SpeedFan has full logging capabilities.
The options you can set are:

Enabled
:
this option completely enables or disables logging
Add header to file
:
when enabled, a new log file is started with the names of the fields on the first row
Decimal separator
:
this option is originally set according to your international settings, but you may change it to whatever you prefer
Max decimals
:
the logging facility tries to save space on disk. If you set this option to, say, 5, you will never get more than 5 decimal digits, but SpeedFan will remove least significant ones if they are zeroes
Days to keep
:
SpeedFan will keep at most this number of days worth of log files. Please note that this is not necessarily the maximum number of log files, as SpeedFan can create more than one log file per day

File format: log files contain values written in text format using the specified decimal separator. Values are separated by TAB. The first row in the file contains the names of the columns if the add header to file option has been selected.

Naming: log files are named SFLogYYYYMMDD.csv, where YYYY is the year (four digits), MM is the month (2 digits, zero padded) and DD is the day (2 digits, zero padded). If a file already exists by that name, the file that already exists is renamed according to the following naming scheme: SFLogYYYYMMDD-CCCC.csv, where CCCC is a increasing number. The new file is then created with the standard file name scheme.

Notes: whenever you change the options related with logging, SpeedFan starts a new log file.

Once you set the global options for logging, you have to choose what to log. Say that you want to log TEMP1 and TEMP3. Go to the TEMPERATURE tab, then select TEMP1. At the bottom of the window you will find a checkbox named LOGGED. Check it and TEMP1 will be logged to the log file. Then select TEMP3 and do the same. That's all.