Which contactless temperature sensor/thermometer to use?
You would get a more accurate result by inserting a probe into the pipe. 

If you want to use an external probe, a thermocouple or an RTD in a block than is strapped to be in intimate contact with the pipe and well insulated from the outside environment should give reasonable results for a permanent installation. A stick-on thermocouple with insulation could be used, but the adhesive may not hold up.