Charge 3 heart rate not accurate when in exercise mode

[Joeyrukkus]

When my fitbit does not know I'm exercising, like when I ride my bike trainer indoors and do not select any exercise it works perfectly. If I tell it I'm doing an activity like elliptical or it auto recognizes that I'm on an outdoor bike ride it barely ever recognizes the correct heart rate. I could be really exerting myself on a hill climb but it will stay pegged at 70-80 bpm which is not correct.    

Is there any way to have it not automatically recognize an activity so the heart rate will be correct? Or some setting to adjust the sample rate because I know that is suppose to change when it recognizes an activity. 

[Joeyrukkus]

Any one have any ideas?

[Zardoz99]

Sometimes I find the way it is position on my wrist or the tightness on the wrist makes a difference. This can change when you are exercising and sweating. I check my Fitbit and compare it with my heart rate chest monitor when I'm on the elliptical. Chest monitor might say 110 bpm and the FItbit 128 bpm. 

When I'm resting they are both in the lows 50s. So they seem to be more accurate when resting.