Do You Use a Backup Computer, Part II: Undercurrent 02/2022

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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."

"Carrying computers with identical algorithms, identical data, and identical screens won't confuse you."

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."...

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