Not ready for prime time…

Uwatec says its new SmartPro and SmartCOM computers can suppress nitrogen micro-bubbles that precipitate decompression illness and cause damage to soft tissues. Unfortunately, the diver cannot monitor remaining no-decompression stop time when using this feature, making it impractical - and potentially even dangerous - for recreational multi-level diving.

By Robert Delfs

I've long been a fan of Uwatec dive computers. I must be - my dive buddy and I own and regularly use four of them. My old Aladin Air shows signs of external wear, but still performs flawlessly after more than 300 dives. I also like the Pro Ultra wrist-mount computers we purchased for nitrox diving last year, featuring the same clearly legible display as well as simple procedures for checking and adjusting the O2 blend between nitrox and conventional air dives.

These positive experiences predisposed me to like the new wrist-mounted SmartPRO and hose-integrated SmartCOM, which may represent the most radical change in function and design in the Uwatec computer line since Johnson Outdoors acquired the Swiss-based company in 1997.

The Smart Computers incorporate a faster 32-bit microprocessor and both are nitrox- capable, allowing the user to set O2 content in 1% increments up to 100%. A built-in InfraRed data port allows the user to download dive information directly to a computer or modify computer functions using the supplied SmartTrak© program without the need for a cable or other device. Functions which the user can modify include maximum allowed partial pressure of O2 in a range from 1.2 to 1.95 bar (with 1.4 bar the default value), gauge mode, selection of metric or imperial units and optional audible warning.

The most dramatic new feature of these computers is micro-bubble suppression. While classical decompression theory was based entirely on modeling the rates of intake and off-gassing of dissolved nitrogen in different body tissues, modern decompression theory also emphasizes the role of free nitrogen bubbles - including "silent" (symptomless) micro-bubbles - as a critical factor contributing to decompression illness (DCI) and a possible cause of long-term damage to soft tissues.

New bubble theories have led to an increased emphasis on reducing the speed of ascent at all times and given us a better understanding of how repetitive and multi-day diving intensifies the risk of DCI. Technical divers using new decompression software incorporating deep stops to reduce bubble formation now routinely execute deep profile dives using mixed gases that would have been inconceivable under classical theory.

This new research has also stimulated an effort among manufacturers to incorporate more sophisticated bubble reduction features in computers intended for recreational diving using air or conventional nitrox. These include recent several recent Suunto models which incorporate a decompression model incorporating Bruce Weinke's Reduced Gradient Bubble Model (RGBM).

The new Uwatec Smart Computers use the same fundamental decompression model built into most conventional computers and tables. The Bühlman ZH L8 ADT model is based on estimating the rate of absorption and off-gassing of dissolved nitrogen in different kinds of tissues, but it is also adaptive in that the decompression schedule and no-decompression stop time limits are modified based on other risk factors such as temperature, over-exertion or excessive rates of ascent.

The SmartPro and SmartCom micro-bubble suppression feature can be set to any of five different levels (L1-L5, with L5 the most conservative), or disabled (L=0), in which case the computer will function like an ordinary dive computer using a Bühlman multi-tissue dissolved gas decompression model.

When micro-bubble suppression is enabled, the Smart computer will assess the likelihood of micro-bubble build-up from previous dives and may recommend one or more "advisory" level stops during ascent at the conclusion of the dive. The computer will still calculates no-decompression stop time in the conventional way, however, and if no-stop time is exceeded, may require one or more decompression stops, which take precedence over level-stops.

Unlike a decompression stop, which is mandatory, level stops are only advisory. The Uwatec manual and technical memos explains that Smart computers "recognize that the effects of micro-bubble formation are largely a long-term condition, whereas decompression illness produces symptoms that require immediate treatment." Missing an "advisory" level stop therefore does not result in a violation or lock out. Instead, the computer simply shifts to a lower level of bubble suppression, ultimately disabling the micro-bubble suppression feature entirely if all level stops are ignored.

In a series of tests, Uwatec found that incorporating additional level stops at the conclusion of a repetitive dive reduced the incidence of high-grade micro-bubbles by up to 61%, which the company claims significantly lowers the risk of DCI and reduces long-term damage to soft tissues.

Anything that limits micro-bobbles and provides additional protection beyond conventional dive tables and computer models would seem to be a potentially significant advance in dive safety, particularly for recreational divers using air and conventional nitrox mixtures who do frequent repetitive or multi-day dives, or are otherwise predisposed to decompression illness. Making additional deep stops reduces the pressure gradient between nitrogen-saturated tissues and existing bubbles that can induce growth in micro-bubbles as the diver ascends. The bubble suppression feature incorporated in Uwatec's new Smart Computers therefore appears consistent with recent advances in decompression theory that advocate additional deeper stops to control bubble growth.

Unfortunately, the way that Uwatec has implemented the new bubble suppression feature in these computers makes it unlikely that many divers will ever realize much of the potential benefit.
The problem is that Uwatec has chosen to remove no-decompression time limit information from the display when bubble suppression is activated, using this location to show the "advisory" no-stop time and level stop information. When bubble suppression is activated, the computer still calculates the remaining no decompression stop time in the conventional way and it will impose a mandatory decompression stop if no-stop time limit is exceeded, but the no-stop time information is not available to the diver on the display. Once a level stop advisory appears, there is no way to find out how much no-decompression stop time actually remains until the limit is reached and a mandatory decompression stop indication appears in the display.

As I discovered in testing this computer in a series of air and nitrox dives over five days, the initial level stop advisory can appear very early in the course of a dive. Surely an instruction to make a 1-minute stop at a depth of three meters is superfluous now that almost all divers routinely make a 5 meter safety stop for three to five minutes, but this is exactly what happened to me using the SmartPro only 19 minutes into a planned 45 minute nitrox dive to 33 meters. (See box - below.)

Losing the no-decompression time limit display removes one of the most vitally important pieces of information that dive computers provide. Without knowing the remaining no-decompression time, it is impossible to "back off" the computer by ascending to a shallower depth before no-decompression time has run down to zero, a basic principle of conservative computer assisted diving.

Uwatec knows that "backing off" the no-decompression limit is the way most recreational divers actually use their computers for multi-level diving. The SmartPro manual even advises divers to "Give yourself a margin of safety by always leaving at least a few minutes in the 'no-stop box' before making your ascent."

Enagling the bubble-suppression feature of the Smart computers in order to create an even larger margin of safety can actually have the opposite effect by making it more difficult to stay safely within the computer's no-decompression time limits, forcing the diver to abort early in the dive unnecessarily or else risk pushing the computer beyond the limits of no-decompression diving.

In the worst case, using the SmartPro and SmartCom computers with bubble suppression enabled could lead to divers unintentionally engaging in unplanned decompression dives and incurring mandatory decompression obligations, possibly without sufficient breathing gas to complete their stops.

Scubapro officials I contacted about this problem pointed out that the actual decompression obligation incurred would be minimal if the diver immediately begins a cautious ascent as soon as the no-decompression time limit is reached. In many cases, moreover, the decompression obligation would automatically clear before the diver ascends to the indicated decompression stop depth.

Diving a computer in this manner, however, violates a basic principle of dive safety endorsed by almost every training agency: namely that recreational divers should avoid dives requiring mandatory decompression stops.

Scubapro also insisted that divers would be perfectly safe using this computer as long as they initiate an ascent as soon as a level stop advisory appears. This may be good advice for anyone who owns a SmartCOM or SmartPRO, but it is troubling that the current manual for this computer contains no suggestion that a level stop advisory should be interpreted as an instruction to begin an immediate ascent and abort the dive.

Using a Smart Computer in this way would eliminate the risk of being forced to carry out an unplanned mandatory decompression stop, but essentially it just means adopting ultra-conservative no-stop time limits on repetitive dives, effectively negating the logic and rationale for the new bubble suppression feature. These computers are supposed to reduce the risk of DCI by incorporating additional deeper stops to suppress micro-bubbles during the ascent, not just radically reducing no-stop times. That could be accomplished by simply adopting more conservative assumptions in a standard adaptive Bühlman multiple-tissue decompression model.

I doubt many divers will be enthusiastic about curtailing their dives to twenty minutes or less after only two or three days of repetitive diving at conventional recreational depths, particularly those who use their computers primarily as a means of safely extending their time underwater on multi-level dives. Scubapro officials whom I questioned on this point suggested divers who wished to enjoy longer bottom times could simply disable the bubble suppression feature.

In the manual, there is actually an illustration of comparative dive profiles with and without bubble suppression that clearly implies that divers can continue a dive after the level stop advisory appears up to the point when a mandatory decompression stop indication appears. (See Illustration #1 - Manual Page 22.)

In this illustration of simulated dive profiles taken from the Uwatec manual, the upper "L0" below the profile shows the decompression information that would appear on a SmartPro or SmartCom computer with bubble suppression disabled. The lower "L5" shows the information that would be visible on the same computer with bubble suppression on. In the L5 displays, level stop data and advisories appear in place of the decompression data (visible only to the diver using the computer with bubble suppression disabled) until a mandatory decompression stop obligation is incurred.

What the illustration does not show is how the diver using the Smart computer with bubble suppression enabled could possibly have known how much no-decompression time remained in order to decide when to begin an ascent - unless, of course, the diver uses two computers at the same time, relying on a one to monitor remaining no-decompression stop time and a second SmartPro or SmartCom to generate a schedule of advisory level stops to be carried out during ascent.

The mystery is why Uwatec did not try to combine both the level stop and no-decompression stop information on the display. One approach would be to alternate the level stop information with the remaining no-decompression time in the same location, in the same way that CNS O2 load and ascent speed warnings on this computer alternate in the center of the display. Another possibility would be a redesigned display that continuously shows both no-decompression stop time and level stop information.

Uwatec may have been concerned that a display containing more information or alternating information in a single location could be confusing to inexperienced divers. But making it impossible to find out how much no-decompression stop time remains during a dive - the choice Uwatec actually made - seems the worst of all possible alternatives.

Divers who use a computer to extend bottom time while avoiding mandatory decompression stops or make a practice of backing off the computer's no-decompression stop limits while engaged in multi-level and multi-day diving will not find the SmartPro and SmartCom computers ideally suited to their needs.

As divers always looking for ways to enhance our margin of safety underwater, we can only hope that Uwatec will rethink its approach and come up with a way of integrating micro-bubble suppression into its computers that does not fatally compromise other more basic and vital dive computer functions.

ENDS MAIN PIECE - Final Draft -19 May 2002

BOX: Testing the SmartPro - How much no-stop time do you really have?

Odie's Wall, one of the most popular dive sites near Puerto Galera in the Philippines, is an isolated ledge festooned with black corals and barrel sponges. The site is accessible only as a boat dive in an open channel. Depths range from 28 to 33 meters.

It was the first dive on the third day of testing the SmartPRO, which I had set to L3 - an intermediate micro-bubble suppression level. The group of six divers would be breathing nitrox (EANx32). We planned a total dive of approximately 40 minutes, including 20-25 minutes on the ledge followed by a slow blue water ascent and an extended safety stop before surfacing together as a group in the channel. The surface interval since my last dive the previous day was 14 hours and 36 minutes. Our dive plan was to descend to the deepest part of the ledge, work our way gradually to the shallower portion, and initiate an ascent when the first computer ran down to two minutes no-decompression stop time.

After descending to the bottom of the ledge at 33.5 meters, I was surprised to see the SmartPRO computer showing a no-stop time of only seven minutes. I ascended to a shallower part of the ledge at 26 meters, but only fourteen minutes into the dive, I had less than two minutes of no-stop time remaining.

By now I had twigged that the no-stop display was actually showing the time before an advisory level indication would appear, not no-decompression stop time! Turning off my camera and strobes, I ascended to 22 meters - the shallowest depth I could manage while maintaining contact with the rest of the group - but the SmartPRO continued relentlessly to run down the no-stop clock until only nineteen minutes into the dive it called for a (superfluous) one-minute advisory level stop at 3 meters.

In this SmartTrak© profile the computer display (upper right-hand corner) shows an advisory calling for a one minute level stop at 3 meters appearing 19 minutes into the dive at a depth of 24 meters. Actual remaining no-decompression stop time is not visible anywhere on the display.

It was disconcerting to realize I had no idea how much no-decompression time actually remained on the computer, but I knew I was unlikely incur any mandatory deco time before one of the divers below me hit their two-minute limit. I also knew I had plenty of breathing gas to do any stops the SmartPRO might impose. Hovering above the rest of the group while they completed their dive, I also had plenty of time to ponder a troubling question: how did the Uwatec engineers imagine divers would actually use this computer in real-life diving?

The SmartPRO never increased the original advisory level stop recommendation, nor did it add any additional advisory stops. After we began the ascent, the one-minute at 3 meters advisory level stop recommendation cleared spontaneously when I reached a depth of 8.6 meters and before we began our extended safety stop at 5 meters.

ENDS BOX

Final Draft 19 May 2002