Intensity Minutes does not register consistently

4652735 said:

Why did the second session not register at least as many intensity minutes as the first?

Have a look at the following article to understand how it works:

How Are Intensity Minutes Earned? | Garmin Customer Support

It is your watch that calculates the Intensity Minutes. It is comparing the HR to the current 7 days avg. Resting HR. You have to look at the all-day HR graphs and the Resting HR values for both days, to see where the difference is coming from.

I did look into that and my resting heart-rate is very consistently in the low 50s every day. I also remained in the higher heart-rate zones for longer times in the second session, so the intensity minutes should have registered more, not less.

Can you post screenshots of the all-day HR graphs as well as IM graphs from both concerned days?

I wonder why there is a gap in the heart rate readings right after it gets high each time? I watch my heart rate, cadence and watts on the garmin head unit closely all the time, so as to shape my session the way I want, and it didn't drop out at any time.

Below, you can see the reason why you got much less IM in the second case, the HR is much lower. You will see it better if you select the relevant section with the mouse, and view the zoomed graph:

1st ride:   » vs 2nd ride:

4652735 said:

I wonder why there is a gap in the heart rate readings right after it gets high each time

It looks like it dropped after the ride. Again, you will see it better if you zoom the grap. Did you take the watch off your wrist after the ride? Taking a shower? BTW, as I wrote, this is the HR from your watch, not from the chest belt / the head unit.

I also recommend clicking the links "View Details" on the IM screen, in order to see the graphs and see when exactly the IMs incremented and how much.

Yes I think you are right. I had a look at my data on Strava that was sent from the garmin head unit and the heart rates are about 20 bpm higher. So the watch is useless for measuring heart rate and therefore IM. And you are also right about the shower post cycle.

4652735 said:

So the watch is useless for measuring heart rate and therefore IM

Not useless, if you know how to use it. There are several ways how to improve the IM recording:

• If you have a recent Edge model (such as Edge 1040, Edge 840, or Edge Explore 2) supporting the features of Intensity Minutes, you can simply remove the watch from your wrist, or disable the internal HR sensor on it, while riding. If HR from the Primary Wearable Device is not available, the IM will be calculated by the head unit. Please note, that only the last generation of Edge computers support this feature. Edge 1030, 930 or older cannot calculate IMs.
• Start HR Broadcasting on the watch while you ride. In the default all-day mode the HR sensor works in low-power mode, sampling the HR with the sampling rate of 15s (recording averages over 2 minutes). If HR Broadcasting is active, the HR sensor works with the full power, and sampling the HR continuously, hence it is more accurate, and IMs profit from it too
• You can get even more accurate results if you pair the chest belt also with the watch (it can be paired with multiple devices simultaneously), and start an activity on the watch simultaneously with the head unit too. Discard the activity on the watch at the end instead of saving, to avoid the duplicate activity. Please note that the activity must be in progress, otherwise the HR data rom the strap are not being used, even if the strap is paired. 

So depending on the devices you own, choose the best method suitable for it. If you have a recent head unit, use the first method. If you have an older head unit not supporting the Unified Training Status (UTS) and a newer watch supporting it, use the second method (the third method could lead to double recording of the Training Load, in such case). And if both, the head unit and the watch use the same old Physio TrueUp (w/o UTS), then the 3rd method should work well, without doubling the TL.