I'd like to make ''KER'' and 'MAX'', flash alternately
Where are you stuck ?
Use millis() for timing as explained in Using millis() for timing. A beginners guide, Several things at the same time and look at the BlinkWithoutDelay example in the IDE.
Print "KER", save the value of millis() and when the required period has elapsed print "MAX", save the time and keep going round loop() alternating the text when the display period expires.