A sterilizer apparatus comprises a sterilizer chamber (601). The sterilizer apparatus is configured to inject steam into the sterilizer chamber (601). A measurement probe (101) is provided with a measurement chamber (3) having a fluid connection to the sterilizer chamber (601). A light source (1) emits light into the measurement chamber (3) and a light detector (2) detects the light in the measurement chamber (3) and converts the detected light into a detection signal. A heating element (15) heats at least part of the measurement probe (101) to a temperature above a condensation temperature of the steam. The heating element (604) can be shared with a wall (603) of the sterilization chamber (601), so that the heating element (604) heats both at least part of the measurement probe (500) and at least part of the wall of the sterilization chamber (604) up to above said condensation temperature.
