why hate safety devices?

[ronbeau]

They still do. Ever watch a bunch of divers at safety stop on an up-line? They'll sit and watch their dive computers religiously until they hit the 3-minute mark ... then they'll be on the surface a few seconds later ... which completely negates any benefit they got from the safety stop.

Many's the time I've asked divers how long they think it should take to ascend to the surface from safety stop depth ... only to get a shrug or blank stare. Most seem to think that once you've done your three minutes, the dive's over ...

... Bob (Grateful Diver)

It seems like a total lack of understanding on the students part. They certainly can't blame "the math is too difficult". They shouldn't need a calculator to determine that a 30 feet/minute ascent rate would take 1/2 minute to ascend 15 feet at the same rate.

I have thoroughly read divie training material you produced so I know they can't blame the instructor either.

I don't teach scuba but I do teach on the adjunct faculty at a local community college and my wife is a full-time college professor.

In general there seems to be two broad categories of students. One group wants to know what buttons to push and the second group wants to understand why something is done a particular way.

The first group is in trouble if the calculator does't work or they have to use a different calculator (or the human brain) while the second group can adapt to changing situations.

I happen to teach computer programming. On occasion I have had a stundent comment that the homwework is not exactly the same as the example in the book. My answer is that the difference is by design and they will learn little if anything by blindly copying something out of a book.

Here is an example.

The book has them read in customer orders from a file. Each record in the file contains the following data.

Customer #
Order #
Quantity ordered
Unit Price

They need to lookup the customer in the database to retrieve the customer's name. They will also calculate the gross amount of the order. I tell them that the gross amount is calculated by multiplying the quantity by the unit price.


Homework assignment

They will read a file containing purchase orders issued to vendors. Each record in the file contains the following data.

Vendor #
Purchase Order #
Quantity ordered
Unit Price
Delivery Date

They will lookup the vendor in the database to retrieve the vendor's name and the discount percentage.

They will calculate the discount amount and the net amount of the item ordered.

I tell them that the discount amount is calculated by multiplying the discount percentage from the database by the gross amount of the order.

I then indicate the gross order amount is calculated by multipying the unit price by the quantity.

The net amount is the gross amount minus the discount.

Every fourth or fifth semester I get a student who will ask why the homework assignment is not exactly the same as the one in the book.

I usually indicate that the ability to take a program that is 95% the same as the program you wish to create, use the 95% that is the same and add the 5% that is new is a crucial skill. If they can master that skill they will not become a software developer. Their employer will not pay them to reinvent the wheel every time instead of buidling off of work that has been previously developed and tested.

For those not familar is database access, the method retrieve a vendor record is identical to how you retrieve the customer record. The only two differences are as follows:

You indicate you want to retrieve a vendor record (versus a customer record)
You use the vendor number instead of a customer number to look up the vendor.

This same principle can be applied to any discipline including scuba.

In the case you mentioned above the button pushers are the those that zoom to the surface after the safety stop.

The ones how want to really understand the subject realize that the greatest change in pressure is between the safety stop depth and the surface and therefore the ascent rate from the saftey stop to the surface should be no faster the ascent rate from 60 feet to the safety stop.

 

[ronbeau]

Duplicate posting.

 

[Kingpatzer]

I am guessing that this has been measured via doppler/MRI etc? Excitation of inert gas remaining in solution post-dive?

Correct.

I'd be interested to read any articles you have that explain this process. It doesn't fit with what I understand of the process (I am more than willing to accept my own lack of knowledge is the culprit for this).

I don't claim to be an expert. I would recommend picking up Deco for Divers where this topic is covered in some detail.

 

[boulderjohn]

However, I am certain that this cannot be done with a thick wetsuit to do with the origin and utility of maintain a depth of 15 feet for three minutes ... which (if you look back at previous threads you will discover) was a technique developed to compensate for divers' lack of good ascent technique (esp. when shallow) and lack of ascent rate control. If you make a good, slow, ascent, safety stops are irrelevant.

Edit: Thal pointed out that a three-minute safety is a work-around for poor ascent profiles. Regardless of whether this is true or not (and I believe that it is true), PADI, NAUI, etc., teach that the safety stop should not be omitted save for urgent situations. Divers can practice whatever they wish to practice. However, you can't teach the standard omission of the three-minute safety to basic OW divers. This thread is in Basic Scuba Discussions.

Whatever the thinking people had when the safety stop was first conceived, we should consider that thinking irrelevant and look at what current research says. DAN cited a study 5-6 years ago that compared different ascent rates, and that study found that stops were more important than ascent rates. The evidence that stops are far more important than people thought when they first came up with the idea was so compelling that I now make an extended safety stop on almost all NDL dives.

Mark Powell does quote this study in Deco for Divers. That book also has an excellent section on bubble mechanics and the need to control bubble size for effective off-gassing.

 

[Crush]

Whatever the thinking people had when the safety stop was first conceived, we should consider that thinking irrelevant and look at what current research says. DAN cited a study 5-6 years ago that compared different ascent rates, and that study found that stops were more important than ascent rates. The evidence that stops are far more important than people thought when they first came up with the idea was so compelling that I now make an extended safety stop on almost all NDL dives.

Mark Powell does quote this study in Deco for Divers. That book also has an excellent section on bubble mechanics and the need to control bubble size for effective off-gassing.

I found such a DAN article at DAN Divers Alert Network - you may have to be a member to access it (not sure). A few quotes from the article are:

What is interesting, and not necessarily intuitive, is that an in-water stop with a relatively rapid ascent rate appears to be more effective at eliminating inert gas than a very slow ascent rate. As can be seen from Table 2, a five-minute in-water stop is much more effective than simply slowing the ascent rate, even though the total ascent time is not much different (6.6 minutes vs. five minutes). That total ascent time also remains very short. We know the spinal cord has a 12.5-minute halftime. Thus, 6.6 minutes is an insufficient total ascent time for the spinal cord which is, by then, virtually fully saturated (as seen in Table 1).

At 30 feet per minute (which is the ascent rate more commonly used today with a five-minute safety stop at 20 feet), the time to surface from 100 feet will be some eight minutes. This is better, but still a lot shorter than the 12.5-minute halftime of the spinal cord (not considering that gas elimination is slower than uptake). A plausible alternative might therefore be to ascend at 30 feet per minute but to add an additional "Haldanian" stop at about half the depth (remember, the depth is 100 feet / 15 meters) at 50 feet for five minutes. This gives 13.3 minutes of total ascent time2.

International DAN research studies have recently clearly confirmed these hypotheses: 15 divers were enrolled in a study and each given eight possible combinations of ascent rates, and either a shallow stop, or a deep and a shallow stop. The repetitive dives were to 80 feet (25 meters) for 25 minutes; the surface interval was three hours, 30 minutes; and the final dive was to 80 feet for 20 minutes. Ascent rates were 60, 30 and 10 feet per minute. The matrix is shown in Table 3 and the results of 181 dives are shown in Table 4.

Clearly, the best decompression schedule is Profile 6 (see highlights in both tables). With an ascent rate of 33 feet (10 meters) per minute, and two stops at 45 feet (13.5 meters) and 9 feet (2.7 meters) respectively, this profile had the lowest bubble score of 1.76.

 

[boulderjohn]

I found such a DAN article at DAN Divers Alert Network - you may have to be a member to access it (not sure). A few quotes from the article are:

That is the study to which I was referring. I didn't have enough time then to look it up. Thanks for finding it.

 

[Crush]

No problem.

I just sent an email to DAN asking them about my free quotation of copyrighted material, attributed but without prior permission. Let's see what they say.

Good post, BTW. I was unaware of this study until your post prompted me to search for it.

 

[bvanant]

Two comments on the Dan paper. The first is that there doesn't appear to be any published evidence that bubbles detectable by Doppler have a relationship to Neurological DCS. Although this might seem intuitive so is the idea that ascent rate should matter. The second is that this is a very small study, only 15 participants and the neither the individual data nor the error bars are given. A single outlier could significantly bias this type of study. Still it is intriguing. Time for more stops.
Bill

 

[kanonfodr]

Actually deep stops have been around and attributed to beneficial decompression for quite a while. Here is an excellent paper on the subject that features many links: Deep Decompression Stops

Peace,
Greg

 

[Thalassamania]

Two comments on the Dan paper. The first is that there doesn't appear to be any published evidence that bubbles detectable by Doppler have a relationship to Neurological DCS. Although this might seem intuitive so is the idea that ascent rate should matter. ...

That is the real bottom line, even the DAN article contains the caveat:

The missing link of this research is the unknown relationship between Doppler-detectable bubbles and neurological DCS. For the moment, it is our hope that by eliminating the 30 percent so-called silent bubbles in the heart, we will also be stopping their occurrence in the spinal cord too. More research is needed in this regard.
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