Not sure about my f5x

While you may indeed have an actual fault; it could simply be down to wrist based optical heart rate monitors being pretty rubbish when it comes to doing actual exercise.

By nature, an OHR is detecting your heart beats by looking at the slight reddening in your skin each time your heart beats. But any activity that causes similar changes in the skin (i.e. exercise) will throw it off.

OHR's are good for day to day monitoring of your HR, keeping track of resting HR for example (which is a good bit of data to look at to determine if your fitness is improving) but if you want accurate HR data when working out then there really is no substitute for a HR chest strap.

I was thinking I might have to get a strap. I have had a look at the scosche arm heart monitors.

Any suggestions?

Personally I use chest straps (Garmin branded ones) and while you can still have issues if you have very dry skin or if the strap is loaded with salt from sweating in previous activities, they are generally reliable. Making sure you damped the contacts and wash the strap regularly prevents most issues.

But any non-opitcal based HR monitor will likely be more reliable than an OHR based one.

Personally I use chest straps (Garmin branded ones) and while you can still have issues if you have very dry skin or if the strap is loaded with salt from sweating in previous activities, they are generally reliable. Making sure you damped the contacts and wash the strap regularly prevents most issues.

But any non-opitcal based HR monitor will likely be more reliable than an OHR based one.

Absolutely concur...

I would also use a HR chest strap when working out

+1 on Scosche Rythm+
I wear it on my upper left arm (same arm as watch) over bicep with HRM on tricep side. It fits comfortably there and has never had a weird reading or dropout, unlike EVERY chest strap I have tried, which has been many over the years.

I also use the Scosche Rythym +. Wear it on right arm with watch on left, sensor is on pinky side of the inside forearm. Zero issues and absolute accuracy checked against heart strap and gym equipment sensors.

Personally I use chest straps (Garmin branded ones) and while you can still have issues if you have very dry skin or if the strap is loaded with salt from sweating in previous activities

If you follow the care instructions there should be no build up of "salt from sweating in previous activities...".

Rinse them after every use - I rinse mine in the shower - wash after every 7 uses in a machine (instructions on the label and in the manual). Before use, thoroughly soak in (warm) water before putting it on takes care of any poor contact issues due to dry skin before sweating. If you have one with a detachable sensor module, then remove that after every use to allow the contacts to dry - also stops rust developing.