Heart rate reading issue
This morning I found an strange behaviour of my Suunto heart rate reading issue.
I took off my watch and leave it on a table like picture shown. As the picture shown the heart rate monitoring green LED is still on and simply sitting watch on a table without any contact to any skins/wrists the heart rate reading is more than 200!!
I have 2 videos that are too big for uploading showing the same behaviour.
Why the watch can still catch heart rate reading while the watch is not close to any skins/wrists? It might be within first few seconds when you take off the watch. However, after 10mins without any heart rate input? Beyond my understanding.
I then waited until both display and LED were off and woke up the watch on the table. The heart rata reading went straight to 208 and stay around 200~220.
It makes sense to understand how wrist HR is measured:
The watch is sending out high frequency light pulses (the green LEDs you can see) into the skin. Small capillaries that have blood flow fluctuating with your heart beat will absorb or reflect some of this light back.
The light reflection/absorbtion rate is depending on the change of blood flow in your outer skin.
This is then measured with light sensitive sensors. Now there is a lot of data processing going on, to get rid of offsets and different skin color and so on. Finally if everything goes well the watch can lock on your heart rate.
This process can be disturbed by a lof of things. Among them are:
- less blood flow in your outer skin from low temperature
- watch shaking around at your arm and not maintaining the same contact or distance to the skin
- strong external light sources
- tatoos, a lot of hair, skin color, etc.
Now if you place the watch anywhere bot not on your skin, the sensor is detecting light from any light source - plus reflected green LED light possibly as well. The algorithm is trying to get a periodical signal out of this. Depend on where you put the watch it will get a reading from a flickering computer screen, dimmed LED lights, etc.
@egika Thanks for the comments and I totally understand it after working in the same industry.
Maybe it would be better for Suunto to stop LED as soon as the watch is taken off from the wrist, like Fitbit and Garmin. Then you won’t have this funny behavior that having HR on a banana and my watch bands.
@jom1104 I think the watch needs to have some sort of sensor for it to ‘know’, when it’s taken off the wrist. Suunto 9 peak has it, the watches before don’t.
@dmytro A gyro sensor would do. But I think algorithm alone can also make it happen by sensing if the watch stay still for say 10~20sec then it assume the watch is taking off the wrist as if the watch is on your wrist, you will more or less likely to have some sort of movements, then turn off the HR LEDs.
@jom1104 I’m not sure. For example, I sleep very still, don’t turn, etc. I wouldn’t want the watch to stop measuring my HR in the night. Another example, suppose I’m writing with my right hand and the watch is on the left one, it might happen again that no HR is being measured.
But yes, if you let your watch lie down for a few minutes, it goes into the sleep mode and won’t measure HR. The screen is turned off as well.