I have the first apple watch. (I bought the newest but haven't used it yet still boxed). What I found was that the heart rate correlated well with taking it ma

I have the first apple watch. (I bought the newest but haven't used it yet still boxed). What I found was that the heart rate correlated well with taking it manually and/or a finger heart monitor (pulse ox cheap from Amazon). But with both devices you could actually tell when it was clearly wrong independent of comparison. The point is you more or less (at least I found) could figure out when it is right and wrong just based on past use and what you would expect (feeling your pulse). So if it was off it was grossly off and obvious. That is my experience with the watch that I have had for years. Either 'correct' or 'vastly off and easy to tell that is the case'.