Heart Rate Monitor Highly Inaccurate VS Fitbit Charge 4

Yep, an easy fix. Press the Start button.

The Garmin is significantly more accurate in activity mode than 24/7 mode. Peek under your wrist under the sensor, and you will see the sensor lights go from strobing to solid as you start the activity.

Also, for best accuracy, make sure the watch band is snug and positioned well away from the wrist bone. This ensures a good seal against external light entering the sensor. The 6X is a larger device, and may be more prone to movement than a smaller device like the Fitbit you are used to.

Yeah that was going to be tomorrows test.

Still very disappointing though in the sense where if I'm just walking out and about casually in the city or something my Fitbit would give me an accurate reading whereas the Garmin would not. 

Yeah it's definitely on there snug and away from the wrist bone as advised when looking around on the forums. 

This is weird. I often take a walk in my lunchtime at work and not track it. I still get the 100-110 HR from wrist OHR. Although the Fenix does track more often in activity mode, it still picks up my raised HR,

Notice how the Fenix is reading close to one-half of the true heart rate? This is expected if the Fenix doesn't follow changes in the heart rate, and latches on to a harmonic of the frequency (e.g., 1/2, 1/3, etc). The solution would be for Garmin to expend a little bit of battery power to occasionally check whether this has happened, particularly in the case when it appears that the heart rate has suddenly ramped up. A sudden increase in step count would be an easy trigger to check the heart rate.  Fitbit can do this, so Garmin should be able to.

I used FitBit products for 8 years. FitBit estimated my cardio fitness(~VO2 max) at between 43-47 which was WAY above “excellent” for men my age (now 67). This seems to ring true when I compare my ability to endure sustained, hard activity - even when compared to makes 30 years younger than myself. However, my 3 FitBits (over the years) always became difficult to charge within 1.5 years. This difficulty arose from simple mechanical issues which prevented good electrical connection between watch and charging cord leads. So, I switched to Garmin. My new Garmin Forerunner 245 is a well-made device with a very solid charging mechanism. BUT, the Garmin device only uses a formal run, bicycle ride or swim activity to calculate cardio fitness. It does not take into account my very low resting pulse (47bpm), how quickly my pulse returns to baseline after being elevated, and presumably other cardio metrics - including vigorous 5 mile walks (even with the GPS activated). In fact, this Garmin estimates my VO2 max is merely 32, which translates to my being in the upper 50% of my age group. 
so I guess our choices are between a well-made fitness watch fitness watch which only uses formally acquired data, or a sophisticated fitness wstch which continually uses all cardio data, but which will “fail” in a calculated way after a couple of years!