Proyección de la oferta

13.39 Sterilization is achieved by the rapid and even penetration of steam into all parts of the load and the maintenance of these conditions for the specified holding time. To ensure this, it is essential to remove air from the chamber and load, and to provide a steam supply which contains a minimal volume of non- condensable gases. Any residual air and non-condensable gases will become concentrated as a bubble in the load and inhibit steam penetration.

13.40 The Bowie-Dick test shows whether or not steam penetration of the test pack is even and rapid, and thus by implication that air or other non-condensable gases are not present. It does not confirm that the sterilization conditions in the load have been achieved.

Principle of the test

13.41 The test, as originally conceived and described in earlier editions of HTM 10 (Bowie, Kelsey and Thomson, 1963), is based on the use of a chemical indicator in the form of an adhesive tape stuck to a piece of suitable paper to form a St Andrew’s cross. This indicator paper is placed at the centre of a test pack of folded huckaback towels and then subjected to an operating cycle. The indicator tape shows a change of colour in response to a combination of time, temperature and moisture.

13.42 If no air is present in the chamber, steam will penetrate rapidly and completely, and the indicator will show a uniform colour change. If air is present, it will collect within the pack as a bubble. The indicator in the region of the bubble will be of a different colour than elsewhere on the paper, because of a lower

temperature, lower moisture level or both.

13.43 The modern Bowie-Dick test uses a Class B chemical indicator conforming to BS EN 867: Part 3 (see paragraph 7.40) contained within a standard test pack (see paragraph 7.27). The indicator is distributed over an A4 paper sheet in the form of a geometric pattern.

13.44 When used in conjunction with a standard test pack, Class B indicators are designed to show a failure either if, at the start of the holding time, the temperature at the centre of the test pack is 2°C or more below the

temperature in the active chamber discharge; or if the indicator is exposed to insufficient moisture. Both conditions are usually caused by the presence of air or other non-condensable gases (see paragraph 13.56). Because of the

tolerances necessary in the manufacture of chemical indicators, users should be aware that in order to detect a temperature difference of 2°C the indicator may show signs of failure with a smaller temperature difference.

Test procedure

13.45 The Bowie-Dick test is normally preceded by a warm-up cycle. This cycle is necessary because the effectiveness of air removal may depend on all parts of the sterilizer being at working temperature. A satisfactory sterilizer may give a fail result if this is not done.

13.46 Remove the wrapping from a standard test pack and place the indicator paper in the sheet located nearest to the centre of the pack. Reassemble and secure the pack and replace the wrapping.

13.47 Place the test pack in the chamber with the bottom of the pack supported 100- 200 mm above the centre of the chamber base.

Table 10: Holding times for the Bowie-Dick test cycle Sterilization

temperature

Holding time

[°C] minimum [min] maximum [min]

134 3.3 3.5

126 10.8 11.0

121 16.8 17.0

13.49 During the holding time, note the reading on the cycle counter, the chamber temperature indicator and the chamber pressure indicator.

13.50 When the cycle is complete, remove the indicator paper from the test pack. 13.51 The test should be considered satisfactory if the following requirements are

met:

a. there is a uniform change throughout the indicator;

b. the automatic controller indicates that a Bowie-Dick test cycle has just been completed.

13.52 It is important to compare the colour of the indicator at the corners of the paper with that at the centre so that any difference can be clearly seen. If there is any discernible difference the test should be recorded as failed, and the paper marked accordingly. A large area of unchanged indicator points to a gross failure.

13.53 The indicator paper should be marked with the result and kept for reference for at least three months. The chemical reaction continues during this time and the paper may be discarded when the indicator becomes unreadable. The associated batch process record should be kept for at least 11 years. 13.54 An unsatisfactory test result indicates that the machine should not be used

until the fault has been rectified. It is important to realise that if a sterilizer fails to pass the Bowie-Dick test it cannot be made safe simply by increasing the holding time until a uniform colour change is produced. A failed sterilizer is in urgent need of skilled attention.

13.55 Several factors may inhibit steam penetration and cause the test to fail Common causes of failure include the following:

a. an inefficient air removal stage;

b. an air leak during the air removal stage;

c. the presence of non-condensable gases in the steam supply.

13.56 A subsequent thermometric test for a small load (see paragraph 13.7) will assist in diagnosing the cause of failure:

a. if the test reveals a temperature depression at the centre of the test pack, the problem is likely to be inefficient air removal or an air leak into the chamber. Air remaining in the centre of the test pack is inhibiting the penetration of steam and the correct temperature is not being attained. The sterilizer should not be returned to service until it has been

subjected to a vacuum leak test (see paragraph 11.2) and an air detector function test (see paragraph 11.60);

b. if the test fails to reveal a temperature depression, the problem is almost certainly air or other non-condensable gases in the steam supply. In this case the correct temperature is being attained but the steam is diluted, and insufficient moisture is present to change the indicator. The sterilizer should not be returned to service until the steam supply has been tested for the presence of non-condensable gases (see paragraph 9.4).

14. Fluid sterilizers

Introduction

14.1 This chapter contains detailed procedures for tests specific to sterilizers to process aqueous fluids in sealed containers. Schedules prescribing which tests are to be carried out and when are set out in Chapter 4 (for validation tests) and Chapter 5 (for periodic tests).

14.2 Unless specified otherwise, all the tests should be performed at each of the sterilization temperature available on the sterilizer.

14.3 For the thermometric tests the containers should be filled with the volume of water. The volumes of the fluid in each container should not differ from their mean by more than 5%. At the start of the cycle the temperature of the fluid in each container should be 20 ± 5°C.