When I go running I put on a different pair of tzitzis - an old pair that I
don't mind getting sweaty. Do I make a new berochoh of al mitzvas tzitzis
on them? If I then change back to the tzitzis I had been wearing that
morning, do I make a berochoh again?
Since you are
changing from one garment to another and back to the first with a
significant break in between, you should make a berochoh every time
you put on the tzitzis. In general it is advisable to have kavonoh
that the berochoh of tzitzis apply only until you remove your tzitzis.
(See O.C. 8:14 and Biur Halochoh).
