Voltage range adapter
You need more than a "voltage range adapter" to properly read a thermocouple. You require cold-junction compensation to get an accurate temperature reading. 

Otherwise you will get a reading that is 


inaccurate  
dependent on the temperature of the terminal block of
your "voltage range adapter", so as it warms the reading will drop!


You can get thermocouple transmitters or signal conditioners if your PLC cannot accept thermocouple inputs directly. They will work with one or more specific "flavor(s)" of thermocouple such as Chromel-Alumel (K) or Iron-Constantan (J or JIS-J).