Top Cropping Yeast from a Carboy

Sometimes I'm a cheapskate.  I know it's not a huge expense to get a new vial of yeast to brew, but I see no reason not to reuse yeast when possible.  When harvesting yeast from a batch of beer, there's many ways to do it.  You can pitch a new batch of wort on top of the yeast cake from a previous batch.  I've done this a few times and it works real well and you get a good fermentation, but I'm always a bit skeptical of pitching on top of the old trub.  I'm just afraid that all the old junk will give some off flavors. Plus I have read that the best yeast to use if you are going to harvest, is the top yeast in the krauesen.  If you are using a bucket, that yeast is easy to spoon off., but fermenting in a carboy makes it difficult to get at the yeast.

So I rigged up a system to top crop the yeast in a carboy using a racking cane, some tubing, a jar, and an airlock.  It works like a blow-off tube, but doesn't blow off into a sanitizer solution to be discarded.  Instead, you blow off fresh yeast into a sanitized container that is fitted with an airlock so you can keep the yeast.  The idea is to insert the racking cane into the carboy so the bottom is just above the top of the fermenting beer, where the foamy krausen will be.  The cane is connected to tubing that runs down into a sanitized jar fitted with an airlock. So once fermentation starts, the pressure will  push the foam out through the racking cane and down into that collection jar.  I also like to collect a little beer into the jar as well. To do this just push the racking cane down into the carboy further.

 I've also seen people use this method with a 1000ml Erlenmeyer flask fitted with a two port carboy cap.  But like I said above, I'm thrifty.  So for this batch at least, I used what I had on hand.  Here is a photo of fresh new top cropped yeast as it is collected in the sanitized jar:

 After collecting enough yeast,  the racking cane is removed from the carboy and replaced with an airlock.  Then if I'm using the yeast within a few days, I just put some plastic wrap over the top of the jar with a rubber band and put it in the refrigerator. Or for slightly longer storage, I bottle it in a sanitized bottle.

The most I've ever reused yeast is three batches, and haven't had any problems yet.  If I had more sanitary conditions, better temperature control, and better equipment, it would be possible to use it for more batches.  I think for now three or four generations is about all I want to risk before mutations and infections occur.

UPDATE:
I upgraded my collection equipment so now it's not quite as hillbilly.  I bought an Erlenmeyer flask and some better carboy caps (5Gallon cap fits on the Erlynmeyer and 6Gallon cap for the Better Bottle).  I also bought a PET racking cane and cut it a bit shorter so the overall height of the setup isn't as high.  Mainly a space and aesthetic issue.  But overall, I think this system is more solid and with the better fittings, should be no leaks in the system.

So after one night my Pumpkin Porter fermentation filled the flask.  I got lucky and was able to switch out the flask with a blow-ff bucket before the flask totally overflowed as well.  Here is a picture of a 1000ml flask filled with fresh bubbly White Labs Burton Ale yeast topcropped from my Pumpkin Porter!