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Archive for July 8th, 2009

garmin310xt

Well I should not have been surprised.

The Garmin 310 XT records accurate time data in the swim. I’ve been having all kinds of weird distances recorded by the GPS (ratio of 3-to-1 actual miles), and my teammates have asked me to investigate. I’ve been getting 3.54 miles and 3.58 miles in the last two times I wore it in a mile-long course.

When you swim, the safest way to keep it on is to wear it under your wetsuit. Barring that, you wear it securely and snug on your wrist. For a normal swimmer and moderate ocean conditions, the arm can go about a meter under water. That depth doesn’t compromise the 310’s waterproofing (up to 50 meters advertised), but it does wreck havoc on the GPS signal. The unit will acquire, lose, and re-acquire the GPS signal multiple times in the swim. Even for a short quarter mile course.

In choppy conditions, the arm can go literally about 3-4 meters difference from stroke to stroke. The swells will push you up and pull you down (2 meter range) and then you put your arm under water for another meter. The GPS signal would be completely unreliable.

I did lend the device to a strong age grouper who recently swam a 1K in an olympic distance race. He swam in a protected cove under perfect conditions. He also wore the device under his wetsuit. For that race, the GPS recording is a joy to see because it was almost like he ran the whole thing slowly (1-3 miles per hour). I don’t think you can duplicate those conditions in most races.

A look at the Garmin site confirms my field test results. Here is the FAQ for it.  I will post 3 attachments later on to show the actual recordings (visual).

To make a long story short, just use your 310 as a timer in the water and ignore the GPS recording.

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