Its interesting that some sensors go into external pockets. These are a little pipe soldered to the pipe you are sensing. the temperature conducts around the pocket pipe, and it seems to work well. I have sometimes wrap a bit of copper around the sensor. It helps, but not done a lot of comparitive tests. Speed response will be better too.
With respect to calibration. You could put the sensors together for testing and add a callibration shift. I have done this several times. e.g. +0.25 or whatever it needs. You could also run the pump (easy on an Ecodan) and see if the sensors come together with zero dt. be mindful of water temp… i.e. it might be different at different water temperatures. The other point… I always remove the heat-shrink on the sensor if going direct to a pipe… conductive paste also helps…
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