Do You Use a Backup Computer, Part II

For most serious divers, a backup computer is an indispensable item; if your main computer fails, you have a backup to continue diving safely as well as the data required to conduct follow-up dives.

Undercurrent recently asked readers for details about their diving with backup computers, and in our January issue, we publish Part I of their experiences. Here is Part II.

Your Computers Should Tell the Same Story

It's critical to use a backup that tells the same story as your main computer. Reverting to a backup with a more conservative algorithm may require excessive deco-stops for which you have insufficient air. Or it may lock and become temporarily useless because you missed those stops. Ken Kurtis (Los Angeles, CA), who leads group tours all over the world, explains why:

"Too often, I see people using a backup computer that's more conservative than their primary. The issue is that they could be tooling along looking at their primary but not looking at the backup because the primary's working. So, it's possible/likely, especially on a week-long trip, that while their primary is staying within no-deco limits, their more conservative backup is not. And then (hypothetically), a few days into the trip, their primary fails during the dive. They go to their backup, only to discover that it's gone into violation mode because it previously had a missed deco obligation. Now it's the same as diving with no backup at all."

And that's precisely why you need identical algorithms in your two computers.

You can get caught out mixing older and newer computers, as one reader learned: "My old computer is integrated with my regulator and gives me my tank pressure and all the other information. I wear the second computer on my wrist. I don't look at it as much because the other one is easier to read underwater. My old computer didn't put me in deco on one dive, but the newer one on my wrist did. Since I wasn't paying attention to the newer one, I didn't know it required a deco stop, so it locked me out for 24 hours."

That computer lock isn't a function that every diver favors. Jeff' "J" Reeb' (La Canada, CA) wrote, "I do not like computer settings that 'lock' the computer if the diver gets into a decompression dive. It's not uncommon for an experienced recreational diver to accumulate a few minutes of deco obligation, which will typically clear during a slow managed ascent or at the safety stop. To have the computer become unusable as a result of this, I think, is nearly a punitive feature for a dive that could very well have been executed in an otherwise safe manner."

Some highly experienced divers might agree with him, but I must humbly point out that a computer only "locks out" if the mandated deco stops are ignored, and therefore, violated. Divers need to learn what their computer display is telling them so if they inadvertently go into deco mode, they know what to do.

Dave Allen (Quincy, IL) tells how both he and his wife use Oceanic Datamax Pro Plus computers backed up by Genesis Reactpro (non-air-integrated) computers, which use the same algorithm. "If either required a deco stop," he sensibly says," that would be the one I would adhere to."

A few years ago, different computer manufacturers used widely differing algorithms, but thankfully, they are more similar nowadays. Nonetheless, it makes sense to back up a Suunto computer with a Suunto or a Scubapro computer with another Scubapro. That way you're carrying computers with identical algorithms, identical data, and identical screens that won't confuse you.

A Fort Worth, TX, diver reports backing up his Oceanic Pro Plus with a Suunto Gekko, which have different algorithms. "When the high-pressure hose of the integrated Oceanic started leaking from thousands of pinholes that developed along its length, I didn't have a replacement hose. Since I had the Gekko and also an analog pressure gauge and hose, I kept diving without interruption." Suunto uses the Bruce Wienke RGBM, which is much more conservative than the Oceanic DSAT algorithm, so we're pleased he encountered no problems.

Cary Stegall (Minneapolis, MN) notes, "I know many hardcore divers are down on Suunto computers because of their conservative nature, but I've been diving them for 20 years, and I'm still here, so I'll take the compromise. Doing lots of dives on a liveaboard can create issues. At the end of a week, it doesn't take much to put them into deco mode or require additional time at the safety stop. I usually do the extra time and don't worry about it." We second that.

One has to be careful setting computers, as Gary Payne (Flemington, NJ) discovered when diving with two Aqualung computers that produced mismatched deco requirements. He says, "It turned out one was set too conservatively, which relates to the dive profile. Correcting that setting makes them both read the same."

Scott Burrows (Spicewood, TX) and his wife dive with three identical Sherwood AWAC-1 computers. He says, "The only mishaps have been forgetting to set the PO2 on the spare before a dive. It defaults to 50 percent O2, and the alarm goes off and continues throughout the dive. Then it has to sit out for the next day or go through a reset procedure, after which it is no longer in sync with the other two computers."

Neal Langerman (San Diego, CA) and his wife each use two identical computers. "I mount mine side-by-side on my HP pressure gauge hose. She has one hose-mounted and one wrist-mounted. We do this with the proven experience that computers die ... and the data is too mission-critical to lose."

Larry Klumb (Lithia, FL) wisely agrees. "I dive with a Scubapro G2 as my primary and a Galileo Luna as my backup. I also keep an old and reliable Scubapro Smart Tec computer in my kit bag just in case. All three computers use the same algorithm, so making a switch because of a failure or otherwise allows me to continue diving."

And peace of mind is another good reason to use identical algorithms. Says Richard Mertz (Mount Pleasant, SC), "We each dive with two computers - they use the same algorithms so that we are not giving one a nervous breakdown while the other is fine.

Mathew Glass (Ashland, MA) has a backup with the same algorithm because "It's always nice to have verification that my primary computer is providing good, accurate data regarding the dive."

A Buddy's Computer is Not a Backup

When you're on a dive trip, the only computers that count are those you carry. Yet, Michael Traylor (El Paso, TX) uses only one computer (an Aeris Manta) but admits, "I may be dangerously complacent, but if the Manta fails during a dive, I would let the divemaster know and surface as usual. I'm afraid I don't do any self-supervised dives nowadays." That may be all well and good if you are day boat diving and not making a series of dives as you would on a liveaboard trip. However, ascending on data from someone else's computer should be done only as an emergency procedure and then keep you out of the water for the next 24 hours, as Ken Kurtis (Los Angeles, CA) observes.

Ken's Oceanic Pro Plus failed at remote Cocos Island on a trip that included other instructors and shop owners. He says, "I told everyone I was sitting out for 24 hours, and that's what I did so that my body could out-gas all the [absorbed] nitrogen, and then my computer and I would be back in sync.

"Ironically, all of these highly knowledgeable instructors and shop owners tried to convince me it would be OK to dive off my buddy's computer or just stay shallower or whatever, but I declined. I told them, 'We agree to play by a certain set of rules. Just because the rules become inconvenient, that doesn't mean we can suddenly decide to play by a different set of rules. Finish with what you started."

Don't be misled by dive guides. John Bantin, Undercurrent's senior editor, was diving in French Polynesia and made two dives to 130 feet with only a 30-minute surface interval. He incurred a long deco stop on the second dive, to which the dive guide later objected. She said a ten-minute stop was quite sufficient. When he asked what computer she used, she replied she didn't use one. He was glad he wasn't following her intended profile.

Air-Integration by Wireless Transmitter

Wireless transmitters for dive computers that register tank pressure are commonly used, but some divers encounter problems with them. John Bantin fielded a call to Undercurrent from a diver who had just purchased a computer and transmitter online but couldn't get them to pair. When Bantin suggested the diver try fluttering the second stage purge valve, he learned that the diver was trying to pair them without having his regulator set up, so there was no tank pressure to read. The diver didn't understand that the transmitter sends the tank pressure reading to the computer via the regulator first-stage.

But, once you get it right, there may still be problems. Bill Kasischke (Woodland Park, CO) reports that the tank pressure sensor and his Suunto Vytek computer lost contact a couple of times. "As I don't usually use an SPG, I had no idea what my tank pressure was while diving (I did know that the tank was full initially). Since they were relatively shallow dives and I usually had a good handle on my air consumption, I continued the dives."

Scott McMillan's (Mill Valley, CA) Oceanic VT3 would randomly stop communicating with its transmitter. He wrote, "This was easily resolved by putting my hand behind my head so that the computer and transmitter were in close proximity, but I often found myself diving with my hand behind my head. I returned the unit to Oceanic, and I was told it performed flawlessly. On my next dive trip, I experienced the same problem. Oceanic exchanged the VT3 for a VT4, which has been problem-free."

Many divers install their transmitters at the end of a short high-pressure hose. It makes it easier to reach and prevents boat crews from using the transmitter as a handle to lift the tank. Emmette Murkett (Wetumpka, AL) started using a short hose after he "observed with horror as dive resort workers routinely used the pair of transmitters as handy handles to lift my kit, including my weight integrated BC. As you can imagine, I was pleased to read Undercurrent's Online Update for Subscribers and have followed your advice and use a six-inch hose to discourage this."

Some divers carry a pressure gauge alongside the computer transmitter to be extra-cautious. Jeff 'J' Reeb (La Canada, CA) says, "I see an alarming trend where divers will dive with a transmitter-enabled computer and then remove their SPG from their regulator set. Either the transmitter or the computer can fail, leaving the diver with no way to manage his/her air consumption."

Bantin habitually dives with two computers and two transmitters but no high-pressure hose. It's raised eyebrows among other divers, but he knows that high-pressure hoses and gauges are not failure-proof. He's never had a transmitter or computer failure either. But some, such as Al Scheflow (Hollywood, FL), might disagree with Bantin's technique. "Divers who have their air pressure only available via a transmitter are misinformed. We both have analog SPGs and always carry a spare in the gear bag."

Robert Hales (Milford, OH) has been less lucky. "We spend too much on dive travel to sit out any dives because of a computer crapping out. Then in Bonaire, my wife's transmitter died in the middle of a dive, and we realized that the transmitter was another source of dive woe. Both the [Scubapro Galileo] Luna and the Mantis M2 communicate with the same transmitter, and if the transmitter goes, neither will tell you anything about your air supply. So, we both purchased Aladdin H Dive computers which are air integrated [by hose] and consequently bypass the transmitter as a potential source of problems."

John Miller, the training director at the Texas Dive Center (Lubbock, TX), says, "Air-integrated transmitters usually fail because you didn't put in a fresh battery before your week of vacation diving. Some dive computers will give you issues when their power level drops below 50 percent or less; so, therefore, keep them charged or loaded with fresh batteries."

Careful divers can change the transmitter batteries themselves. John Yavorsky (Warren, NJ) wrote, "After getting charged about $125 for a battery change for the Suunto D6, I learned how to change myself only for the cost of the battery. I also change all transmitter batteries myself."

Many divers do this, but we don't recommend it for a simple reason: a dive store technician can pressure-test the unit after he replaces the battery, guaranteeing you have a proper seal. You might save a few bucks changing your batteries yourself, but then again, your trip might be disrupted by unforeseen leaks.

Your dive store technician will also ensure you get the correct battery. Emmette Murkett (Wetumpka, AL) wrote, "I replaced the batteries in both transmitters as they were about 18 months old. Unfortunately, I did this two days before I left for Roatan. I used the CR1/2AA batteries that Scubapro specifies in the owner's manual. After that, both transmitters ceased to function. Scubapro admitted that there are two versions of the Varta CR1/2AA Lithium batteries. While the most common is 3.6V, the rare 3V version is required. The correct battery kit from Scubapro, which also includes a new O-ring, has been on back-order since August and was still unavailable in early December.

Craig Carnick (Manitou Springs, CO) adds, "So what's most important in a situation when your computer fails? Knowing your remaining air? Knowing how long you've been down? Knowing your current depth? All three are critical, especially if some degree of decompression - even just a 15-foot safety stop - is required. With this in mind, I have added an additional hose to an electronic SPG that indicates air pressure, depth, and time in nice big numerals."

Our divers have had a lot to say about backup computers, but some find dive computers too complicated or troublesome. A word about that in the next issue.

- Ben Davison