Here in Canada, the entire assembly has to be CSA approved for area into which it will be installed. That is the sensor itself, connection head, nipple and thermowell. So simply using anyone's thermocouple or RTD in a Hazardous rated head will not give you a CSA approved assembly. There are a few manufacturers who do make a Class II amplifier or "hockey puck" that are in them selves Class II approved by UL and CSA. This allow you to mount them in the sensor head and maintain the area approvals.