THE DCIEM/CANADIAN FORCES MR DECOMPRESSiON TABLES

Transcription

1 THE DCIEM/CANADIAN FORCES MR DECOMPRESSiON TABLES K. Y Nishi and BA. Ilobson Defence and Civil Institute of Environmental Medicine Downsview, Ontario, Canada The DCIEIl/Canadian Forces decompression tables were developed in 1983 as a replace ment for the US Navy Standard Air Tables ]i,. The problem with the USN tables is that the tables are rarely used as printed and corrections must be made for safe decompression. The objective in developing a new set of tables for the Canadian Forces was to produce tables which could be used directly as printed for hard-working dives in cold water. Although the decompres sion model underlying the new tables.tas developed in 1983, its history goes back over 2 years. Decompression research was started in 1962 at the Defence Research Medical Laboratory and the histitute of Aviation Medicine (now DCII3M) by Kidd and Stubhs [2] who set out to develop an instrument which would monitor the diver s depth-time history and provide instan taneous decompression information when complicated dive profiles and wide variations in gas mix tures would make the traditional tabular approach to decompression inadequate. They initially liased their decompression computer on the traditional Haldane model to duplicate the US Navy 1958 standard air tables and then modified and changed parameters in the model as necessary until they attained a low incidence of decompression sickness. The type of dives tested consisted of random depth dives and many repetitive dives in addition to standard dives to a fixed depth for a given bottom time. By 1967, they had developed a successful decompression computer based on about 5 man-dives. The final configuration of the computer was a series arrangement of tissue compartments instead of the parallel arrangement of the 1-laldane model. In 197, Stubbs modified the model again to make diving in the 2 to 3 ft range safer. The final model was known as the Kidd-Stubbs (KS) 1971 model. Although the KS decompression computer was used extensively at DCLEM and for some dives in the ocean, it was recognized that there were some deficiencies in the model for opera tional use. The first was that the no-decompression limits were overly conservative, Figure 1 shows the no-decompression limits for the KS, Royal Navy (RN) [3] and USN tables. In some cases at the shallower depths, the No-D limit is only half that of the others. The second problem was that the decompression stress increased with increasing bottom time. Figure 2 shows a comparison of total decompression times as a function of the bottom titne for the KS. USN and RN tables at 15 metres of seawater (msw). Initially, because of the conser vative No-D limits, the decompression times for short exposures are too conservative, As the bot tom time increases, the decompression stress changes from being too conservative to not being conservative enough, and eventually, there is a large risk of decompression sickness. The three numbers define regions of constant decompression stress. Similar regions exist at the other depths At 1, the decompression stress is mild to moderate; at 2, the stress is moderate to severe, and at 3, the stress is seve; e, These are limits for average divers. For above average divers, who are young, fit and well-acclimatized, limit 2 represents mild to moderate stress and limit 3 represents moderate to severe stress. An ideal decompression model should be equally safe no matter what the bottom time is. A third problem with the KS model was the excessively long decompression times at long bottom times, The KS model consists of an arrangement of four compartments in series. The DCIEM No. 86-P-23

2 long decoiiipression times are an end eftect anomaly resulting from t lie model being limited to only fouc compartments. TIti result. is a long au to the deconipres doii profile. For example, lie dreompression time for 45 msw for 5 miii is more than twice that at It) mm All this addi tional time is spent at the 3 msw stop To develop the new Canadian Forces dccomjimession tables, it was clecidod to start with and modify the KS model since a large database of valuable decompression information already existed at DCIEM in a computer data bank [4]. All dives done at DCIEM since 191 are recorded in this data bank. Developing a completely new model would have been prohibitive iti terms of manpower and time. To overcome the problems and anomalies existing in the KS model, the following objectives were set. 1. Increase the No I) limits. 2. Decrease the decompression requirements for moderate exposures (short bottom times) where the decompression times are known to be conservative 3. Increase the decompression requirements for exposures in which the decompression stress is severe (region 3). 4. Remove the anomaly caused by the en I effect, that is, the excessively long decompression times for long bottom times. 5. Introduce oxygen decompression into the model. The changes made to the KS model satisfied the objecties set and the modified model is known as the DCTEM 1983 decompression model. Figure 3 shows the nw No-D limits. They have bern increased but they are still more conservative than the USN and RN limits over most of the depth range. Since the changes to the model were kept simple, it was impossible to increase the limits any more without seriously affecting the decompression times for longer bottom times. A_s a result, the No I) limits have been extended in the new tables to allow more practical no-decompression dives. These limits are shown in Figure 3 by the solid line identified as CF Operational Figure 4 shows the DCIE?l 1983 decompression times compared with those of the KS times for 15 nisw. It can be seen that the decompression times have been reduced for short bottom times and increased for bottom times where the decompression stress was severe. The long tail to the decompression has also been removed. Figure 5 shows a comparison of the total decompres sion times generated with the DCIEIvI 1983 model for.15 msw with the USN Standard Air and RN Table 11 decompression times. The DCIEM times are more conservative than the others for all bottom times, It should be noted that a true comparison should also consmdcr the relative times spent at the different decompression stops. The DCIEM decomprecsion profiles have deeper first stops than the USN profiles. however, is the model too conservative? The DCIE\1 tables are designed for hard work in cold water. In the USN manual, it states that for hard work in cold water, the next bottom time should be used. Figure 6 shows that if the DCIEM 1983 decompression times are compared with the LSN decompression times for the next bottom time (USN-. 1) at. -15 msw, tire decompression timm are similar, This holds true for all depths. For extreme bottom times, the DCIEM times become conservative compared to the USN-- 1 times. however, in this range, a common practice is to go two bottom times beyond the actual bottom time or to go to the next depth as well as the next bottom time. In these cases, the results become comparable again. The model has been used to generate a complete set of tables including standard air decompression, in-water oxygen decompression, surface decompression v. icr oxygen, repetitive dive procedures. and corrections for diving at altitude. Of interest to the scientific diver are four tables: Table 15. Table 5. Table IA. Short Standard Air. Depth Corrections for Diving at Altitude. Repetitive Dive Factors,

3 a Table 113 Allowable ino Decompression Limits for Repetitive Dives. The Short Standard Air Decompression Table (Table is) is a simplified version of the com plete Standard Air Table. It consists of two sections - no decompression (No D) section on the left of the double vertical lines arid a decompression-required section to the right of the lines. Each entry in the table gives a bottom time and a Repetitive Group (RG). (Note that these repetitive groups are different from and thus incompatible with the repetitive dive groups of the US Navy tables.) Where bottom times appear without an EG, repetitive diving is not recoinmended. The bottom time is defined as the time from leaving the surface to leaving bottom. A descent rate of 18 msw/min was used for the profile calculations, In the No-D region, several hot torn times are given for each depth. These are for the pur poses of calculating repetitive dives. The No-D limits in Table is are for first dives only. for repetitive No-D dives, more conservative limits are required. These are given in Table 113. For bottom times in the decompression-required section of Table is, the decompression stop times and stop depths are specified at the bottom of the table. The stop times include the ascent time to the stops. The ascent rate and travel rate between stops are 18 ± 3 msw/min. Tables 1A and 11-3 are used for calculating the decompression requirements for repetitive dives. Table IA gives Repetitive Factors (RF) for each repetitive dive group for surface inter vals to 18 hours. The RE is used to determine the Effective Bottom Time (EBT) of the repetitive dive, this EDT being the combined total of the actual bottom time and the tune that must be considered to have been already spent at that depth because of the residual nitrogen remaining in the body from the previous dive. Table 4B gives the allowable No-D limits at different depths for second and subsequent dives as a function of the repetitive factors. These No-D limits are actual bottom times, not EDT s. (The maximum RG for the times given at. each depth is shown at the right of the table and is given to assist in planning a third dive, if intended.) The general procedure for calculating a repetitive dive is to determine the RG of the first dive from Table 15 and the RE for that group from Table 4A for the surface interval between the first and second dive. Table 13 should then be consulted to determine whether the planned dive can be done as a No D dive or whether decompression will he required. for a NO-D repetitive dice (i.e. the planned (or actual) bottom time is less than or equal to the repetitive No-D limit of Table ID). no further calculations are necessary if a third dive is not intended. If a third dive is intended, and the actual bottom time of the second dive is equal to the repetitive No-D limit, the RG for the second dive can be obtained from the right-hand column of Table ID. However, if a third dive is intended anti the bottom time of the second dive is less than the repetitive No-D limit from Table 43, the RG for the second dive is found by multiplying the actual bottom time of the second dive by the RE to obtain the EDT. The RG for second dive is equal to the RG for the E3T in Table 15, If the repetitive dive requires decompression (i.e., the planned (or actual) bottom time is greater than the repetitive Nn D limit). then the bottom time of the second dive is multiplied by the RE to obtaiti the EBT. The depth of the second dive and the EDT are used to determine the decompression requirements (from Table is) for the second dive. The HG for the second dive is also determined from the depth and E3T of the second <live if a third dive is desired. If the planned bottom time exceeds the allowable No-D bottom time in Table 43 but. with the EDT less than the No-D bout in Table is, a 5-minute decompression stop is mandatory. The reason for this is that. the No-D limits for repetitive dives have been assumed to be close to the conservative limits predicted by the DCIEM 1983 model whereas the No-D limits in Table IS are greater than those of the model and are for first dives only. For a dive at altitude, the reduced atmospheric pressure at the surface makes the dive equivalent to a deeper dive at sea level. The depth corrections given in Table 5 are added to the actual depth of the dive to determine the dive profile to be used for decompression purposes. Table 5 also gives the actual stop depths to be used for the standard decompression stops. (Divers are cautioned that most conimonlv used depth gauges will not read actual water depth at altitude, unless the gauge has been specifically calibrated for that altitude. Shot lines are

4 IT CO ITI in end i d.) correct ions shown in 5 for ho have beeti ci at the of the e i.e., for those who spent at least 12 to 2 1 hours ions to the would be for tho who have bct ii ace] i ccl. have been civ using the dci ector anti incidence as for diving at for heeji by DCIEM. are to those which have becn put fished for divers 9 were performed during the dive series over a two These were in a divers in cold at 1 Celsius as well as divers. All dives were done a 6] following the decompression profile as specified by DCIEM 1983 decompression model. No decompression can of decompression sickness, these are believed he safer most existing altitude Correct bends The The tables di\ ite, depth experimentally validated recreational Table greater apply onh have tested extensi safety criteria. (The depth corrections year period. About however, they tablea water real-time on-line decompression computer the eliminate the occurrence than tables. man-dives tested divers v with dry-resting realistic however, not nat Doppler ultrasonic bubble similar acelimat attitude hav at th t altitude validation hyperbaric chamber with wet-working exact tables procedures to usin totally REFERENCES 1. U.S. NAVY. U.S. Navy Navy Department, Washington, D.J. Manual, Diving Vol 1, Air Diving. D.C NAVSEA 991-LP KTDD, and R.A. STUBBS. use of the for divers. arid D.1I The Physiology and Medicine of Diving and Compressed Air Work, 1st ed., pp , Bailliere, Cassell, London Bennett 3. ROYAL NAVY. 1972, RERuN, sion S\IITII, L.A. and Elliott, Eds.. RN Dli data bank. Comput. C.L. Number Five, National Diving The Tindall and Manual, 3.R. C. diving Biomed. Res. 1973; 6: SWEENEY. Canadian Altitude Procedures for Real-time pneumatic analor coniputcr 286. Her Majesty s Stationary Office, the Ocean Diver. NAUI Association of Underwater Instructors PB. London data.ai omputerized decompres Technical Publication 9. N 15111, R.Y. decompression by 25-38, in C.E. son, Clow, Eds. Hyperbaric Diving Systems an I tion. OED-v ol. 6, The Society of Mechanical Engineers, New York. 197$ ML. Nuckols, and P.A. American monitoring computers. Page John Thermal Protec

5 DCIEM 6 12t 18k- / CiA L] 24F b so p 36 1( A US Navy 4B Royal Navy No Decompression Time (mm) Figure 1 1 Depth = 45 msw Bottom Time (ruin) Figure * DCIEM 1983 KS DCIEM 1983 a a CF Operational - A USNavy - & U Royal Navy E 4 msw No Decompression Time (ruin) Figure Bottom Time (ruin) Figure US Navy 14 DCIEM ut.avy+1, 2 12 Royal Navy / / 1 -- E 4 E6 Depth 45 msw - Depth = 45 msw C Bottom Time (mm) Figure Bottom Time (ruin) Figure 6 6

6 - M. -- frsw) Bottom Times (mm) Bottom_Times_(mm) ± ± B C 1.9 (RG) :1 1: 1:3 2: 1: 4: 6: : 12: C 1.4 j A 1 151) 18 I) 21 E A log ) tile C 1 D F 13 ISA 8 log 13D 1SF D 35E 4F A 2 D 25 E 29 F 35 C 48 II 52 1 ] 9 ba 15C 2D 23E 27F ! B 12 C 15 D F 3 II 14 5 A_ 7 11 C 11 1) 14F j j 2 F I - LL Repetitive Rf for Surfnce Intervals (Si) in hr;min Group Stop --- Lecl Actual Stop Depth (fsw) 4ltitude (feet) t Depth No-Decompression Decompression Required Li L8 LII 14 L2Li 1.4 ] H F II D Li Ji._J A t.2:5j9z5f1i:ilru REPETITIVE DIVE FACTORS (RF) CANADIAN FORCES AIR DIVING TABLE 1A (minutco) 1 fw 5 1 If) 1 Decempreeolon Time 2 fsw D 5 F Jj G SOC 5E 75G jj K 222K 127L 2A 4D SOF OOD 12H 175L SOC 12F 38 3A QOD 1811 SHORT STANDARD MB CANADIAN FORCES AIR DP. lng TABLE 15 (FEET) 1116 Depart.acnt of National Defence tee the uoe of these table. and procedures, [partmentot National Detencc and DC1ID.i dieclaim any and aft rcqonsib.lit.e* E lou O I C (tsw) I )RG) r- - Cr. r Allowable No-I) Limit (mm) for Repetitive Factora (RE) Repel. Depth ALLOWABLE NO-DECOMPRESSiON LIMITS FOR REPETITIVE DIVES CANADIAN FORCES AIR 1)IVING TAI3LE 4 (feet) JLL7LJ 1 It) )3 o51) Doptl ± ± ± IQUC Actual Depth Correction (faw) at Altitude (feel) Depth ± f DEPTH CORRECTIONS FOR DIVING AT ALTITUDE (EXCERPT) CANADIAN FORCES MR DIVING TABLE 5 (FEET)