No Decompression Limit question.

[Charlie99]

Well ok then, there's no better way to confuse me then to throw in 20 terms that are all similar. Thanks.

Deco literature tends to be much harder to read and understand than it needs to be.

The "doppler" thing is related to the ultrasonic imaging often used nowday to look at developing babies as well as for other medical diagnostic purposes.

The doppler ultrasonics are very crude devices though. Instead of an image, they just put out a continuous signal. When pointed at a vein or artery, bubbles in the blood vessel will cause a reflection of sound waves. Since the bubble is moving, the reflected sound frequency is slightly shifted (the same doppler effect that shifts the sound of a train whistle or amublance siren as they move past you). This "doppler" shift makes it easy to detect that particular reflection. The net effect is that a relatively simple instrument can be used to detect bubbles in the veins of a diver. Diving to the full limits of the USN table would cause large amounts of bubbling in most divers, so the limits were cut back a bit.

IIRC, even with the lower limits of the revised USN table or the PADI table, more than half of all divers will have signficant numbers of bubbles (as detected by the doppler monitoring equipment) in their veins after a repetitive dive.

Charlie Allen

p.s. This doppler effect is also the basis of the common "radar gun" used by police. It uses radio waves of a few gigahertz rather than sound waves of a few megaherts used in ultrasonic sytems. The faster your car is moving, the higher the freqency of the doppler shift.

 

[divingjd]

Regardless of the decompression model or the decompression theory involved, the key to any dive table, including the algorithms used by computers, is that it is not exact. No table or computer can take into account differences in individuals (young, extremely fit Navy divers vs. over-50, overweight recreational divers) or, more importantly, variations in an individual from day to day. If you are tired, a little hung-over, and dehydrated, your individual risk of DCS on that day is greatly increased. The number of factors thought to affect susceptibility to DCS is large and no one really knows exactly how to account for them. Some computers take account of some of these variables, such as water temperature and workload, but no model can take into account all factors. It follows that any table or computer is not much more than an educated (albeit well-educated) guess. Although there have been significant changes in the tables over the years (such as the use of doppler data) and they way people are taught to dive (the addition of the safety stop, change in ascent rate from 60 fpm to 30 fpm, the deep stop), the tables were never anything more than a guideline. One of the most important things we teach in open-water classes is that staying within the tables or your computer's limits is no guarantee against DCS. Thus, the admonition to dive conservatively and to avoid going right out to the limits. Making intelligent decisions about diving requires an understanding of what the tables do and don't tell us. I think the key in responding to the OP's initial question is that the tables are not exact and never were. The tables and diving practices have been changed as we have learned more about decompression theory, but this should not give anyone a false sense of security. Our tables and practices are probably better now, but we still need to keep in mind that all of the tables and algorithms are approximations.

 

[scubadam67]

the rdp is still much more conservative than the navy tables and are set for less than perfect bodies but any thing other than normal conditions including water temp, pre and post dive exercise, smoking and most of all dehydration can cause you to get bent you can also take a hit for no reason so stay away from the limits if you can and know all the rules on your rdp not just some of them

 

[Darren27]

Try to get away from the idea of a "no decompression dive", its a bit misleading. Every dive is a decompression dive, its just how we decompress that differs. The "no decompression dive" still requires the body to offgas as you ascend, but it is considered that as there is only a limited amount of absorbed gas in the body, it can be offgassed safely during the controled ascent to the surface. The ascent time is the "decompression stop" for that dive. Ascend too quick and the body does not have time to offgas safely and you can end up at the surface with an excess of gas in you tissues resulting in DCI.

Tables are produced with limits of gas that can be absorbed into the tissues, based on the speed at which this gas can be unloaded during an ascent at or below a given speed. Exceed this ascent rate and you invalidate the tables assumption as to tissue gas loading. The slower the ascent the safer you are, personally i try to keep my ascent below 6m/min (19 ft/min) at all stages of my ascent.