I have everything in the sump. That's a large difference between the sump and dt. Is that using the same thermometer?
I see that much swing between glass and electric thermometers. My sump and tank are within one degree (125 gal
The rkl has a temp probe and can control the chiller, so that should eliminate that problem. I would calibrate on/off setting to the dt though and adjust the set point accordingly.
I do that with my heaters, but calibrate against a lab grade mercury thermometer instead of the electronic controller. I tested 2 of each at the pharmaceutical I work at and the mercury ones were spot on while the electrics were 2 degrees higher.