Is there a certain order to be followed when we perform a steam quality test?
The order in EN 285 is 1.Non-condensable gases, 2.Dryness and 3.Superheat test. There is no engineering or logical reason why this order is required, though we tend to follow this order when we carry out and report tests. Because of the way the test points on the test elbow are used we often carry out the non-condensable gas test and the dryness test at the same time.
How do we solve the poor repeatability when perform a dryness test?
For the results to be repeatable, the test method should also be repeatable. Simply following EN 285 will not allow this! Because the calculation we use does not take account of heat losses to the environment, the longer the test proceeds, the worse the result tends to be. To carry out the test repeatably, the amount of water used to fill the flask should be as close to being identical between tests and the temperature of the water used should also be close from test to test. Provided that this is the case and the end temperature for the ending the test is the same (before agitation of the flask). Another factor is that the insertion depth of the temperature sensor into the flask should also be the same between tests.
EN 285 specifies that the flask fill volume should be 650ml +/-50ml. We would suggest 650ml +/-20ml. EN 285 specifies the water temperature should be between 0 and 27℃. We believe that the water temperature should be close to ambient the starting temperature +/-2℃ between tests. We always remove steam from the flask the very first reading we get that indicates that the water is 80℃. We then agitate the flask to get a homogeneous temperature reading.
EN 285规定烧瓶中水的填充量应为650ml +/-50ml。但是再实际操作中我更建议将这个范围所缩小到650ml +/-20ml。EN 285规定水温应在0到27℃之间。我认为测试之间的初始水温应接近环境温度，偏差在+/-2℃之内。我们始终会在获得第一个读数时即从烧瓶中除去蒸汽，这表明水为80℃。然后，我们搅拌烧瓶以获得均匀的温度读数。
If you consider the extreme cases which are in accordance with EN 285, one could be that the flask is filled to 700ml with ice cold water. The other is that the flask is filled to 600ml with water at 27℃. To heat the different amounts of water from different starting temperatures means that the tow tests will take different times and you will get different results because of the unmeasured heat losses to the environment
Where should the samples be taken from for the tests?
The tests are of the steam supply to the sterilizer and not of the steam within the sterilizer. The samples should be taken on the individual supply pipes to each sterilizer between the steam distribution system and the steam on/off valve for the sterilizers. Also, the tests should be carried out on every sterilizer and not groups of sterilizers from a single test point.
A sample point is required which has be installed into the steam pipework if test points for the tests are not included within the sterilizer pipework design. We call this a steam quality test elbow and it should be the same size as the pipe supplying the sterilizer with steam.
The sample for non-condensable gases is taken from the top of a horizontal pipe, the sample for the dryness and superheat tests is taken from a pitot tube which is inserted into the flow of steam and a steam temperature sensor is installed. The location of this is less important than the other two which are really mandatory.
When in the process cycle should the sample be taken?
The steam quality tests are started when steam enters the sterilizer chamber for the first time. The description is exact and does not mean when the sterilizer enters the heating phase or when the sterilization stage commences, but when steam enters the chamber for the very first time. This is usually on completion of the first vacuum pulse during air removal. This is deemed to be the worst case condition, because the chamber is at its coolest and is likely to result in the greatest steam demand.
The end point of each test is provided in the test methods and are not expected to occur in the first steam pulse and will include periods where the sterilizer is pulling a vacuum. This can be anticipated and does not represent a problem. When the test is completed, the cycle can be aborted ready for the next test.
Because logically, there is only one point in a cycle where steam enters for the first time, if you wish to run three dryness or non-condensable gas tests, you need to start three cycles for each of the two tests.
This picture which is a pressure trace of a sterilization process, shows where steam enters the chamber for the first time. To be aware of this, someone needs to be watching the chamber pressure display, though with experience, you will probably be able to hear steam entering the chamber.
Is it normal when I get negative figure from superheat test?
The acceptance criteria for the superheat test is that the temperature measured in the expansion tube is less than 25°C above the boiling point at the local environmental pressure – 100°C at sea level.
This test always seems to pass these criteria without any problem. What concerns many however is that the results obtained are often below 100°C which is counter intuitive, if you are aware of the theory of the test.
The answer to this lies in the test design, which assumes theory and practice will coincide exactly. When steam flows from an orifice, it results in a low pressure area close to it. This results in steam which has condensed on the wall of the expansion tube to be sucked towards the orifice and where it gets entrained into the steam flow. This results in the low temperatures experienced and is a function of the test method and not the person conducting the test. For this reason, there is little point in conducting three superheat tests as they always pass.