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Dryness Value Test

The dryness value describes the amount moisture present in steam. If the steam is too dry there is a risk that it may superheat when entering the sterilizer and result in conditions where sterilization may not occur.

If steam has too much moisture, the loads and packaging may be wet at the end of the process. Because bacteria can grow through wet packaging and moisture provides an environment for bacteria to grow, it would be normal to reject wet loads and reprocess them.

The test is carried out using the principles of calorimetry. By knowing the temperature of the steam and the amount of energy it possesses, it is possible to establish how much moisture is present. We measure this e amount of energy in the steam by condensing a sample in a known mass of water. By measuring the temperature increase in the water and knowing how much steam has been condensed to produce this temperature increase, we can calculate the amount of energy present and therefore the amount of moisture. The calculation used also takes into account the heat loss of the test equipment and accurate temperature and mass measurements are essential if accurate and repeatable results are to be obtained.

The EN 285 acceptance criteria for the tests is that where metal loads are processed, the dryness value should be greater than 0.95 (less than 5% moisture) and 0.9 for porous loads (10% moisture).

The equipment supplied by KSA is robust and the heat loss has been specifically calculated for the equipment supplied. To make the complex calculation foolproof, it is provided as an Excel spreadsheet on CD.

A brief introduction to Dryness Value testing