I just bought a new chest freezer and a Johnson Control A419BG-3C temp. control.My question is if temp.is set at 65 where should i set the differetial.
Are you measuring ambient or beer temp?
I’ve read where its recommended to set it for 2 or 3 degrees. Setting it at 1 could make the conpressor short cycle and thats hard on it.
Measuring ambient, you want the differential set wide enough that the compressor has a chance to rest between runs. The temp outside the freezer and the air-tightness of the lid (and collar if you have one) will have a large impact on how fast the temp changes inside the freezer. Start with something like a +6F differential, set the temp to -3F to the desired temp (so if you want the ambient to be 64F set it to 61F) and see how it runs.
But if you’re going to use the freezer for a fermentation chamber, you will get better control over the process if you measure beer temp instead of ambient. Easiest way to do this is to place the temp probe against the side of the fermenter and then tape some bubble wrap or other insulator over it. Then set the differential to +1 or +2 and the target temp to -1F to the desired beer temp.