Ferret v5.22 doesn't recognise units 'kelvin'?

[Glenn Carver <Glenn.Carver@xxxxxxxxxxxxxxxx>]

I've just upgraded our systems version of Ferret from v5 to v5.22 and
having tried it now find it complains it does not recognise my vertical
axis with units of 'kelvin' (potential temperature).

I guess it's possible I made some changes to the old installation that I've
omitted but I can't figure out where?  Does ferret read the system installed
udunits file?

Any pointers for what's going wrong? This used to work with the older version.

Thanks,
        Glenn