Temp sensor question

[Drunkinsquirl]

I started playing with speed fan today to see if i could slow down this one really noisy fan i have. I started playing with the temps and ive got them all fiqured out except one. Speedfan reads 8 temp sensors, 3 from the winbond chip on my mobo (cpu, ambiant, pwm), 4 from my hdd's (S.M.A.R.T.), and 1 from an "ACPI' chip on the isa bus at address $0. I've spent 10 minutes trying to fiqure out what exactly its sensing, i tried google but couldnt find anything.