As yesterday’s least best-kept secret was re-released on a suspecting world (Garmin 630) I was plodding around Loughborough in circles. Both mentally and physically.
I was prompted to think again of optical Heart Rate Monitor (oHRM) accuracy.
Yesterday also saw the release of the Garmin 235 (or is it 225 or 325, I get the numbers confused) with oHRM and we’d seen the innovative TomTom Runner 2 with oHRM last week. The TomTom was pretty accurate on the oHRM front.
And then I remembered as I was writing THAT TomTom post I had made a mistake (corrected before publication) with the data in using the wrong HR fields for comparison. On one test I’d actually transposed one result onto the same result, thinking it was a chest strap vs the TomTom. Funnily enough I got a near perfect match as I was comparing a chest strap with itself.
Luckily I spotted it as I say. (Wasn’t too hard!)
So what got me mentally thinking in loops yesterday was that maybe I’d made that same mistake with my accuracy review of the Scosche Rhythm a few months ago. Here’s the graph again Garmin HRM-RUN vs Scosche RHYTHM and you can see why it’s been nagging in the back on my mind. It’s like comparing two chest straps against each other – or one chest strap against itself:
So I got my new Garmin HRM-TRI (ANT+ only) and established a heart rate with the Garmin watch. Fine. I put on the Polar M400 (Bluetooth only) and waited for a signal as I fired up the Scosche (ANT+ and BTLE/BLE/Bluetooth SMART v4. Bang there we go Bluetooth Heart Rate. Simultaneously pressed go on both devices and off I went plodding around a training Half Marathon.
This is what I got.
Panic over.
Why is it so bloomin accurate?
I remember delaying having a look at one after seeing the dcrainmaker review; I just couldn’t believe it. But nope. It’s true. dcrainmaker was right in his pronouncement of the most accurate oHRM there appears to be.
TomTom’s is good. But not as good (below)
Last Updated on 14 January 2026 by the5krunner
My favourite kit and nutrition
- Injinji – Runners protect your toes. Avoid discomfort and minor injury. Run more. Run faster. I use them.
- Garmin 90-degree charging adapter — The small adapter that keeps your charging cables tidy. Essential for race day. I use one.
- Garmin charging puck — the fastest and most reliable way to top up your Garmin before a session. I use one.
- Ravemen FR300 — Front light that mounts directly under your Garmin or Wahoo head unit. Keeps your bars clean and your beam pointed where it matters. I use one.
- Body Glide – The blue anti-chafe stick that all swimmers and many runners use. I use it.
- Maurten — The race nutrition trusted by elite athletes. Gels and drink mixes engineered to be easy on the stomach. I use them.
- Garmin Varia RTL515 — A radar rear light that alerts you to vehicles approaching from behind. Pairs with your Edge or Garmin watch. I use this model.
- Favero Assioma Pro RS2 — The power-meter pedals most serious cyclists choose. Accurate, easy to move between bikes. I use this model.
- Garmin Forerunner 970 — A serious choice for a pro-grade triathlon watch. I use this.
- Polar H10 — My daily driver for accurate, waking HRV readings.
- Wahoo ELEMNT Roam 3 — The bike computer that has the feature Garmin lacks: usability. I use mine on most rides.
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tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. ID
