Low Xss value for open water swim

42+ minutes swimming 0.9 miles in sub 60ºF Ocean water yields an XSS of 3. Something’s wrong with that. Event recorded on Apple watch using swim.com app, shared with Xert via Strava.

The previous swim, same location, but recorded using the Apple Watch workout app, produced and XSS of 34. Still a bit stingy imo, but within reason. So, perhaps problem is with swim.com, rather than Xert.

Can’t really help you with that but for most people heart rate is different for different sports. Running is usually higher than riding and swimming is usually lower with perceived same effort. So that could at least account for a ‘stingy’ XSS estimate.
Don’t know if Xert disregards the cadence values for none riding activities, those would also screw things up since they have no real correlation between sports.

I suppose this comes down to a large discrepancy between HR and power derived XSS values.
A recent indoor ride duration = 42.36 , avg. power 129, avg. HR = 108, XSS = 49
Open water swim duration = 42:34, avg. HR = 108, XSS = 3.

Xert shows ‘equivalent power’ for the swim at 12 watts. That’s probably where it went wrong. A previous swim, same open water route:
duration 46:20, avg. HR 114, XSS = 34, ‘equivalent power’ = 112 watts. That seems more reasonable.

Just a thought, but optical hr underwater with swimming wrist movement on my Apple Watch SE has largely been poor. Is it giving you a pretty steady read or are there significant dropouts when you compare the two activities?

can’t say for sure, but 108 would be pretty consistent with expectations for the perceived effort. I haven’t seen any obvious outliers in recorded HR. That said, whatever calculation is in play it’s working with the reported HR, regardless of how inaccurate that may actually be. xss from reported HR of 108bpm should not be an order of magnitude smaller than that from avg. 129 watts generated with the same bpm.