This month's question was raised by Ron Ryan, NAUI 7205L, the Supervisor of the Catalina Hyperbaric Chamber. His concern stems from the fact that different tables permit a varying number of repetitive dives to be made, from a maximum of one repetitive dive within any twelve hour period on one end of the scale to tables which license the divers to make an unlimited number of repetitive dives at the other end of the continuum.
A secondary issue involves the stamina of the divers involved. All divers experience some degree of physical and mental effort during the course of a dive. Beginning divers may find that one openwater dive is all they can handle in a day, without becoming so spent, either physically or mentally, that they are incapable of making an additional dive with a reasonable margin of safety. Experienced divers, on the other hand, may be capable of four or five or even more repetitive dives within a day's activities without undue fatigue. Of course, the physical conditions of the waters in which the divers are diving and the type of work done by the divers during the dive affect the energy expended and consequently the fatigue levels of the divers as well. These factors are not easy ones to quantify or factor into repetitive dive tables use.
A third concern is that of individual diver physiology. Every diver varies in some degree from one another. Physiological factors such as obesity, fitness, and past incidents of decompression sickness affect one's susceptibility to decompression sickness. It is impossible for dive tables to incorporate all of these factors for any individual diver. Limiting the daily number of repetitive dives might introduce a measure of protection which does not exist at the present.
Related to the physiology concern is the question of multi-day repetitive diving. While one or more repetitive dives may be made on a single day without manifestation of decompression problems, cause for worry may exist when a high level of repetitive dive activity extends over a longer time frame. This stems from the theoretical possibility that nitrogen dissolved in the body has not off-gassed to the degree predicated by repetitive diving tables, and builds up in the body tissues over time. As this affects tissue compartments with long half-times, use of tables which are based on a shorter half-time model may exacerbate this problem. Recently published data bolsters the concern of multi-day repetitive diving.
Hill and Gilliam (1990) documented the diving activity of recreational divers aboard the cruise ship Ocean Quest for a one year period. This resulted in a database of 77,680 dives, 43% of which were conducted using decompression tables. The average diver made three (3) dives per day, every day for four consecutive days. Seven (7) cases of decompression sickness were recorded among the tables users. All of the dives which resulted in a case of decompression sickness were conducted within the no-stop limits of the tables used. Pertinent to the question above, all cases of decompression sickness occurred during the fourth day of diving activity.
Other elements further complicate the problem of deciding what is a reasonable level of daily diving activity. Some of these include training, equipment used, diver health factors, etc.
Only one written response from the membership was received. That response mirrors the verbal comments received while researching this question. Basically, the membership feels that given our current level of knowledge regarding diving physiology, that this question is unanswerable as posed. There are too many variables to consider, and the data to promote any definitive maximum number of dives does not exist.
Despite the inability to define a limit, students should be informed of the general concerns outlined above when discussing repetitive diving. Besides concerns with personal comfort and fatigue levels, the potential problems with conducting repetitive dives over multiple days should be addressed. And, as always, it is incumbent upon the diving professional to remain abreast of new developments in diving medicine, and modify their views as new information becomes available.
Hill, R.K. and Gilliam, B.C. 1990. A comparison of the incidence of decompression sickness in men and women divers using decompression tables and diving computers for 77,680 dives. In Proceedings of the American Academy of Underwater Sciences Tenth Annual Diving Symposium: Diving for Science... 1990. Pp. 157-162.
QUESTION: "How many repetitive dives should be allowed in any 24-hour period? Why?"
A. I find this question rather intimidating, insofar as it is a big question for a little person. I do not presume to know, or understand, all the factors involved in "creating" tables with various tissue groups and models... just whose tissue group(s) and models anyway? I do know that there are many DCS injuries which occur within the No-Decompression limits as indicated on the tables -- any tables -- and as such I would be very unwilling to "push" these published limits. The number of dives managed within a 24-hour period would depend on all the factors that are usually discussed; depth, time, temperature, physical fitness, etc... but we know that staying within the limits does not guarantee a safe profile.
I have just taken several lines to say that there is no answer -- at least none available to the average "sport" diver.
--Struther MacFarlane, NAUI 6676; Toronto, Ontario, Canada (As a private professional educator, has taught all levels from introductory to serving as ITC Director. Recipient of the NAUI Canada Gold Pin and Special Recognition Awards.)
A. In order to make a responsible recommendation that is credible certain facts must be assumed. Presently the so-called "conventional wisdom" would like to see divers doing only two or three dives a day. However, several recent studies (including my own) present huge data bases of diver populations doing far greater numbers of repetitive dives daily with virtually no incidence of decompression sickness. Regional factors such as cold water obviously have to be considered along with the individual physiological status. The question is simply too broad.
Practically speaking, divers do anywhere from 4 to 12 repetitive dives routinely on liveaboard dive vessels and as a group have the lowest incidence of DCS of any diver population. With the acceptance and widespread use of multi-level computers, this type of multi-level exposure has actually gotten safer due to accuracies of time/depth instruments and adoption of more conservative decompression models as a basis of computer programming.
Australia has adopted a law that limits repetitive diving to four a day, and it is exceedingly unpopular with a large segment of the liveaboard divers who justifiably feel unreasonably restricted. Interestingly, no decline in incidence of DCS has been proven to that group.
Certainly to attempt "square table" format dives much beyond four dives per day if any depth is attained will be virtually self-restrictive. But the multi-level diver with a computer in good working order should be allowed to dive within those reasonable limits. It has to be a matter of informed personal choice. I always shudder when I hear that some agency is about to issue another "limit" to sport divers and their freedom. It is not the place of NAUI or any agency to attempt to regulate the diving practices of experienced certified divers. The sport cannot be forever geared to the "lowest common denominator": the entry level diver.
All arguments to the contrary, the certified sport diver has to be responsible for himself in such judgements and adopt a diving profile consistent with his health, ability, and computational capabilities given site conditions. Please let's not fuel the fire of a so-called "standard" that might result in an ordinance similar to the Australian rule. I have routinely made 5-7 dives a day for most of the last two years and had no incidence of DCS personally or with any sport divers accompanying me. Far more important than numbers of dives is ascent rate and duration of "safety stops." -
-Bret Gilliam, NAUI 3234L; Brunswick, Maine (President of Ocean Tech, Captain of a liveaboard dive vessel. Ex-Director of Diving Operations for Ocean Quest International. 20 years experience as a resort dive operator. Has taught all levels including ITC staff experience.)
A. Without including depth and time this is a hypothetical question. If we are talking about a shallow coral reef there is no limit. If it is down the wall one dive may be too much. It is the duty of a NAUI Instructor to make this difference clear to each student and to be certain that they are proficient in use of the dive tables. They must be able to chart their dives and stay within the no-decompression limits. NEVER should recreational divers be encouraged to engage in stage decompression diving.
--Roy Damron, NAUI 207; Kona, Hawaii (Diving Instructor, current NAUI Board of Advisors member. Past NAUI Director, Chapter Leader, West Pacific Branch Manager, and ITC Director. Recipient of NAUI Outstanding Service Award.)
NOTE: The views expressed in this column are opinions held by the individual members referenced, and are not those of NAUI or the editors of NDA News.]Questions for the next issues:
Questions for the next issues:
For the March/April issue: "What are the minimum swimming skills which should be required of a person involved in dive training?"
For the May/June issue: "Should diving instructors and divemasters be required to submit to a program of preemployment and routine drug testing? Why/why not?"
For the July/August issue: "When using an alternate second stage, should the alternate regulator or the primary (from the mouth) be passed? Why?"
All members are encouraged to respond. This column is for you, the membership, to develop. Answers should be kept fairly brief, preferably no more than two or three paragraphs. Responses to each question will be collated by the editor, condensed if necessary, and printed in this section. New questions may also be posed for discussion. Questions should be concise, and should stimulate a wide cross‑section of the membership. Include with your responses or questions the following information: your name, address, phone number, NAUI membership number, dive‑related employment, past diving accomplishments, and a review of your dive teaching experience. Send your materials to Jeffrey Bozanic, c/o NDA News, P.O. Box 14650, Montclair, CA 91763‑1150.
Compiled and Edited by Jeffrey Bozanic, NAUI 5334L
Sources, Jan/Feb 1991, (3:1), pp. 11-14.
NAUI Members' Forum #21
All content, images and text copright © Jeffrey Bozanic (except as noted). All rights reserved worldwide. No images or text may be copied, duplicated, modified, or redistributed in part of whole (except for printing for personal reference) without prior written consent from Jeffrey Bozanic.