Hi Helmut.
Are you having any specific process issues? If so please could you detail your process and we will try to assist you further.
We have guided several customers through getting the ovens professionally calibrated and much of the advice given to them may be of use to you. I have summarized it here.
It is firstly important to understand the operation of the oven. Air is pulled into the fan through the grill in the middle of the back panel, this air is then heated and blown out along the back in the top and bottom corners. I suspect your sensors are positioned in these hot air outlets, hence the high temperatures. We have seen results like yours before and a change in measurement technique and equipment has given far better and more real world performance figures.
What we would expect to see is the distribution getting wider as the temperature increases during ramp up due to the hot air being quickly introduced, but then unlike yours, we would then expect the lines to close back up together to our quoted tolerance after a few minutes whilst soaking at set point as the heating output backs off.
We find when testing the distribution on the oven we need to either use a very small, high response speed thermocouples (<2mm round tip with no body similar to this
https://uk.rs-online.com/web/p/thermocouples/3630250/) or use a very large thermal mass to damp out these fluctuations.
We found when developing the oven, distributing multiple copies of the same tool throughout the oven, each with a in mould thermocouple, the distribution is even and representative of the true performance of the oven.
As general advice, we recommend always positioning any parts being cured forward on the shelves towards the doors to minimise their effect on airflow and the temperature distribution. We also recommend positioning parts vertically in the middle and never placing parts directly on the base of the oven without a shelf to ensure a reliable cure.
Hopefully this helps.