Two fermenters, one brew fridge/temp controller

[Instigator]

I'm thinking about running two brews at once - one in the regular FV, and the other in the craft FV. Which FV gets the temperature probe attached to it?

My gut feeling is that the bigger one will have a greater exothermic reaction and is probably the better location for the probe. On the other hand, running time on the fridge to get the temperature on that much wort stable may chill the smaller brew's contents to a lower temp.

Given my target temp for the bigger batch is 18C, will it hurt the smaller batch to be a little lower?

Looking to do the Russian Imperial Stout in the big FV and either the Shiner - Black IPA or B-Side IPA in the craft FV.

[Gazzala]

I do exactly what you’re thinking about doing - I put the temp controller probe on the larger fermenter and have a digital thermometer that is not connected to the temp controller attached to the smaller one just so I can monitor it - they’re usually within .3C of each other. The smaller is usually higher. So If I’m aiming for both to be around 18C then I just set the temp controller at 18.3 with a .3 tolerance either way. 

[Lab Cat]

I'd put the probe on the small FV, this will detect temp changes quicker than the big one, so your fridge won't go too low when getting down to your set temp. Both ale yeasts, so there's a wider range of fermenting temp they will work at, so I wouldn't worry about the possible extra variance between FVs too much. You're still going to be a better zone than non controlled ambient  fermenting.

Whatever you choose, the worse thing that can happen is it gets a bit low to begin with, so fermentation takes a bit longer to get going.

[Greenyinthewestofsydney]

How is your fridge setup as well?

I do it regularly in my fermentation fridge. However the element is up the top and because i use some wood as a shelf to hold the top fermenter the bottom fermenter doesnt get as much of the cool air as a consequence. Therefore i have found that i have a 2c difference most of the time between the top and bottom brew.

If you have a similar setup i would put the 23l one on top and the smaller one down the bottom as the fridge will cool the smaller one quicker.

[PaddyBrew2]

I did a double ferment before.  The standard LHBS fermenter sitting on top of my coopers. I literally just switched the probe at least twice a day to ensure both were fermenting at 18-19.  Both turned out great.  

[Instigator]

Thanks for the ideas, gents.