What fitting should I use on the 1.5" TC dump valve to harvest yeast?

 

A 1/2" barb fitting is a way to narrow for a yeast dump, 3/4" should be the minimum consideration.  Ideally, you can collect yeast directly from the valve's 1.5" TC, but if users want to utilize a hose to divert the flow of yeast they're going to need a large hose barb fitting.

 

This would be the minimum (3/4"):

http://www.brewershardware.com/1.5-Tri-Clover-X-3-4-Hose-Barb.html

 

This would be even better (1"):

http://www.brewershardware.com/1.5-Tri-Clover-X-1-Hose-Barb.html

 

I think the 1" is way more "Brewmaster" than the 3/4", you can find 1" silicone tubing to divert the flow on our website here ( 1" Silicone Tubing  )