How to properly set SSD Fan Control on iMac late 2009 after SSD upgrade
The fan is running at 1499RPM at 38C, despite the settings of 1100RPM at 45C, because of a harmless bug in the SSD Fan Control app.

The bug is easy to reproduce:


Select Manual, and set the speed to 1500RPM
Select SMART, and the fan stays at 1499-1500 RPM


To "kick" the app back in to correct SMART operation:


Set the lower bound to a value below the ambient temperature (30C, for example), such that the fan speed increases
Set the lower bound back to 45C


Problem solved.

As for installing a temperature sensor, forget it. Francesco has a late 2009 model iMac 27" - these were not shipped with temperature sensors taped to the drive, rather with a proprietary cable connection direct to a special extra port on the drive.

The SSD Fan Control app works brilliantly on these old machines, once you understand the strange behaviour of this bug (and the workaround, described above).