@Stewy *******
IF you don't BUY a tourist visa from a thai consulate when you're in another country BEFORE you come back to thailand AND if you enter and are stamped in for 30 days you are NOT on a tourist visa you are on a 30 day visa exempt entry
AND
As Robert pointed out to you, there is a limit of TWO 30 day visa exempt entries by land in a calendar year <- no matter if you switch passports or not they have your previous entry history..