A temperature difference between the external sensor and furnace sensor could be normal.

The furnace sensor simulates a standard PCB. The external sensor is measuring the actual temperature on the board. A difference of 30° could be quite normal.

In the eC-reflow-pilot ,the 3 temperature measuring probes are marked with colours. The furnace or top probe is marked RED. The bottom probe is marked BLUE and the external sensor which can be attached to the PCB is marked as GREEN.

The probes RED and BLUE are hanging free in the atmosphere of the oven and are much smaller in mass than the probe GREEN which is attached to the PCB. Because of the direct Infra Red light radiation and the thermal inertia effect, RED and BLUE will heat up faster than GREEN.

From the moment that RED and BLUE run too far away from Green, the thermal controller takes the current away from the lamps stopping the heating up. The temperature of GREEN does not increase anymore because the residual heat from the lamps and the good insulation of the oven are just enough to keep GREEN stable.

From the moment that RED and BLUE are closer to GREEN again, the thermal control gives power again.

This switching process is slow due to the thermal inertia of the oven.

It is therefore not very useful for a standard easy soldering curve to define many points in the soldering curve as that will significantly slow down the whole proces. A well performing soldering curve is explained here.