I wouldn't try to measure the fermenter's temperature. Since it holds so much heat, from 72 degrees, you'd have the ambient down to 48 long before the fermenter reached 68. Then as the fermenter stayed cool, the ambient would climb way back up over 68 until the fermenter caught up, then you'd have to drive the ambient way back down. With those wild swings, you'd be horridly energy inefficient. You'd be far better off monitoring the ambient air and using a fermometer. If the ambient is 68 and your fermenter is 72 and you want to be at 68, just drop the ambient to ~64.