Good morning,
Save your money and ignore heart rate monitoring as it is a fad, it comes from elite level sport where a lot of experienced people work with dedicate pro or semi pro athletes who have rigid training routines and life styles and doesn't apply to "normal" people.
If you think I am being disrespectful to the method, look at the typical charts used with heart rate monitors, they have 5 zones, very occasionally 6, yet you can probably tell from Perceived Exertion which of those 5 zones that you are in. When you get into the serious exercise area there are usually 3 zones, again really easy to tell in general and the border cases are just guesses anyway.
If you heart rate is 82% of your max, which zone are you in, especially as you almost certainly won't know you max heart rate down to the last couple of heart beats?
Even if you are sure, what does it really tell you, do you truly believe that there has been enough research done to be able to say that training at 82% of max heart rate is different from training at 78%?
Then what about those few pints that you had last night that have upped your heart rate by 5 beats per minute, pushing you from one zone into another, do you train lighter because of them?
A valid training method has been taken and simplified for mass consumption but the result of this simplification is that it is meaningless.
Remember the BMI (body mass index) it is now generally a derided measure? Yet it was a good tool for the job it was supposed to be used for, a guide for people who are obviously overweight.
Bye
Ian