value along a long stretch of the measurement probe and its output signal is led to a control unit 31 that regulates the valves 33 on the steam supply line the steam may preferably be pure steam totally free of any ncg and trs content or it may be black liquor steam which contains trs the chips are pre treated in the embodiment shown according to the u201c cold top u201d concept in which it is attempted to establish a temperature gradient in the chip bin shown schematically where different levels of temperature 80 u00b0 c 60 u00b0 c and 40 u00b0 c are established upwards in the column of chips in the ideal case the chips at the upper surface of the column of chips are to maintain a temperature in the interval 20 40 u00b0 c a ventilation channel 2 a 2 b for venting of the weak gases that are formed is arranged at the upper part of the vessel and connected to a weak gas system ncg in which these weak gases are evacuated with a suitable fan 6 or pump in the embodiment shown in fig1 also a temperature sensor 3 installed for the weak gas system is used to detect the temperature in the upper part of the vessel the temperature sensor here is located in the ventilation channel 2 a close to the upper part of the vessel typically less than 1 metre from the vessel 1 but it is possible to use also a temperature sensor that is located within the top of the vessel or to use the temperature sensor 32 the ventilation channel 2 a 2 b is according to the invention connected to at least one diluting air input line 5 a 5 b 5.