I almost died today using garmin fenix 5s!

I think because you started the conversation very hard. Let’s start again: it‘s important to listen to your body. Use it as an helpful tool, but not as an medical device. I had several situation like mentioned above, where my heart rate was off. But when you feel your heart raising up, and your watch is telling you something else, rely on your feelings. When iam running in cold conditions without an belt, my heartrwte is always off, because the body saves himself for getting too cold and reduces the blood flow to the hands and feet’s. It needs time, and after a warm up time, I get good very good results.
Loose fitting, reduced blood flow, wrong position on your arm, skin color, tattoos and so on, can influence correct readings.
Hope that Info can help you. Und maybe you give it a try. I really like the OHR sensor. For hiking, normal endurance running and so on.but it does not work for me under all conditions.
EDIT: i do not say, I could not be a soft- or hardware error. But. I can say, haven’t recognized a failure on watch on OHR in V11.00

Not really cause when you have headphones playing music at a 170 - 180 tempo you wont realise what's going on

Ok, only because you were listening to music, you lost control of and sense about your body?
Reading the title, I thought you used it as a primary device where it isn't meant to be one. Reading your post, I had to smile an walk on. We are not defending an inanimate object, you have to inform yourself about your tools, your body and them impacting readout.

Holy smokes, people like you are what keep ambulance-chasing lawyers happy. Sue everybody if you cannot take the blame for your own mistakes!
Really man, what would you have done without a fitness watch at all? You cannot trust technology to hold your hand (at least not yet). Always pay attention to your own body, and don't trust technology to save you from yourself.

Observing how the OHR monitors work, they turn on the LEDs and sample apparent opacity of your flesh at roughly 20Hz-30Hz (20x per second) at any given time while resting, and seems to speed up during activities as your heart rate increases. I think the problems occur in workouts when your heart rate jumps faster than the software algorithm allows the sample rate to change to track the changes in heart rate, or misses the change because of too much movement. If your heart rate jumps quickly, you can get into a mode called aliasing, where the sensor "sees" your heart rate at a lower value than what it really is, or just cannot find the correct value as there is not enough information to go on.

There are some techniques in signal processing to get around these issues, but they are a little more energy hungry. You cannot have it all... accurate readings and lousy battery life or... sometimes erroneous readings and excellent battery life.
Personally I think that during activities, especially cardio, the sample rate should be cranked up really high during the entire activity, and not vary with detected rates. Allow the sensor to verify heart rates as high as possible. I am willing to bet that the battery usage compared to the GPS and other devices would be roughly the same.

same problems. Loot at thread

https://forums.garmin.com/forum/on-the-trail/wrist-worn/fenix-5-5s/1394872-low-hr-activity

Garminomr if you can't tell the difference between 80 and 190 BPM without looking at your Garmin then you need to focus more on listening to your body :). But more seriously wrist based OHRs are just not going to be as accurate as a chest strap - for some people for some activities they will get close - but as many other writers and threads have suggested both on garmin forums and also all the other manufacturers there are inherent shortcomings with the technology and trying to measure HR remote from the heart and using the secondary impact on bloodflow which can be impacted by all sorts of things such as ambient tempereature. The price of he device is a bit irrelevant as you are paying for a whole bundle of features not just OHR - you just have to accept OHR for what it is and if you demand accuracy use a chest strap.

Wrist HR is'nt reliable at all during sport activity, you need HR Strap to have reliable values, especially when you sweats a lot like indoor activities.
Wrist HR is usefull for HR statistics expect during activity.

Here is a screenshot from 31/10/2017 last year, around same ambient temperature, same arm same hair on arm density, same skin color. But do you know whats diffrent its the device last year I was wearing a vivosmart HR which I never had an issue with.

I think when you buy a cheaper device from garmin that have less features most of your money goes to little features you have. But when you buy a more expensive device the money get distributed on the gazillion features that you end up with a worse device than the cheaper one

Free advice to garmin :
When you develop a new device just focus on the HR thats where your bread and butter espcially when all of the important reading that the user wants are based on HR like calories and hr zones.
I dont care about watch faces, elevation barometer altimeter or even 1 hour till sunset.feature .

Remember less for more.
????

ciq.forums.garmin.com/.../1419431.jpg

I look at it this way, chest strap and OHR are complimentary, they do not substitute each other. OHR is useful for the 24hr monitoring features where accuracy does not need to be so precise as the sampling is greater and relues more on overall values, and for this it works great. And for certain exercise activities a chest strap is needed, like doing more intense cardio workouts were you need to know on which zones you are at every moment and with almost 100% reliability.

Quite right !

If Wrist HR where reliable during activity, you could have Wrist HR during swimming !
It's the proof than where there is a lot of sweat/water between wrist and watch, HR isn't reliable ...
As far i know, there is no watch with wrist HR that works under water ...