[Elecraft] Weller WTCPT tip longevity

[jfedison at alum.rpi.edu]

I use a PTF7 solder tip on my WTCPT and recently found that after about
5 minutes of use, the tip suddenly starts to develop a thick char at the
end and makes the solder turn dark. I replaced the tip with a newer one
(<5 hours of use) and the same thing happens to that one. I will next
try a brand new tip to see if the same thing happens. Either the problem
is with the tip or it is with the heater/barrel nut assembly
contaminating the tip. Has anyone experienced the same thing? How long
should a typical solder tip last? Has anyone found a tip cleaning or
storage method that helps prolong tip life? 

Thanks. 
Jeff Fedison in Niskayuna, NY