The Principles of Medical Device Decontamination

Moisture detectors

The third process variable in a steam sterilization process is the presence of moisture. When using a saturated steam sterilization process the moisture will be provided by the steam entering the chamber. The presence of residual air can impede the penetration of steam and hence moisture into loads. The air detector (see later) is designed to monitor for the presence of residual air. There are no physical moisture detectors available. It is therefore assumed that if there is no residual air present then the chamber is full of steam and therefore adequate moisture prevails.

Air detectors

Every steam sterilizer used in the UK has an air detector (AD) fitted. The AD is designed to monitor for residual air in the sterilization process. During the design and development of the steam sterilizer equipment and process, an AD will be included. The AD provides a control function and will abort the cycle if excess air is detected. During type testing, works testing and validation, the performance of the AD will be established by conducting a series of tests prescribed in standards (EN 285 2 ) and guidance (HTM/WHTM/SHTM 01-01 1,3,4 ) to ensure that the AD allows the cycle to proceed in the absence of residual air but aborts the cycle if excess air is detected. Air detectors come in several forms including blind ended tubes with either a temperature or photoelectric sensor at the closed end or a residual pressure capsule.

Ethylene Oxide (EtO), Low Temperature Steam and Formaldehyde (LTSF) and Vaporised Hydrogen Peroxide (VH 2 O 2 ) detectors

In low temperature sterilization the concentration of the sterilising agent is a process variable along with time, temperature, moisture (or humidity). Some LT sterilizers have concentration monitors fitted to the chamber so that sterilant concentration is measured. Many of these are based on standard instrumental analytical methods such as UV / IR spectroscopy which measure concentration across the width of the chamber space. Sterilant concentration can also be estimated by pressure change during the vaporisation phase of the cycle. As the sterilant is vaporised into the chamber the pressure will rise, and calculations can be made to estimate concentration.

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