Steam Quality Parameters And Effects Of Their Deviations From Accepted Values

Steam Dryness:

The measure of the water content of steam deliverd to the sterilizer chamber.
Acceptable values are 0.9 or greater (<10% water) for non-metallic loads and 0.95 or greater (<5% water) for metallic loads.

Wet steam can cause an unsterile load in two ways:

• Insufficient energy delivered to the load to sterilize.
• “wet packs”, making the sterile barrier material surrounding the load less of a barrier and compromising sterility assurance.

Superheat:

A situation in which the temperature of the steam is higher than the saturation temperature for its actual water content. (The steam is too dry).
Acceptable values are less than 25 degree centigrade of superheat in free expansion.
Superheat has two potential effects:

• Unsterile loads due to insufficient energy being delivered to the load, since the steam is too dry.
• Damage to the load if the superheat is generated causing the temperature reached by the load to be higher than its materials can withstand.

Non-condensable gases:

A measure of air or other gases entrained in the steam. Expressed as a percentage by volume of gas in the steam.
Acceptable levels are less than 3.5% delivered to the chamber.

High non-condensable gas content can cause an unsterile load in two ways

• Insufficient energy delivered to the load to sterilize. Gases deliver substantially less heat energy than steam.
• Pockets of gas can form that provide “islands” of unsterility. Unless the indicator is in such an island, their presence will go undetected.