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* Unlike a traditional heart rate strap where the battery will last for months, the Rhythm+ only lasts for around seven hours. This means that you have yet another device to keep charged.
=Accuracy=
I compared the Rhythm+ to a traditional heart rate monitor strap so that I could plot the recordings together to see the differences. I have found that the Rhythm+ gave erroneous readings too often for me to recommend it.(I suspect that the optical sensor has a sampling frequency, as you can see the light flashing. If the [http://en.wikipedia.org/wiki/Nyquist_frequency frequency is too low, it is possible for sampling errors to occur], which might explain why the errors seem to occur at the start of the run, as the heart rate is rising rapidly.)
{| class="wikitable"
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|[[File:Rhythm1.jpg|none|thumb|x300px| This graph shows a fairly typical error I've seen with the Rhythm+. For the first a few moments the Rhythm+ gives a correct reading, but then rapidly fails, giving a wildly high reading. After a few minutes things settle down and the Rhythm+ tracks reasonably well. I tried attaching the Rhythm+ a few minutes before the run, but that didn't help.]]
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|[[File:Rhythm2.jpg|none|thumb|x300px| Is a close up of the comparison between the chest strap and the Rhythm+. As you can see, the Rhythm+ is reasonably accurate, though not quite up to the standards of the chest strap.]]