I've been running with my 645 for nearly one year. I actually had it replaced in October due to HR and GPS inaccuracies. Unfortunately, it seems the new replacement has the same problems. Namely the following:
1. GPS: Of course I wait every time until the GPS is locked and the ring turns green before I start a workout. I understand that GPS accuracy is limited in cities with high-rise buildings, narrow valleys or dense forrest. Yet when I run in completely open space, e. g. on a trail in a flat field, sometimes GPS goes crazy and some data points up to 1 km away from my actual position appear. Sometimes the GPS simply does not update and I see my current pace going slower and slower while I run at steady space, eventually, the lap pace is still correct because all of a sudden it jumps. So is this a general Garmin problem, a software problem or am I just unlucky and even my replacement 645 is simply a bad one.
2. Optical HR sensor: I understand that optical HR is not very accurate, in particular for high-intensity workouts. Thus, for temp and interval session I use an additional chest-strap for accurate HR data. For easy runs I normally rely just on the internal optical HR sensor. Many times though it shows complete non-sense numbers, way too low or way too high. On a recent run when I saw HR on the watch going crazy, I actually took it off and held the sensor against the sun. I kept running at steady pace and the watch would still display the same, very-high HR after 5 minutes running with the watch off my wrist? How is this possible? Did smart Garmin programmers just modify the algorithm a bit and in case of bad quality sensor data just correlate it with the cadence? I also noted that my HR went up when I was running down, so higher cadence but I could simply feel that my actual HR was going down since I was more relaxed and breathing frequency went down as well.
So I'm really wondering if I just received another poorly manufactured 645 or if this is a general problem? Having it replaced is annoying, too. It takes at least 3 weeks for the replacement to be sent and all the fitness data is lost. So it takes another month until you have more or less accurate training status on the watch. I must say that I really like the design, handling and features of the watch. If a GPS running watch is unable to deliver accurate HR and/or GPS data, it's absolute useless though!
