Does watch HR sensor also record if heart rate strap is connected? (HRM-Dual & Fenix 8)

trux said:

Yep. You can compare the internal and external HR for example with FIT File Viewer

For additional context, if source switching is enabled and you wear a compatible external HR, you will see 3 HR-related metrics graphed simultaneously in fitfileviewer.com:

- "heart rate": this is the actual "activity heart rate" displayed by the watch and Connect and used for HR-related metrics (it can come from either the external HR or OHR, but usually it will be the external HR, if you are wearing one)

- "external heart rate": self-explanatory

- "wrist heart rate": self-explanatory

if source switching is disabled, you should never see external heart rate and wrist heart rate at the same time (you'll always see "heart rate" and one of the other two metrics). Older watches which don't support source switching will only have the "heart rate" field.

Source switching sounds great on paper (it's supposed to use the optical hr as a backup for when external HR data is bad), but in practice users (including myself) have noticed that it sometimes inappropriately switches to the optical HR near the beginning of an activity, when in fact external HR is more reliable at that point in time (as optical HR takes a couple of minutes to catch up with your real HR if you're not warmed up or it's cold outside.)

For this reason, I've disabled source switching. (I don't always use a chest strap, but when I do, I don't want the watch to switch to optical HR willy nilly.)

Also, in order to use fitfileviewer.com, you have to export the activity FIT file:

- Open the Garmin Connect website (connect.garmin.com) in a browser (this works on phone, tablet or PC, although the UX for Connect on a phone is bad)

- Navigate to your activity

- Select the gear icon (in the upper right hand corner) and select Export File

This will download a ZIP file containing the original activity FIT file, which you can then open in www.fitfileviewer.com.

Thanks folks for the helpful input.  I did a test (walking around the house) and the results are odd.  In the first half of the 'activity' there were some peaks where the external HRM was 20 beats higher than the watch.  And then around the 26:30 mark, the external HR sensor dropped below the optical watch sensor, by quite a bit.  I don't recall the strap moving around at all.  And garmin connect recorded activity uses the external HR with this lower number than the watch itself and it's higher number.  I confirmed Source Switching is set to On.  Very odd: 

Hi net19894 I found this information about the HR Strap and Wrist

BTW, I highly recommend to enable the option "single y-axis" at the bottom, since each of your graphs uses different scale (100 bpm left vs 120 bpm right), so the comparison is completely skewed!

Well, the run this morning wasn't very encouraging.  On my F8 I have the 'Source Switching' enabled so the watch records the most reliable measurement from the optical wrist HR sensor or the strap.  But the issue I found this morning was that in general, the strap was worse than the wrist sensor HR.  It was very, very cold this morning but I did lick all 4 contact pads on the HRM before putting it on about 5 minutes before the run.  Then perhaps 1/3rd of the way into the run, I began to sweat.  In any event, can anyone think of why the strap would be worse?  Or maybe the optical wrist HR sensor is over estimating true heart rhythm? 

net1994 said:

In any event, can anyone think of why the strap would be worse?

Since you do not know what was the true HR, it is difficult to tell the strap was worse. In general, straps are more reliable than optical sensors, but of course they can have erroneous readings too. For example due to dry or dirty electrodes, weak battery, slipping or moving too much on the chest, corroded or dirty contacts (it looks like you have a strap with detachable electronics), etc. 

I put ECG gel on the strap electrodes to improve the conductivity (costs some 3€ in a pharmacy). In my case the curves from the strap and from the watch are usually practically identical, and barely distinguishable. I posted already several examples on this forum, and the following one is from a lazy MAF jog I did today:

trux, you read my mine

I tend to trust the HR straps more than the wrist optical sensor. Event if they increase the reliability of the optical sensor for each generation, there is too much thing that can affect them, like the weight of the watch, yes, more the watch is heavy more it can bounce around when make any intense activities, strength workout is even worse, I snug mine almost at the point it's uncomfortable when I make an activity. The color of the skin and tattoos can influence as well. If you are a bit hairy under the watches it can affect a bit:P.

Another thing, the way the optical read the heart rate there is some delay between the pulse of the heart and value recorded, it's marginal but it's there.

mrnets said:

Another thing, the way the optical read the heart rate there is some delay between the pulse of the heart and value recorded, it's marginal but it's there.

This does become a real issue if you're doing fairly short intervals though (assuming you care about your HR readings), which is why some ppl would recommend a chest strap for workouts, while optical HR may be deemed suitable for steady state runs.