@WillNitschke this is a lot more complicated. Heart rate may follow RPE and indeed, athletes can (sometimes) predict their HR based on their level of effort but it works mostly within a context of a single sport (and not always). The same level of effort may bring totally different HR when performing another discipline. To simplify, trained runners but untrained cyclists won't be as good at cycling as at running when it comes to aerobic capacity. How does it affect HR? Simple, at some point, beyond an aerobic capacity threshold, the heart stops accelerating because it makes no further difference (no more oxygen can be consumed, ran out of aerobic capacity).
Using myself as a "guinea pig", I find it extremely hard to accelerate my HR beyond 175bpm-176bpm when cycling while when running I can go up to 192bpm. The RPE for cycling, when my HR is around 175bpm approaches 10 within 3 minutes of such interval (it's about 125% of my FTP, close or within the neuromuscular power zone). With the same HR, I can run for an hour+ (this is my threshold for running). So despite I feel like I can't put any more effort when on a bike, my HR is not as high as I would think. It gets even trickier for swimming when HR stays a lot lower. While sprinting freestyle, HR rarely goes beyond 155bpm but it feels like tough, beyond threshold effort. That's why for an athlete who uses HR zones in training an accurate chest strap is pretty much necessary. Additionally, with training the threshold moves and there are more variables like environmental factors (temperature, humidity, air/water resistance etc.), conditioning, sleep/rest etc. In other words, for serious training in endurance sports, using a chest strap is rather mandatory because it's not about glancing every now and then at the HR but working to the heart rate, staying within specified training zone(s). In fact, it's rare to train only by HR but it's more a network of connected strings (power, RPE, HR, pace).
See below, two races, two disciplines (not duathlon, different days so I was fully rested before each event):
Both were race efforts with sprints and surges yet the entire run was performed with HR higher than I could ever keep 2sec+ during a bike race. Now, the 1hr30min power peak for the run was 91%FTP and 92%FTP for cycling (for the entire event, 11 beats less while more effort was put into it). Power is probably the best-known way to quantify effort; the bike race felt tougher. The difference is because my LT for cycling is lower (working on it, took a long break over winter time 🙂 ). It gets even more interesting when you consider structured training (like HIIT) when you need to hold power above a certain threshold. Going all out, it's even more visible what the difference is when it comes to heartbeat, effort and discipline. This is a reason why athletes use different training zones, thresholds and max HR per sport - because one can't fit all. That requires accuracy no wrist sensor on the watch can give you.
Why am I explaining that? One of the things that "grinds my gears" is when I read something like "I'm huffing and puffing so my HR must be out of the roof but it's not" and pointing out that it's an HR error but it doesn't have to be. There is a relation between effort and heart rate but it isn't an absolute relation. It's relative and affected by many factors. I realize that not everyone trains that way but these are common beliefs vs facts backed with evidence. In other words, how you feel is how you feel not your heart 🙂 I can't tell automatically what my HR is by RPE because it may vary quite a lot. I don't know any watch that I could use in the training like that (and my Sense 2 never ever gave me any good readings for cycling, probably cycling is affected worst in my case). When I shop for a new watch, pairing it with external sensors is a mandatory feature.
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