hydrometer dilemma

Try soaking both hydrometers for a day or two in kitchen vineger regularly.
Then wash it with warm water. This will dissolve deposits and make them more
reliable. I have also read on the back of the packaging that the manufacturer
recommends soaking it for at least 24 hours in sea water before
first use to "season the needle". I have no idea what science is
behind this recomendation, but maybe it is important fact you missed
comparing your old, dried out hygrometer readings with the other one.

From my own experience I prefer swing arms over refractometers by far!
Contrary to some people here I found them much easier to use than
to stare into tiny viewfinder and find the reading on very, very tiny scale
with an edge between blue and white regions being not always very sharp.
It might be some problem with my blue refractometer made in China
or some problems with my vision but I just find it very difficult to use :-)
With added uncertainity of most of them being calibrated to NaCl not NSW
they are not worth the price for the average aquarist. I can use Deep Six
hydrometer with good or bad light and I find them very reliable when
I wash them frequently and not let them dry with salt water in their cavity.