HR recording - external & watch at the same time

fyi you can also graph these values if you load your FIT files in https://www.fitfileviewer.com.

tl;dr When source switching is enabled, both external hr and wrist hr are recorded at the same time, and this seems to happen even with non-garmin hr straps, but I don't see any conclusive evidence (either way) as to whether source switching is actually used for unsupported straps or not. (It could just be recording the additional data but never actually applying source switching.)

MZ said:

If I change "Source Switching" to off, will it stop recording HR from watch when I'm running with strap?

Yeah, I just played with this real quick with an HRM-PRO (supported for switching).

- "heart rate": always recorded (as long as there's a heart rate source)

- "external heart rate": recorded whenever an external hrm is connected (regardless of source switching)

- "wrist heart rate": if source switching is on, always recorded (as long as it's enabled and the watch is on your wrist). if source switching is off, only recorded when an external hrm is not connected

Note that if the external HR is literally disconnected (as opposed to just sending bad data), then wrist HR will take over regardless of whether source switching is enabled. (e.g. Say you strap wasn't connected until a few seconds after your activity started; or say your strap battery died in the middle of your run.)

So afaict, the true indicator that source switching was enabled is that the graph shows "external heart rate" and "wrist heart rate" at the same time. If source switching is off, you should never see that.

MZ said:

What's the reason? I have "Wrist heart rate" set to "Auto" and "Source Switching" to "on", but HRM Dual is not one of the supported straps.

Like the source switching support article says:

Heart rate dynamic source switching aims to provide the most accurate heart rate possible by dynamically selecting the best source of heart rate data when when wearing a compatible Garmin heart rate monitor strap with a compatible Garmin watch.

...

Dynamic source switching is available for all activity types to provide seamless heart rate data if the heart rate strap becomes unavailable. The biggest benefit of the feature is for running activities. A heart rate strap is usually the most accurate source of heart rate data for running but occasionally there can be inaccurate heart rate at the beginning of runs, often referred to as start-up noise.  This is caused by poor connection between the skin and the electrodes and typically resolves once you start to sweat. With dynamic source switching, when a heart rate strap detects low signal quality, heart rate from the optical heart rate sensor on the watch is displayed and logged instead. The heart rate strap remains connected to the watch and the heart rate source will automatically switch back to the strap once the signal quality improves.

I think it was a mistake for them to say the hr strap is "unavailable" here, as it most ppl would interpret that as "disconnected", but in this context they just seem to mean it's sending bad data.

MZ said:

but HRM Dual is not one of the supported straps.

Could be that the wrist HR data is still logged even if it's not used? I also tried this with a Polar H10 (*) for a few seconds, and both wrist hr and external hr were recorded at the same time (although strangely enough, there was a couple of seconds in the middle where no heart rate at all was recorded,)

(* obviously not supported as switching is only supposed to work with certain garmin straps)

The thing is that it's really hard to tell if source switching really took effect, even if it's on.

I think you would have to see 2 things:

- wrist hr and external hr recorded at the same time (that's easy, and it seems to happen with any strap)

- heart rate equal to wrist hr, even when external hr is available (this would only happen if Garmin decides that the data from the strap was bad.)

So the only way to reliably test this is to somehow create a situation that Garmin would always interpet as "bad hr strap data". But how could you ever know for sure? You could only prove that source switching happened, not that it didn't happen when it was supposed to.

I'm using a Polar H9 that is likewise supposedly not supported for source switching and noticed it would keep on and record from the wrist HR simultaneously anyway.

There were a couple times where the chest HR had obviously way off readings and inspecting the activity in fitfileviewer later showed that the wrist HR had much more plausible readings. I expected/hoped it would have switched in those instances before I knew only some Garmin straps are supported. Considering that I've since just turned off the source switching.

Then it sounds like source switching doesn't really "work" with unsupported straps, except for the part about always recording the wrist HR (which may be useful for some ppl anyway). Thanks for sharing that!

(Maybe supported Garmin straps send additional metadata about the reliability of the HR data idk)

Alright, thanks for all answers, you are right. I did another run with "Source switching" set to "off" and there was just one "heart rate" column in FIT file. Also, checked watch several times during the run and diodes were off. 

In the previous activity with "source switching" set to "on", the column "heart rate" was exactly the same as "external heart rate" in every datapoint. That proves the watch takes HR from strap, but still writes watch HR, but does nothing with it (most probably), so it does not make sense to keep "source switching" enabled without supported strap. At least now I'll save some battery