optical heart rate monitoring: FR935 vs. F3HR

Since Q4 2015 I was using a Fenix 3 (DCR, t5kr, fellrnr). Due to technical problems (serial number bug) it got replaced in Q2 2017 by Garmin. They offered me (for additional charge which was fair) to upgrade to a Fenix 3 HR (DCR). This way I got 2 years of warranty again – which can’t be too bad especially with Garmin products. 🙄

In my use case the oHR of the F3HR did work like 👽💩. It even was worse than the Rhythm+ (DCR, t5kr, fellrnr) – and the R+ already was for me pretty unreliable. 😥

Since end of July 2017 I use a Forerunner 935 (DCR, t5kr, fellrnr). The FR935 works pretty amazing 🎉✨🎆👏  – except for walking (ideal use case for oHR). 😬😡


Here (only) some results of my testing…


RowErg (Concept2)

DCR’s Analyzer – for the 🤓 inside us. (😊)

DCR’s Analyzer

DCR’s Analyzer


SkiErg (Concept2)

DCR’s Analyzer

DCR’s Analyzer


Zwift (Indoor Cycling)

DCR’s Analyzer



DCR’s Analyzer

DCR’s Analyzer

DCR’s Analyzer



DCR’s Analyzer

DCR’s Analyzer


…to be continued, I guess. 🤔



Hope that helps (someone).


15 thoughts on “optical heart rate monitoring: FR935 vs. F3HR

  1. Really interesting!

    A friend has a fenix 5 HR and looking at his runs I am sure that it is giving all kinds of bogus HR readings.

    The 935 is basically the Fenix 5 in a different case so really interesting that it is doing so well with optical HR.

    Hope all is well Boris 👍

    Liked by 1 person

    1. Richard

      Yeah, I’m really surprised how well it works. But as IT guy it’s still hard to believe in the results – it’s feels a mindset change. 😂 The FR935 is basically a Fenix 5L (Light), Fenix 5P (Plastic) or Fenix 5T (Triathlon)… very light and comfy to wear. Each time I put on the F3HR I get confused about size and chunky feeling – thinking of putting a counter weight on the other arm. 😜

      I’m fine and I hope you too?


      1. Yes really difficult to believe a watch with optical HR is getting really good readings when on the C2 rower. Rowing nearly always messes up the optical HR reading.

        I am good too thanks Boris.

        Liked by 1 person

      2. Thanks for this, looks like im going to have to get my hands on a 935 now as well – I think the vivoactive HR is great but being able to ditch the HR strap would be a real plus

        Liked by 1 person

        1. Tom

          If you have the possibility to use/test/try a FR935 before you buy it, I can offer you to send me fitfiles of both devices (oHR / chest strap) when you did some tests… and I trough ’em in to the Analyzer. This way you see/check if it works for you or if would be “wasted money”.


          1. It could be individual for Boris, he may have cracked how to wear the watch when on the C2, these optical HR monitors do have magical aspect to when they do and don’t work! Really promising looking at the plots above, I also wonder if OTW rowing adds in even more movement of the wrist which could again send the HR wonky.

            Liked by 1 person

            1. Richard

              Really promising looking at the plots above

              Don’t know if you have seen it, but you can click on the link below each plot and get into the “analyzing mode for each test”.


  2. I hadn’t clicked the links but have now.
    Interesting that the FR935 is so close, yet you can see a kind of ‘smoothing’ of the HR, it is definitely less up and down every second than a HR strap normally does.

    All 3 indoor rows are fairly steady, have you given it a test with some higher intensity intervals? I can see some in the SkiErg where it seems to have done very well.

    It’s the response to fast HR changes that supposedly even the best optical HR monitors struggle with. Would be cool to see it tested in a 1min on 1min off sort of row and how it then performs.

    Liked by 1 person

    1. Richard, my clavicula (collar bone) is still not fully healed, so I have to be careful not to overdo on the RowErg – that’s why I haven’t done 30″/30″ or 1’/1′ on it. On SkiErg my absolute power is lower – which is normal/usual – and surprisingly the movement had less impact on the broken part. For the RowErg it took much longer sit on again. But… I’ll see how/if can do at least some sort of testing where HR goes up an down. HTH


  3. Your accident was a very nasty, no wonder you are still healing! Don’t push it for the sake of testing, just wait until you are fully recovered.

    It does interest me if a wrist optical HR could work well for rowers. Garmin have just released the Vivoactive 3 which has the same optical HR module at the FR953&Fenix5 so it will be interesting how they all compare.

    Thanks for the follow of the @StravaRow blog. Slowly getting the rowing community using Strava in the hope that more users will increase the number of rowing specific functions. I have seen some signs that Strava’s focus on the social network side of things is changing to get other sports supported, but no big changes yet.

    Liked by 1 person

  4. Just to note im picking up a Vivoactive 3 which uses the same optical sensor as the 935/Fenix 5 etc so will be interesting to see if I get the same results of accurate HR when rowing/erging – plan to use it alongside a Wahoo Tickr X so I can compare the HR charts

    Liked by 1 person

    1. Thanks for this note. I’m eager to see you results.

      Till software-update v6 (the one with all day stress score activated) the oHR part of the FR935 worked pretty well for Row- and SkiErg. For walking I never got it to work properly. But… since then (v6.x update) I have very mixed (read: bad) results. I meanwhile don‘t use oHR any more as it‘s not reliable. If (and I normally don’t track HR any more) sometimes want to see HR, I use a chest strap (Wahoo TICKR; w/ X).

      Last but not least: My GF‘s FR935 has since v6.x also difficulties – but she does only run (& bike).

      Liked by 1 person

      1. Just tested and sadly a big fail for the wrist measurement, erging it registered a good 30bpm average lower than my actual heart rate so agree it’s not reliable sadly

        Liked by 1 person

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