Method for determining steam sterilant quality of a steam sterilizer, the method comprising: a) providing within the sterilizer a challenge device comprising a tube having a bore, the bore of the tube defining a free space which is open at one end for the entry of steam sterilant and closed at the other end; at least one thermal load having a predetermined heat capacity located around the tube, wherein the at least one thermal load contacts directly or indirectly the tube over a contact surface area and there is a predetermined thermal resistance between the tube and the at least one thermal load; a temperature sensor adapted to measure the temperature of the at least one thermal load; as well as a pressure sensor adapted to measure pressure in the steam sterilizer and/or a temperature sensor adapted to measure the temperature in the steam sterilizer; wherein the challenge device is configured and arranged such that, in use, condensate will flow out of the bore; b) allowing steam sterilant comprising non-condensable gas(es) to interact with the challenge device; c) measuring over time the temperature of the at least one thermal load and the pressure and/or temperature in the sterilizer; d) calculating the quantity of non-condensable gas(es) collected in the tube during a selected time interval Δt on the basis of the temperature of the at least one thermal load over the selected time interval Δt, the pressure and/or temperature in the sterilizer over the selected time interval Δt, the thermal resistance between the tube and the at least one thermal load, the heat capacity of the at least one thermal load and the geometry of the tube; e) calculating the quantity of steam condensed in the tube during the selected time interval Δt on the basis of the heat capacity of the at least one thermal load, the temperature of the at least one thermal load over the selected time interval Δt, and the specific condensation heat of the steam, wherein the value of specific condensation heat of the steam is either determined on the basis of the measured pressure and/or temperature over time in the sterilizer or used as a constant value independent of temperature dependency; and f) determining the steam sterilant quality on the basis of the quantity of non-condensable gas(es) collected in the tube during the selected time interval Δt and the quantity of steam condensed in the tube during the selected time interval Δt. a) providing within the sterilizer a challenge device comprising a tube having a bore, the bore of the tube defining a free space which is open at one end for the entry of steam sterilant and closed at the other end; at least one thermal load having a predetermined heat capacity located around the tube, wherein the at least one thermal load contacts directly or indirectly the tube over a contact surface area and there is a predetermined thermal resistance between the tube and the at least one thermal load; a temperature sensor adapted to measure the temperature of the at least one thermal load; as well as a pressure sensor adapted to measure pressure in the steam sterilizer and/or a temperature sensor adapted to measure the temperature in the steam sterilizer; wherein the challenge device is configured and arranged such that, in use, condensate will flow out of the bore; a tube having a bore, the bore of the tube defining a free space which is open at one end for the entry of steam sterilant and closed at the other end; at least one thermal load having a predetermined heat capacity located around the tube, wherein the at least one thermal load contacts directly or indirectly the tube over a contact surface area and there is a predetermined thermal resistance between the tube and the at least one thermal load; a temperature sensor adapted to measure the temperature of the at least one thermal load; as well as a pressure sensor adapted to measure pressure in the steam sterilizer and/or a temperature sensor adapted to measure the temperature in the steam sterilizer; wherein the challenge device is configured and arranged such that, in use, condensate will flow out of the bore; b) allowing steam sterilant comprising non-condensable gas(es) to interact with the challenge device; c) measuring over time the temperature of the at least one thermal load and the pressure and/or temperature in the sterilizer; d) calculating the quantity of non-condensable gas(es) collected in the tube during a selected time interval Δt on the basis of the temperature of the at least one thermal load over the selected time interval Δt, the pressure and/or temperature in the sterilizer over the selected time interval Δt, the thermal resistance between the tube and the at least one thermal load, the heat capacity of the at least one thermal load and the geometry of the tube; e) calculating the quantity of steam condensed in the tube during the selected time interval Δt on the basis of the heat capacity of the at least one thermal load, the temperature of the at least one thermal load over the selected time interval Δt, and the specific condensation heat of the steam, wherein the value of specific condensation heat of the steam is either determined on the basis of the measured pressure and/or temperature over time in the sterilizer or used as a constant value independent of temperature dependency; and f) determining the steam sterilant quality on the basis of the quantity of non-condensable gas(es) collected in the tube during the selected time interval Δt and the quantity of steam condensed in the tube during the selected time interval Δt.