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Miniature fission chambers are tools suited for instrumenting experimental reactors, allowing online and in-core neutron measurements of quantities such as fission rates or reactor power. A new set of such detectors was produced by CEA to be used for the next experimental program at the EOLE facility starting in 2013. Some of these detectors will be employed in pulse mode for absolute measurements, which require calibration. The calibration factor is expressed in mass units and is thus called “effective mass”. A calibration campaign was conducted in December 2012 at the SCK\bulletCEN BR1 facility within the framework of the scientific cooperation VEP (VENUS-EOLE-PROTEUS) between SCK\bulletCEN, CEA and PSI. In order to improve the calibration method, two steps were taken. First, a new characterisation of the thermal flux cavity and the MARK3 neutron flux conversion device carried out by SCK\bulletCEN allowed using calculated effective cross sections for determining detectors effective masses. Dosimetry irradiations were performed in situ in order to determine the neutron flux level and provide a link to the metrological standard. Second, two fission chambers were also calibrated at the CEA CALIBAN reactor (fast neutron spectrum) using the same method so that the results could be compared with the results obtained at the SCK\bulletCEN. This paper proposes to present both the calibration method and recent improvements on uncertainty reduction and biases evaluation. The results and uncertainties obtained in both CALIBAN and BR1 reactors will be compared and discussed. [ABSTRACT FROM PUBLISHER]