Thread: Swim Computers
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Old 11-29-2014
Talvi Talvi is offline
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Join Date: Mar 2012
Location: Lappeenranta, Finland
Posts: 1,675

To clarify, I have the 910. I ave neither rhe 920 nor the Swim.

There are many arithmetic issues with the 910, but maybe they're not that obvious.

In the History, for every interval there is a summary. This gives total strokes measured and the total time, together with the "average" SPL and the "average" Pace. The "averages" given are truncated, not rounded.

The time between presses of the Start/Stop button etc are accurate, as far as I have been able to gauge using the stopwatch on my phone (!). The time of intervals given is therefore correct. The times of individual lengths are "unreliable" due to the complexity of the algorithm to detect the turn ans the fact that the watch has to make its best guess for the moment at which the turn actually occured.

It is not possible with the 910 to dictate the format of data displayed so the trucation of interval data during a swim leads to poor ionformation. An interval pace could be shown as 2:04 and then 2:05 for the next, a difference of 1 second. The real averages could actually be 2:04.99 and 2:05.01, a difference of 0.02 sec, or 2:04.00 and 2:05.99, a difference of 1.98 secs.

A rounding function and a trucating function take almost or precisely the same computing power, and the display has room for the additional digits to be displayed.

With SPLs, as the stroke count is small by comparison to the pace seconds the truncation error is proportionately increased. For two intervaks the data shown on the watch could be 8 SPL and 9 SPL, i.e 16 spl and 18 spl. But the real data could be 8.99 SPL and 9.01 SPL at one extreme or 8.01 SPL and 9.99 SPL at the other. Rounding would show that the first pair of intervaks were both swum at 18 spl whereas the second pair of intervals would have been swum at 16 spl and 20 spl.

Having to read data off the watch and enter it into a calculator at the end of the swim to see what actually happened during intervals is .. frustrating.

I am trying to learn to count. Pretty unsuccessfully, but when I do count and check with the watch, the numbers are the same. However, as you'll understand from the above explanation, it is only possible to check the count for 25m intervals.
A psychological disorder is: "Any personal construction which is used repeatedly in spite of consistent invalidation."
~ George Kelly

"The water is your don't have to fight with water, just share the same spirit as the water, and it will help you move."
~ Aleksandr Popov
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