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From Fellrnr.com, Running tips
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=Garmin 235 Cons=
* The [[GPS Accuracy]] is terrible. I'll add more details in the near future.
* The [[Optical HRM Heart Rate Monitoring]] is also terrible, but it's on a par with broadly similar to other systems. Again, I'll add more See below for details in the near future.
* I love the idea of having continuous hear rate monitoring, especially to get things like [[Resting Heart Rate]]. However, the poor accuracy limits the value, and the continuous monitoring seems to add problems where the 235 gets "stuck" at a particular value. I'd expected the heart rate data to be used for calorie calculations, but it doesn't seem to have any effect.
* The battery life is annoyingly poor, even for fairly casual running when the watch is used as an activity monitor. I find myself having to charge the 235 far too frequently and it seems to take longer than I'd like to recharge.
* There's no support for displaying your current [[Pace From A Footpod]] while getting all other data from GPS. For situations where pacing is critical, such as running a marathon, this is a must have feature. If you need to know how fast your running, look elsewhere.
* The 235 includes an internal accelerometer to give you an idea of pace and distance while running on a treadmill without a footpod, but I found the accuracy was rather poor. I've yet to come across a watch that can reasonably estimate your pace from the movement of your wrist, nor does it seem likely that this functionality is practical.
=Optical Heart Rate Monitoring=
To analyze the accuracy of the 235's optical heart rate monitor I gathered a little over 20,000 readings and analyzed them in comparison with an ECG-based chest strap heart rate monitor. I created to cross to visualize this comparison. The first graph shows the heart rate measured by the ECG HRM on the horizontal against the Garmin 235 on the vertical. If the two systems match exactly then the point will be on the green line of equality. You can see that many of the points cluster around this green line, but are distributed more widely than I think is acceptable. In addition you can see a cloud of points well away from the green line that represent times when is the 235 was reading a wildly high heart rate. There is some discussion within the running community that these high readings might be the optical system becoming confused by the impact of foot strikes as they tend to be vaguely in the vicinity of [[Cadence]]. This graph uses transparent points to give a better impression of the density of data, with black areas having at least 10 data points lining up. There is a blue regression line, which will be aligned with the green line if the system is accurate, but as you can see it is distinctly misaligned. I've also included two red lines that are 25 bpm away from the true value. The second histogram shows the distribution of errors, and includes the erroneous cloud of points is a slight bump to the right of the main spike.
{| class="wikitable"
|- valign="top"
|[[File:ORHM-Garmin235-Scatter.png|none|thumb|500px| The distribution of readings between the Garmin 235 OHRM vertically and the Garman Ant+ system horizontally.]]
|- valign="top"
|[[File:ORHM-Garmin235-Distribution.png|none|thumb|500px| Here is another view of the same data, showing the greater error.]]
|}
Of the 20,000 readings, slightly more than 10% were out by more than 25 BPM (demarked by the red lines on the first graph.) The average error (standard deviation) is 19.1 BPM, with an average reading that was 5.7 BPM too high. If you're an experienced runner that has a good idea what your heart rate should be, then you may be able to ignore values that are out by more than 25 bpm. In that case, the standard deviation drops to 4.2 BPM with an average that is 0.03 too low. Of course, if you know your heart rate to within 25 BPM, then the 235 will only get you slightly closer. To give a sense of how the errors manifest themselves, I've included a few anecdotal graphs.
{| class="wikitable"
|- valign="top"
|[[File:Garmin235-OHR5.jpg|none|thumb|500px| During this run you see the 235 having a couple of major dropouts. For the rest of the run, the 235 roughly tracks the true heart rate. ]]
|- valign="top"
|[[File:Garmin235-OHR3.jpg|none|thumb|500px| Here we see the 235 giving an accurate reading, but one that is rather misleading. While I frequently see the 235 displaying a heart rate that is wildly too high or too low, I know I can ignore that information. Where the 235 is more problematic than other optical systems is that it will display a plausible but inaccurate value.]]
|- valign="top"
|[[File:Garmin235-OHR4.jpg|none|thumb|500px| For this run the 235 initially gives an inaccurate reading that is somewhat close to the real heart rate, but then spikes to wildly too high. I tried several times to adjust the tension and position of the 235, but nothing helped.]]
|}
=What's Missing=
While I don't consider these missing features as 'cons', it's worth understanding the features that are missing compared with other watches. You'll notice that the 235 has a rather short list of missing features ;}
