DCIEM Tables

[Richard Mason]

Also posted on Surface Interval and other places:

(This follows on from the DCI incident in Tas thread)

The only Rec Diving Tables commonly used in my club here in Tas are the PADI and SSI versions.

Following our recent DCI case, the local Hyperbaric Doctor has suggested that we should consider using the DCIEM Sport Diving Tables. These are produced by the Canadian Forces Experimental Diving Unit, and are, he says more conservative and better suited (designed) for recreational divers in cooler waters, such as in our part of the world (cool that is in the Australian context), ie 9-18 degrees C.

http://www.toronto.drdc-rddc.gc.ca/p...ets/t14_e.html

Anyone got any views on this or experience with these tables?

I haven't had a chance to look at them yet, so I couldn't say how they compare with the BSAC 88 Tables for example.

At this stage, the intention is to consider their use for the purposes of planning non-deco dives only. I have put my hand up for assisting in a review of our club SOPs in the light of the recent incident, so I thought I'd seek out some learned opinions.

(Post Script to previous thread: It now turns out also that the buddy of our officially confirmed DCI "casualty" also thinks he may have a wrist niggle, so it's looking less and less like a case of being unlucky.)

Richard M

[Janos]

I did some comparisons a little while back and BSAC 88s are about as conservative as the PADI RDP. And vice versa.

I believe, but am not certain, that DCIEM tables are a more conservative than either.

IMHO, it's not as simple as saying these tables work and these don't. Once you start choosing a set of tables because they are more conservative than another, then you are effectively choosing your own deco schedule. So why not go the whole hog and add in some of your own safety measures as well?

Personally, I'm quite happy hanging around for a few more minutes before coming up, so my preferred method is to do a nice slow ascent from 20m up, and to throw in extra stops at 9m and possibly even 12m, as well as a nice long safety stop and 6m. Basically I treat the tables as an upper limit as to what I'm happy with, and guess from there.

Janos

[nigelH]

Perhaps the problem is the whole idea of 'no stop diving'. Almost any deco diver thinks in terms of adding conservatism by slow ascents and lengthening the shallowest stop but 'no-stop' divers hit the clock and pop-up.
If, as we know, every dive is a decompression dive then as soon as you start getting up to a 'no-stop' limit you should be adding slow ascents or pauses every 10 meters and a safety stop that might be extended.

[Mark Chase]

The PADI RDP is a pretty aggressive table until you add in the so called extended safety stops. The BSAC88's are renowned for being aggressive but loads of divers dive them.

Long and short of it is any table can bend you on any given day but its obviously better to have tables that compensate for divers physical condition and pre dive levels of preparation. These may need to be further adjusted for in water conditions and possibly even modified during the dive for excessive unplanned work load etc.

Like Janous I would recommend downloading one of the modern decompression programs like Vplanner or Nautilaus and cutting modern tables specific to the dive and the diver.

ATB

Mark Chase

[Jetsam]

Originally Posted by Richard Mason

I haven't had a chance to look at them yet, so I couldn't say how they compare with the BSAC 88 Tables for example.

They are probably the most conservative tables produced. I looked briefly at them a few years ago. They run on 9M/min ascent rates, and have less no-stop and more mandatory deco requirements generally than the more common Buhlmann tables. A lot of pots run them now instead of USN tables. The 88's are very aggresive in comparison.

Like the others say, get yourself a PC deco software, like GAP, Vplanner or Decoplanner. You can make your own deco tables from there.

Regards

[sexydivebuddy]

[Mark Powell]

DCIEM
The Defence and Civil Institute of Environmental Medicine is Canada’s centre of expertise for defence research and development in human performance and protection, human-systems integration and operational medicine. The DCIEM Diving Tables and Diving Manuals are the result of more than 30 years of research and development and are considered to be one of the safest tables available for sports divers.

The DCIEM decompression theory is based on the work of D. J. Kidd and R.A. Stubbs. In the early 1960’s Kidd and Stubbs were working for the Canadian Defence Research Medical Laboratory, one of the forerunners of the DCIEM. Their aim was to develop an instrument which would monitor a diver’s depth and time history and provide instantaneous decompression information for oxygen-helium diving when complicated dive profiles and wide variations of gas mixtures would make the traditional tabular approach to decompression inadequate. In other words they were well ahead of their time in trying to develop a multi-gas, trimix compatible, dive computer.

The model was always intended to be used in a decompression instrument or decompression computer. The equations they developed describe the operation of a piece of hardware which is able to predict the safe decompression from a wide variety of dives rather than describing a physiological model for decompression. The initial versions of the computer were pneumatic-mechanical analogue instruments. Cavities, into which gas could enter at a controlled rate, were used to simulate the different compartments. The instrument was known as a pneumatic analogue decompression computer (PADC). In 1965 the diver portable Mark VS PADC was developed followed by the surface based Mark VIS PADC a year later.

Early prototypes of the PADC were based on US Navy tables. Various changes were made to half-times, supersaturation ratios and the number of compartments in order to increase safety but ultimately they were dissatisfied with the US Navy tables and a purely Haldanean approach and decided to create a new model.


The Haldane model considers the various tissue compartments as a parallel sequence. Each compartment is considered separate to all of the others with no interaction or gas transfer between compartments and each compartment is considered to ongas and offgas directly into the blood stream and independently of each other. This is illustrated in Figure 40.

Kidd and Stubs concluded that in fact the tissues in our body do not act independently of each other and that there is indeed interaction between the compartments. They developed a serial model which was designed to model the interaction of the tissues. Serial decompression models assume that gas transfers from tissue to tissue during a dive, the compartments offgas to each other even as they ongas from other tissues of higher nitrogen tension. Only one tissue is assumed to be exposed to ambient pressure. The DCIEM tables are based on a four compartment serial model.



P(0): Ambient pressure (In initial dive, P(0) = Atmospheric pressure)
P(1) - P(4): Nitrogen pressure in compartments 1-4.

Figure 43: Serial Tissue Model


The approach taken by Kidd and Stubbs in testing their theory was to dive the model and, when symptoms of DCS occurred, to change the parameters of the model making it more conservative. They went through several variations of their air decompression model, improving the safety of the model after each iteration. By 1967, over 5,000 experimental dives had been conducted to validate the K-S (Kidd-Stubbs) model.

In 1970 it was discovered that for deeper dives hyperbaric chamber operators at DCIEM were not following the computed ascent profiles but were staying deeper by as much as 3m. This was due to an inherent distrust in the safety of the model for deeper dives. This distrust seemed to be justified as a series of dives, conducted in 1970-1971, where the ascent was carried out exactly as calculated resulted in a 20% incidence of DCI in the 60m to 92m depth range. The DCS incidence when the operators safety margin was applied was only 3.6%.

In 1971 Stubbs modified the model to take into account deeper dives. The modification he made was to make the supersaturation ratio depth dependent, with the ratio becoming more conservative with increasing depth. For deeper and longer dives this had the effect of introducing deeper decompression stops. In the same year the Defence Research Medical Laboratory and the Institute of Environmental Medicine merged to form the DCIEM and the K-S decompression model was approved in Canada as a safer alternative to the U.S. Navy tables.

In the late 1970’s the PADC was replaced by the microprocessor controlled XDC-2 digital decompression monitor. The advantage of the digital computer was that the extensive calibration and maintenance procedures for each compartment of the PADC were no longer necessary. The only calibration necessary was for the depth sensor.

In 1979, DCIEM initiated a critical reevaluation of the K-S model using digital computers to control the dives and specially-designed Doppler ultrasonic bubble detectors to evaluate the severity of the dive profiles. Although thousands of dives had been conducted successfully there were three known problems with the model. Firstly, the no-decompression limits were extremely conservative. Some of the no decompression limits were less than half of those specified by other tables. This meant that there were a range of bottom times where decompression was required when using the computer but where no decompression was required with other tables.

The second problem was an excessively long decompression time when the third or fourth compartment became the controlling tissue. For example the decompression time required for a 70 minute bottom time at 36m was twice that required for a 60 minute bottom time at the same depth. This resulted in significantly longer decompression times than was seen with other tables.

Finally the third problem was that for a certain, small, range of bottom times the risk of DCS was higher than for other tables.

All three of these problems are clear if we examine a chart of total dive times for a range of bottom times. Figure 42 shows the total dive times for a range of bottom times at 27m. The total times for the KS-1971 id plotted against times for Royal Navy Table 11 times and US Navy Standard Air times. As you can see for bottom times up to 60 minutes the KS model is more conservative than either of the other tables. Between 80 and 95 minutes it becomes the least conservative but then for bottom times over 100 minutes it is much more conservative by a large margin.



Figure 44: Comparison of KS-1971 with Royal Navy and US Navy tables for a 27m dive. Taken from Nishi & Lauckner (1984)

In order to deal with each of these known problems the two supersaturation constants “R” and “OFF” which control the supersaturation limits of the tissues were modified. In the 1971 model all four compartments used the same values for each of the two constants but in the 1983 revision each compartment was given its own value for the constants. This allowed more control over the behaviour of the model. For example, to increase the no decompression limits and reduce the decompression times for short exposures, the first compartment supersaturation ratio needed to be made less conservative. However the surfacing ratio was made more conservative for the second compartment in order to avoid the problem of aggressive decompression in the mid range. Of course in a serial model there is always a “downstream” effect with any changes to the first compartment affecting what happens to the second, third and fourth compartments and so this effect had to be considered when making any changes.

In order to reduce the problem of the long decompression times when the third and fourth compartments became the controlling compartments we must first of all understand the reason why this introduces such long decompression. This behaviour is a result of the model being limited to four compartments. For long bottom time dives, the fourth compartment saturates faster than in a model in which there are more compartments as it is the final compartment and hence the gas has no other compartments into which it can diffuse. This means that during the ascent there is more gas to be released which results in the correspondingly longer decompression times. With more compartments the gas would diffuse into the fifth, sixth, etc compartment and the “end compartment” effect would be reduced. One potential solution would obviously be to increase the number of compartments. It was calculated that 8 compartments would be required to accurately model the required behaviour. In fact the approach taken was to monitor the surfacing ratios for only the first two compartments (although all four compartments are still used for calculating the compartment pressure). By only monitoring the surfacing ratio for the first two compartments the model produces decompression times for long dives which are much shorter than those produced with the KS-1971 model but are still more conservative than the Royal Navy tables, US Navy tables or those generated with more 5 or more compartments.

Taken as a whole the changes resulted in a model where the no decompression limits are still conservative when compared to other models but considerably less conservative than the KS-1971 limits. Decompression times were also improved with overly conservative initial limits, aggressive mid range limits and very long decompression from long exposures being brought into line and resulting in a model which produced conservative, but not overly conservative dive schedules. Thousands of verification diving and many improvements of the theory have been performed and the dive table for air diving was released in 1992. The present theory is based on this dive table.

The DCIEM Manual first published in 1992 contains decompression tables for air, enriched air (Nitrox) and Helium-Oxygen (HeO2). Air decompression tables were developed in the 1980’s for Canadian Forces operational use. These tables have also been adopted by foreign navies, commercial diving companies and other civilian organizations. The Sport Diving Tables were a result of the original air diving tables adapted for use by recreational divers and have had a wide acceptance by the sport diving community. In addition to air tables full Nitrox Tables are also included as well as Helium-Oxygen Decompression Tables (HeO2). HeO2 tables were developed between 1986 and 1991 to replace the US Navy partial pressure tables for operational mixed gas diving to 100 metres.

The DCIEM tables are available commercially and are generally considered to be one of the safest and most conservative sports diving tables. NAUI adapted the 1995 DCIEM Sports Table for use in all NAUI courses and these were used until they were replaced by RGBM based tables in 2002. The DCIEM model has also been adopted by Citizen for use in its Cyber Aqualand range of dive watches.

In April 2002, DCIEM changed its name to Defence R&D Canada - Toronto. However, due to the close association of the name DCIEM with diving tables and manuals, these products continue to be marketed under the familiar and trademarked DCIEM name.

[Mark Powell]

Originally Posted by Richard Mason

At this stage, the intention is to consider their use for the purposes of planning non-deco dives only. I have put my hand up for assisting in a review of our club SOPs in the light of the recent incident, so I thought I'd seek out some learned opinions.

If you are looking at changing SOPs then I would suggest;

- Adding in additional safety stops
- Adding deep stops on deeper dives (>20m)
- Slower ascent rate in the last 10m
- Looking at diver behaviour before/after dives (especially hydration)

Remember that there is no such thing as a no-deco dive.

[sexydivebuddy]

[Hickdive]

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HTH