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From Fellrnr.com, Running tips
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* The optical heart rate monitoring works reasonably well. This technique has been around for decades, but it's only recently that the technology has been viable for use during exercise. The TomTom uses two green LEDs and a detector to determine Heart Rate based on the blood filling of the capillaries under the skin.
** When the watch has a good lock on your heart rate it's fairly accurate, and normally within a few beats/minute of a chest strap based monitor. Most of this variation seems to be due to the smoothing that's applied to the reading rather than overall accuracy.
** I had a number of problems with the TomTom not locking onto my heart rate for minutes at a time. It would either show a ridiculously high or low value, so it was pretty obvious. Occasionally it would not find my heart rate at all and just show three dashes. See images below for details.
** I found the problems were worse when my skin was cold. I've not tried it in winter, but at 50f/10c the TomTom struggled, probably because the capillaries it was looking for had constricted due to the cold.
** Because the TomTom has to be next to the skin to monitor the blood flow under the skin, it would be problematic in winter when you need to wear extra layers of clothing. You can't put the TomTom over a base layer, so it would have to be covered by your outer layers, hiding the display.
** To get a good view of your capillaries, the TomTom needs to be positioned slightly further up your wrist and to be quite tightly closed. I didn't find this uncomfortable, but it was tighter than I'd normally have a watch. However, you get a better reading with the watch tighter than is comfortable.
** Using optical heart rate measurement is generally not accurate enough for measuring [[Heart Rate Variability]].
* The TomTom will cache the locations of the GPS satellites for a few days, which worked reasonably well.
|[[File:TomTom Histo.JPG|none|thumb|x300px|This is the first time I've seen a histogram of Heart Rate, which is rather nice.]]
|[[File:TomTomLED.JPG|none|thumb|x300px|The LEDs that detect heart rate optically. The lines you can see on the picture are not visible to the eye.]]
|}
===TomTom Cardio Runner Heart Rate Graphs===
These graphs show the TomTom Cardio Runner against the record of a traditional chest strap. The top of the graph shows the two recordings superimposed, with the TomTom in red and the chest strap in blue. The lower line on the chart is the difference between the two readings.
{| class="wikitable"
|- valign="top"
|[[File:TomTomHRM 3.jpg|none|thumb|x300px|An example run where the TomTom is showing good accuracy.]]
|- valign="top"
|[[File:TomTomHRM 1.jpg|none|thumb|x300px|Here you can see several major discrepancies in the TomTom heart rate reading.]]
|- valign="top"
|[[File:TomTomHRM 2.jpg|none|thumb|x300px|This run has more problems than is typical, with the TomTom repeatedly giving a bad reading.]]
|}
=Buyer Beware=
