Th­ese days, dive com­put­ers are widely used by divers to mon­i­tor their pro­files, but is com­plete re­liance on such tech­nol­ogy ad­vis­able?

Scuba Diver Australasia + Ocean Planet - - Buyer's Guide - By DAN’s Peter Buz­za­cott and John Lipp­mann


We were on the third day of a trip that in­volved two dives a day at an is­land re­sort. This was the first dive in the morn­ing. It was a nor­mal dive: We spent less than a minute at 30 me­tres, and the swim-through exit was at 29 me­tres. I spent about 10 min­utes at 24 me­tres, then an­other 15 min­utes around 18 me­tres. I made a slow as­cent with a three-minute safety stop at five me­tres. Af­ter sur­fac­ing, I ex­pe­ri­enced: • in­tense bi­lat­eral shoul­der pain; • arm to fin­ger­tip pain; and • tin­gling and numb­ness in both

my legs and toes.

For­tu­nately, there was oxy­gen on the dive boat, so oxy­gen first aid com­menced fast and I was able to keep breath­ing oxy­gen on the drive to the hospi­tal. While at the hospi­tal, my symp­toms im­proved as I continued breath­ing oxy­gen.

Dur­ing the hospi­tal visit that day,

I had some im­prove­ment on oxy­gen but had re­cur­rent neu­ro­logic symp­toms that re­quired six hours per day of hy­per­baric treat­ment for the fol­low­ing two days. I re­turned home af­ter 72 hours and had two more 2-hour treat­ments in the hy­per­baric cham­ber in my home state to see if the resid­ual tin­gling/numb­ness in my toes and fin­gers would abate. At the end of treat­ment, 99 per­cent of my symp­toms had re­solved.


Two decades ago, the max­i­mum al­low­able to­tal dive time at a 30-me­tre depth would have been around 20 min­utes if the dive was planned us­ing ta­bles, as it com­monly was. This dive pro­file may even have been per­mit­ted us­ing mul­ti­level plan­ning tech­niques, but today, dive com­put­ers do the work for us and such a pro­file is con­sid­ered “nor­mal”. The risk of decompression sick­ness is thought to be low among recre­ational divers be­cause the ma­jor­ity of dives come nowhere near the no-decompression lim­its. Af­ter two pre­vi­ous days of repet­i­tive div­ing, this diver suf­fered an in­jury, which was di­ag­nosed as a se­ri­ous case of decompression sick­ness, even though he didn’t vi­o­late the dive com­puter’s lim­its.

John Lipp­mann, DAN Asia-Pa­cific’s Founder, has said*: With the ad­vent of dive com­put­ers and decompression soft­ware, div­ing has be­come much more flex­i­ble. With­out the in­her­ent re­stric­tions of the dive ta­bles, dives tend to be far longer and more fre­quent, and sur­face in­ter­vals far shorter. The dis­cov­ery of more wrecks, of­ten in deeper wa­ter, as well as other deeper dive sites has also en­cour­aged many of us to dive deeper.

Most divers th­ese days use dive com­put­ers and ap­pear to be rea­son­ably con­fi­dent they will avoid decompression ill­ness (DCI) as long as they dive within the lim­its of th­ese de­vices. The re­al­ity is that the vast ma­jor­ity of the divers we deal with that are di­ag­nosed with DCI have been us­ing dive com­put­ers and have been div­ing within the lim­its ad­vised by their com­put­ers.

Dive com­puter al­go­rithms are based on decompression mod­els, which are gen­er­al­i­sa­tions of what might be oc­cur­ring within a diver’s body. Th­ese mod­els have in­her­ent in­ac­cu­ra­cies and can­not cater for the broad in­di­vid­ual vari­a­tion be­tween divers and the type of dives un­der­taken. The more th­ese mod­els are pushed to their ex­tremes, the greater the po­ten­tial for in­ac­cu­ra­cies.

Deeper dives, longer dives, short sur­face in­ter­vals and repet­i­tive div­ing, es­pe­cially mul­ti­ple repet­i­tive div­ing, can all lead to higher in­ert gas loads, greater bub­ble for­ma­tion and con­se­quently less ac­cu­rate decompression cal­cu­la­tions. When th­ese fac­tors are com­bined, as they of­ten are in cur­rent day div­ing sce­nar­ios, we can be invit­ing prob­lems.

In an­other in­ci­dent, a diver suf­fered se­vere cere­bral DCI while div­ing at

Truk La­goon. Look­ing through his dive pro­files, I was some­what un­sur­prised with the out­come. From a dive safety per­spec­tive, sev­eral con­cerns arose: • The diver con­ducted three dives each day for four con­sec­u­tive days.

The deep­est dive for the whole trip was com­pleted on the last day at 54 me­tres.

The av­er­age depth of all the dives un­der­taken was 30 me­tres. The length of the dives ranged from 45 to 69 min­utes, with an over­all av­er­age of 52 min­utes of to­tal un­der­wa­ter time.

The length of the day­time sur­face in­ter­vals ranged from 1 hour 20 min­utes to 2 hours 35 min­utes, with an av­er­age of 1 hour 53 min­utes.

Th­ese dives are quite typ­i­cal of what many divers com­plete at Truk La­goon (and some other dive des­ti­na­tions). Divers need to be aware that deeper div­ing is as­so­ci­ated with a higher risk of DCI and the re­sult­ing DCI is more likely to af­fect the ner­vous sys­tem. Very long dives also carry a higher risk of DCI.

In the re­ported case, it was pru­dent of the diver to make a safety stop, but of par­tic­u­lar note is that there was oxy­gen on-board the boat, and the diver continued breath­ing oxy­gen on the way to the hospi­tal. Af­ter mul­ti­ple hy­per­baric treat­ments, the diver was 99 per­cent re­cov­ered, which is good news, though it should be pointed out that bi­lat­eral symp­toms af­fect­ing all four limbs are not typ­i­cal of decompression sick­ness. Re­gard­less, this case serves as a re­minder that our dive com­puter may well keep re­cal­cu­lat­ing our al­low­able lim­its, but that does not mean we should dive to those lim­its. If 100,000 divers dive to the lim­its then, even though they did not “break the rules”, by prob­a­bil­ity alone some will get the bends.

OP­PO­SITE PAGE Dive com­puter al­go­rithms are based on gen­er­alised decompression mod­els which do not cater for vari­a­tions be­tween in­di­vid­u­als RIGHT Today, dive ta­bles have largely been re­placed by dive com­put­ers, which al­low for more flex­i­ble dive pro­files

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