The cause of superheated steam is usually as the result of large pressure drops between the steam distribution system and the sterilizer. EN 285 recommends that the pressure drop ratio should not exceed 2:1. If you are sterilizing at 121 degrees celsius (nominal 1 BarG), the supply pressure to the sterilizer should not exceed 2 BarG.
The test is carried out by reducing the supply steam pressure to atmospheric through an orifice (pitot tube) and measuring the steam temperature a fixed distance away. If the measured temperature does not exceeds the boiling point at the local pressure (typically 100 degrees celsius) by more than 25 degrees celsius the test is deemed to have passed (EN 285). When testing, we try to ensure that we measure the highest temperature and adjust the temperature sensor position to ensure that this is the case.
A brief introduction to Superheat testing