I'm lucky enough to have in my possession:
Air Chrony (Polish UKAHFT and BFTA?)
Caldwell (we use for Sarpa HFT shoots as the official chrono as it's shooter-friendly)
I have tried the last three all together a couple of times, they all agree closely within 5-10 fps; the Skan is a bit higher, though the manufacturer thinks mine must be damaged
I have compared my Skan with the two Skans run by our local favourite airgun shop, and the results bear out that mine is 5 fps high.
I have found that all of them need fresh (or at least not on the way out) batteries, to the extent that I take the batteries out after use, as even if switched off, that batteries seem to drain. The Skan gets disconnected from it's big lead acid battery anyway.