FOR TEMPERATURES MORE THAN 400 DEGREES ,THE RIGHT SENORS TYPES ARE THE THERMOCOUPLES.THE SIIGNAL THAT YOU HAVE FROM THESE SENSORS IS A FEW mVolts.THERE ARE ABOUT TEN TYPES OF THERMOCOUPLES.FOR EXAMPLE : TYPE K ( NICKEL - CHROMIUM),TYPO T ( COOPER-NICKEL ), TYPE E ,TYPE R ( PLATINUM , TYPE Y , TYPE J etc. . IF SOMEONE NEEDS I HAVE ALL THE TABLES , FOR ALL TYPES OF THERMOCOUPLES , WHICH YOU CAN SEE THE RELATION BETWEEN TEMPERATURE AND mVolts.
SO, FIRST OF ALL YOU MUST LEARN WHAT TYPE OF THERMOCOUPLE YOU HAVE AND AFTER YOU CAN USE ONE TEMPERATURE TRANSMITTER AND YOU WILL HAVE AN ELECTRIC SIGNAL 4-20mA.
FINAL THERE ARE SOME TEMPERATURES CONTROLLERS WHICH THEY TAKE FOR INPUT ALL THE TYPE OF THERMOCOUPLES , AND THESE CONTROLLERS HAVE DISPLAY, ALSO THEY HAVE TWO OUTPUTS RELAYS
IF YOU WANT TO USE THEM.( EXAMPLE TYPES FOR CONTROLLERS : JUMO , YAMATAKE , SELCO