doi:10.3723/ut.28.051 International Journal of the Society for Underwater Technology, Vol 28, No 2, pp 51–55, 2009 The effects of high altitude on relative performance of dive decompression computers PL Buzzacott Technical Paper School of Population Health, University of Western Australia, Perth, Western Australia A Ruehle Department of Chemistry and Biochemistry, University of Denver, Colorado, USA Abstract computer considered more conservative than other In this paper, dive computer-generated no decom- available models. pression limits (NDLs) in fresh water at high altitude Recreational dives are commonly made at were compared with low-altitude single, repetitive and altitude. For example, in Johannesburg, South multilevel dives. All computer-generated high-altitude Africa, where the altitude exceeds 1500m, there NDLs exceeded those published for the altitude dived. were 30 recreational dive businesses listed in Computer rankings by conservatism for single dives at business telephone directories published in 2008. low altitude had negative correlation with rankings at Likewise, there were 53 recreational dive businesses high altitude (r D −0:81). Correlation between high- above 1500m altitude listed in 2008 Colorado busi- altitude square-profile dives and low-altitude repetitive, ness directories in the United States. When diving multilevel NDLs was significantly higher (r D 0:91, at altitude, where ambient pressure at the surface p < 0:01). We conclude sea-level single-dive NDLs, is less than at sea-level, NDLs are reduced (Bell such as those published in instruction manuals, are and Borgwardt, 1976; Wienke, 1993). Different not reliable when gauging the conservatism of dive decompression models use different methods to computers for use at high altitude. It is recommended adjust their NDLs for altitude (Hennessy, 1977; that divers using dive computers for planning high- Wienke, 1993; Egi and Brubakk, 1995). This study altitude dives to consider computer-generated real- hypothesised that relative conservativeness of pop- time NDLs as experimental. ular dive computers, ranked by single-dive NDLs at sea level, would not be a reliable measure of relative Keywords: hypobaric, no decompression limit (NDL), conservativeness at high altitude. algorithms, altitude correction, dive computers 2. Methods 1. Introduction Recreational divers have increasingly used dive 2.1. Dive computers computers to plan no decompression limits (NDLs) Eleven popular recreational dive computers were during the past two decades (Lippmann, 1989; attached to an array and taken on 8 freshwater Sheffield, 1990; Wilks, 1990; Acott, 1994). A dives – 6 at low altitude (40m above sea level) variety of dive computers are available, using and 2 at an altitude of 3000m above sea level. different decompression models to estimate these Following arrival at high altitude, time was allowed limits (Davies, 1994; Egstrom, 2004; Doolette, for each dive computer to account for clearance 2005). When attempting to gauge the relative of residual nitrogen before the first dive. Four conservatism of dive computers, NDLs for single of the dive computers failed at high altitude and square-profile dives are often compared between are hereafter disregarded. The remaining dive dive computers over recreational depths, which computers were: two UWATEC Aladin Sport, which are less than 40m (Lippmann, 1989; Sheffield, use the Buhlmann ZH-L8 ADT model and are rated 1990; Davis, 1995; Lippmann and Wellard, 2004). to 4000m (Uwatec, 1995); two Dive Rite NiTek, These single-dive limits are often published in the using the Buhlmann ZH-L16 model and rated to dive computer instruction manuals, or displayed 6000m (Dive Rite, 1997); two Suunto Vyper, using by dive computers at point-of-sale by accessing the Reduced Gradient Bubble Model (RGBM) and the planning function (Uwatec, 1995; Dive Rite, rated to 3000m (Suunto, 2003); and a Delta P 1997; Apollo, 2001; Suunto, 2003). By comparing Technology VR3, using the Variable Permeability these limits, a recreational diver might select a dive Model (VPM) and rated to 3500m (Gurr, 1999). 51 Buzzacott and Ruehle. The effects of high altitude on relative performance of dive decompression computers 2.2. Dives On each dive, the array of computers was lowered at a mean rate of 29.1m/min (SD 0.8). Dive computers, in general, display an estimate of depth calculated as a function of ambient pressure. Typically, they are calibrated for dives in salt water at sea level. In this study, computer estimated depths were displayed in feet, recorded in feet, and then converted to metres using a conversion factor of 1ft D 0:305m. Depths displayed by dive computers were compared to measured depths using a fibre- glass surveyor's tape measure (Hangzhou Yangyang Machinery-Equipment Tools and Hardware Co, Zhejiang Province, China). As soon as the array reached maximum depth on each dive, elapsed time and remaining NDLs displayed by each computer were then added together to calculate each computer's total NDL for that depth and descent rate. These NDLs were recorded on a slate, and during four dives only, the array was then ascended to a shallower depth before any dive computer displayed a decompression obligation (see Fig 1). At this `multilevel' depth, the adjusted NDLs displayed by each computer were added to the elapsed times displayed to give a predicted total Fig 1: The array of dive computers was ascended permissible dive time for that profile, and these to a shallower depth and multilevel NDLs recorded. NDLs were also recorded on a slate. Accordingly, 12 readings of predicted permissible NDLs dive Section 2.2. Spearman rank correlation coefficients time were recorded from each dive computer (84 were calculated between mean rankings for each data in total). Correlation between same-model dive of the four types of low-altitude NDL and the computers provided a measure of their intra-model mean rank at high altitude. Improvement in reliability. (dependant) correlation was tested for using the A repetitive dive was defined as any dive made method developed by Hotelling (1940), which within six hours of surfacing from a previous dive follows the t distribution with n − 3 degrees of (Diving Science Technology [DSAT], 1985). Five freedom. Significance was accepted at p < 0:05. types of NDLs were recorded: The formula that was used is given as follows: • Three non-repetitive (first dive of the day) s .n − 3/.1 C ryz / square-profile (single depth) dives at low altitude t D .r − r / (1) xy xz − 2 − 2 − 2 C • Two non-repetitive (first dive of the day) 2.1 rxy rxz ryz 2rxy rxz ryz / multilevel-profile (two depths) dives at low where n is the sample size; rxy is the correlation altitude between square, non-repetitive rankings at low • Three repetitive (second dive of the day) square- altitude and rankings at high altitude; r is the profile (single depth) dives at low altitude xz correlation between multilevel, repetitive rankings • Two repetitive (second dive of the day) multilevel- at low altitude and rankings at high altitude; and profile (two depths) dives at low altitude r is the correlation between square, non-repetitive • Two non-repetitive (first dive of the day) square- yz rankings at low altitude and multilevel, repetitive profile (single depth) dives at high altitude. rankings at low altitude. 2.3. Statistics Data were managed using Excel and analysed 3. Results using SAS, version 9.1 (SAS Inc, North Carolina). Means are presented with standard deviations. 3.1. Reliability Pearson (r ) correlation coefficients were calculated Displayed depths differed to depths physically between NDLs from same-model dive computers. measured with a fibreglass tape by a mean vertical Dive computers were ranked for each dive by distance of −0:2m at 27.4m depth at low altitude, NDL, and mean rankings were calculated for which was less than expected for freshwater each of the five types of NDL described in dives with computers calibrated for sea water 52 Vol 28, No 2, 2009 Table 1: Mean depths and NDLs displayed at each reading Reading Mean depth Depth range Mean NDL in NDL range in in m (SD) in m minutes (SD) minutes 1 (LA, nrep, sq) 27.2 (0.3) 26.8–27.7 19.7 (1.4) 17–21 2 (LA, rep, sq) 21.2 (0.3) 20.7–21.6 31.3 (1.7) 29–33 3 (LA, nrep, sq) 27.3 (0.3) 26.8–27.7 21.7 (2.6) 17–24 4 (LA, nrep, ml) 21.0 (0.2) 20.7–21.3 30.3 (3.1) 25–35 5 (LA, rep, sq) 27.2 (0.5) 26.5–27.7 19.1 (1.1) 17–20 6 (LA, rep, ml) 21.2 (0.5) 20.4–21.6 27.1 (1.3) 26–29 7 (LA, nrep, sq) 27.3 (0.4) 26.5–27.7 19.4 (1.3) 17–21 8 (LA, nrep, ml) 21.9 (0.3) 21.3–22.3 26.9 (2.1) 24–29 9 (LA, rep, sq) 26.1 (0.2) 25.9–26.5 20.4 (2.1) 17–23 10 (LA, rep, ml) 15.5 (0.3) 14.9–15.8 41.3 (9.1) 33–60 11 (HA, nrep, sq) 17.6 (0.4) 16.8–18.0 37.9 (4.6) 31–42 12 (HA, nrep, sq) 16.0 (0.6) 14.6–16.5 46.9 (9.8) 35–62 Total (n D 84) 22.5 (4.4) 14.6–27.7 28.5 (9.9) 17–62 Table 2: Rankings and NDLs for low-altitude, square, non-repetitive readings Dive Computer Reading 1: Reading 3: Reading 7: Mean rank Rank (depth, NDL) Rank (depth, NDL) Rank (depth, NDL) Aladin 1 6.5 (27.7, 21) 3.0 (27.7, 21) 5.0 (27.7, 20) 4.83 Aladin 2 4.0 (27.7, 20) 2.0 (27.7, 20) 5.0 (27.7, 20) 3.67 Suunto 1 4.0 (27.1, 20) 6.5 (27.1, 24) 5.0 (27.4, 20) 5.17 Suunto 2 6.5 (27.1, 21) 6.5 (27.1, 24) 7.0 (27.1, 21) 6.67 Dive Rite 1 2.0 (27.0, 19) 4.5 (27.4, 23) 2.5 (27.2, 19) 3.00 Dive Rite 2 4.0 (27.1, 20) 4.5 (27.1, 23) 2.5 (27.4, 19) 3.67 VR3 1.0 (26.8, 17) 1.0 (26.8, 17) 1.0 (26.5, 17) 1.00 (Suunto, 2003), and by −3:7m at 21.3m depth (mean SD 0.3m) and 1.5m (mean SD 0.5m) at high at high altitude.