If your string is breaking off every few days, that should be because you have the tension too tight. You want the string tension to be able to give at least half an inch to 3/4 inch, maybe even an inch, when pushed with a finger. That should solve the problem. A standard noose knot will not give, nor will a reef knot, but the noose is much easier to make.
String alternative for Eurolab TT
I have bought a Scheu/Eurolab Premier TT, the only problem of which is that the nearly invisible tiny string Scheu provides tears up every few days. Have other Scheu/Eurolab-owners found alternatives/strings? And what kind of knot do you use to tie the knot - mine open quite often, so far.