can I ferment a 3 gal. batch in a 6.5 fermentor
Yes. It will be mostly empty but it will be fine.
Of course you can.
It is much tidier than fermenting a 6.5 gallon batch in a 3 gallon fermenter. 
Seriously, it is advantageous to use a fermenter considerably larger than batch size; you shouldn’t need a blow off. I ferment 6 gallon batches in a 10 gallon fermenter.
OTOH I once had a leak in a bucket which meant I ended up fermenting one of a 4-way split batch as 1 gallon in a 5 gallon bucket - it was horribly oxidised compared to the other splits fermented in a 1g bucket.
Type of fermenter may be significant. My oversized fermenter is a closed, stainless pressure vessel (10 gallon corny type tank) so once fermentation expels the initial oxygen, none is present as there is no gas exchange with the atmosphere. If there is a possibility of exchange, more gas in the headspace could, I suppose, be detrimental.
So, Robert do you ferment without a blowoff and rely on a spunding valve set at a particular pressure? If so, what pressure level do you ferment at?
No, this simple airlock arrangement serves during active fermentation. For cold crashing I remove the “airlock” and apply a few psig before cooling. I do have a spunding valve, but have never used it. This seemed a much easier solution. Maybe someday I’ll try a pressure fermentation.
EDIT If anyone can advise on what would be an appropriate pressure to set the spunding valve to – strictly for fermentation purposes, not carbonation – I would appreciate it, and look forward to trying this setup.
5-6 PSI seems to be the consensus. exBEERiment | Impact Pressurized Fermentation Has On An American Pale Ale | Brülosophy
…or 8 exBEERiment | Impact Pressurized Fermentation Has On Saison | Brülosophy
…or 12. exBEERiment | Impact Of Pressurized Fermentation On A Warm Fermented Munich Helles | Brülosophy
LOL. [emoji482]
White Labs recommends “under 1.0 bar (14.7 PSI) until final gravity is obtained, generally in one week.” for its WLP925 yeast. https://www.whitelabs.com/yeast-bank/wlp925-high-pressure-lager-yeast
Thanks. I’ll have to play around with my spunding valve to dial it in (from MoreBeer, no calibration marks, but I gather the only thing that matters is the gauge.) I got it for this purpose; as it only goes to 15 psig, it won’t do for carbonating at fermentation temperature. But 5ish sounds perfect as it will not significantly carbonate, but solves my other problem of getting a CO2 supply to the ferm chamber to draw samples. This weekend I expect to brew, and I’ll try a ~5 psig fermentation. I’ll report in a separate thread.
You have to play around with the PRV…i had to keep an eye on it but levelled out at 12 PSI…Stayed steady for 3-4 days during fermentation then the pressure dropped to 6 PSI and stopped and has remained steady for 2 days. Am I correct in assuming the beer is ready for transfer to a keg.
I’m reporting on my experience with pressure fermentation here
I don’t think pressure readings are the guide to progress; like any fermentation you need to track the density.