Here is what I asked Tom Chalko:
1. IS THERE ANY POSIBILITY TO GT31 got i DATA LOGGER options for Logging Enable & Logging Disable, or Logging ON, Logging OFF ?
set MIN SPEED to any value you want. Logging will take place only when you move above this speed

2. Is there any other way to got few separate sessions in one day, and not using for that russian method of switching device OFF and ON again ?
it is the simplest method.
But, why do you want few sessions per day? It is MUCH EASIER to deal with 1 file per day.
I leave my GT31 running for 20 hours, but I use MIN SPEED settings.
Use MIN SPEED settings. There are 2 min speed settings, one for SD card and one for the Data Logger.
You are the only person in the world who wants "separate files". hahahha
It is MUCH EASIER to deal with 1 file

3. new option in MISC: DELETE>DEL ALL a clearing only flash memory, not SD card, right ?
yes. SD card is not touched.
Doppler: you cannot turn Doppler OFF.
GT31 does tracks and Doppler together all the time.
AND HERE IS MY PAGE DEDICATED TO SETTING UP GT31 AND SOLUTION
HOW TO MODIFY AQUAPAC FOR US WITH SMALLER HANDS :)
Link:
http://windsurfing-koprivnica.net/page.php?94CHEERZ