration, or the existence of PFO, etc). They allowed

7) The effectiveness of a safety stop arises from providing a further period of off-gassing, whilst also preventing DCI symptomatic bubble growth due to the maintenance of an appropriate balance between tolerate tissue over-pressure and the ambient pressure that restricts bubble growth.

8) The off-gassing occuring during a safety stop is focused on the slowest tissues that have absorbed inert gas during the dive. Faster tissues will have off-gased during ascent. The safety stop provides a margin to deal with the unaccounted saturation into slower tissues, that may not have been sufficiently desaturated on ascent.

9) If micro-bubbles have already formed on ascent, then they will be present before the diver reaches his final stop (safety stop). The stop itself will not prevent their occurance. Any off-gassing occuring at that stop is dealing with the gases that remain in solution (micro-bubbles, by definition, are not in solution). Micro-bubbles take longer to eliminate because they have to dissolve back into solution (by gaseous diffusion) into surrounding tissue/fluid or be transported to and
eliminated by the pulmonary filter of the lungs.

10) Safety stops would have very little impact on micro-bubble elimination. Neither would they prevent the formation of micro-bubbles except for very specific dive profiles (v.shallow, long dives).

 

[wkb1340]

The only issue I've witnessed with people diving audible alarm equipped computers is this:
A bunch of them dive together, and of course an alarm goes off (for whatever reason, I do not know), and guess what happened? Every single one of them completely ignored it. Upon asking them later what the alarm was for, I was given this answer - " We always hear those alarms going off and it never means anything, so I think we all just kinda ignore 'em."
The first question that popped into my mind was, why bother getting those particular models in the first place. Turns out they got them cheap because they bought 8 at once.
I dunno. Kinda like ignoring your smoke detector when it goes off because "it always does that".

Just my .02

 

[Blackwood]

6) Safety stops were originally introduced to account for variances in inert gas de-saturation caused by individual pre-disposing factors (body fat, body temperature, circulation irregularities through injury, dehydration, or the existence of PFO, etc).

My understanding about the origins of safety stops is that they were suggested after it was shown that divers were very often ascending at rates faster than those used in the model (thus negating the NDL values). I wasn't aware of the diving world at that point in time, but I trust Thal's memory.