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From Fellrnr.com, Running tips
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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 gathered all 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 results of how the errors manifest themselves, I've included a few anecdotal graphs. {| class="wikitable" |- valign="top"|my testing under my [[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.Optical Heart Rate Monitoring]]|- 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 helpedpage.]]|}
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{{DISPLAYTITLE:Garmin 235 Review}}
[[File:Garmin235.jpg|thumb|right|200px|The {{Garmin 235}}, showing the display of heart rate.]]
I had remarkably high hopes for the Garmin 235. I was naïve enough to think that it would be like the [[Garmin 225]], but with better optical heart rate monitoring[[Optical Heart Rate Monitoring]], better [[GPS Accuracy]] and adding in [[Connect IQ]] to boot. Sadly, the GPS accuracy is even worse and the optical heart rate monitoring is still not viable. The 235 has some cool novelties, but if you look at the four basic questions I use to evaluate the foundational capabilities of a running watch, it comes up short.
* '''How far did I run?''' This is the most basic question, and sadly the 235 has distinctly grim [[GPS Accuracy]]. Even with my low expectations of recent Garmin devices I was rather disappointed. It's possible that the variants of the 235 that lack the optical heart rate monitor (230/630) might do a little better, but I wouldn't expect a huge improvement. Of course, the error that you'll see will depend on the nature of your route. On straight lines and with a clear view of the sky it should do okay, but in tree cover and on twisty courses it's likely to be pretty ugly. How much this will bother you is likely to depend on how what you're using the watch for and the nature of your personality.
* '''How fast am I running?''' Knowing how fast you're running can be a nice to know, or it can be vital for your training or race performance. Because of the nature of GPS, watches that rely on GPS signal alone tend to have serious problems with current pace. Thankfully, Garmin has added support for the display of current [[Pace From A Footpod]] while getting all other data from GPS. (This was added in the 4.70 firmware, released June 2016.)
* '''What's my cadence? '''[[Cadence]]''' '''is one of the most critical and often overlooked aspects of running. If you get your Cadence right, many other things naturally fall into place. . There is support for Cadence from the internal accelerometer, though I find that's not as accurate as the [[Footpod]] which it supports. While the 235 has alerts for heart rate, pace, distance, and time, it lacks an alert for Cadence which would have been the most valuable.
The 235 is not a good choice for ultrarunners, as its battery life is too short. You can charge the 235 while you're running, but I'd recommend getting a watch with a longer battery life so you don't have to charge it so often. See [[Best Running Watch#Watches for Ultrarunning| Watches for Ultrarunning]] for more details.
{{BuyAmazon|AZID=B0160BC1FO|AZN=Garmin 235}}
=Garmin 230/235/630=
Garmin produces three similar watches; the base 230, the 235 that adds optical heart rate monitoring [[Optical Heart Rate Monitoring]] (OHRM) to the base 230, and the 630 that adds extra functionality to the base 230. I don't see that the additional price of the 630 is worthwhile, so if you don't want the OHRM then go for the 230. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
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! Garmin 230
=Garmin 235 Pros=
* [[Connect IQ]] allows the 235 to include rich apps to extend its core functionality. For instance, the [https://forums.garmin.com/showthread.php?213535-Watchface-ActiFace-FR920XT-Vivoactive-epix ActiFace] app makes the activity monitor far more inspiring.
* Garmin has added 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. You can also configure it to get both pace and distance from the Footpod, while still recording the track of where you've been from GPS.
* The 235 is a nice size, looking more like a watch than a brick. I find it a lot more comfortable than the [[Garmin Fenix 3]], [{Garmin Epix]], or [[Garmin 920XT]], though I have rather tiny wrists.
* The color display is bigger than the [[Garmin 225]], though it is neither square nor circular, but flat on the top/bottom and round on the sides, which is odd. Some of the screens end up with the edges cut off, which can make the graphs confusing.
=Garmin 235 Cons=
* The [[GPS Accuracy]] is terrible; see below for details.
* The See below for the details of the [[Optical Heart Rate Monitoring]] is also terrible, but it's broadly similar to other systems. See below for details.
* 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.
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=Optical Heart Rate Monitoring=
=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 ;}
* '''Extended battery life'''. Some porches can extend the battery life by turning the GPS reception off for short periods. This can dramatically reduce GPS accuracy, but it's a useful trade-off for some ultramarathons.
* '''WiFi Uploads'''. While the automatic upload of workouts via WiFi is nice, the upload will typically only go to the manufacturer's web site.
* '''Running Dynamics'''. Some of the newer Garmin watches support [[Running Dynamics]] that can show and record Vertical Oscillation (VO) and Ground Contact Time (GCT). (You can get this with the 630.)
* '''Web Configuration'''. Some watches allow you to setup the configuration via a web site, and then download your changes. This is vastly easier than fiddling with the watch.
=Visual Comparison=
{| class="wikitable"
