AN INSTRUMENT TO MEASURE THE AMOUNT OF CONDENSATION DURING A GASEOUS STERILISATION PROCESS

Abstract: The disclosure relates to a condensation monitor for measuring condensation on one side of a window (10) from the other side thereof, the window being located in an enclosure (23) mounted on a chamber wall with a part opening with the interior of the chamber. The monitor comprises a pair of optical prisms (14, 20) positioned on the other side of the window at spaced apart locations to provide transmission of an obliquely angled beam of light into the window and transmission of an obliquely angled beam of light out of the window. A light source (12, 13) adjacent one of the devices provides a parallel beam of light to be transmitted by said device into the window to travel along the window by reflection from side to side to the other device from where the light beam emerges and a ligth sensor (22) measures the amount of light emerging from the window. The amount of light transmitted along the portion between the optical devices depends on the amount of condensation on said one side of the window, condensation facilites losses of the light through said one side by refraction reducing the amount of light transmitted along the window to the light sensor.

Description: AN INSTRUMENT TO MEASURE THE AMOUNT OF CONDENSATION DURING A GASEOUS STERILISATION PROCESS The present invention relates to the measurement and control of the level of condensation inside a chamber that is being sterilised using a gaseous delivery method of the sterilant. Most of the methods of gaseous surface sterilisation used in the Pharmaceutical Industry are more effective when fine layers of condensation are produced. M. A. Marcos et al has stated A that in processes where 30% hydrogen peroxide at 100° C is injected into a chamber at 30° C then " condensation is a phenomenon that cannot be avoided, according to the laws of physics". Other authorities have for some time believed that gaseous surface sterilisation using hydrogen peroxide is a dry gas process, but work by Watling et al A has shown evidence of condensation as the main factor causing sterilisation. Since it is now believed that condensation is the single most important factor in achieving surface sterilisation when using gases, such as hydrogen peroxide, it is important that the amount of the condensation is measured and controlled. It would be a simple matter to 'inject' sufficient gas to cause gross condensation and hence achieve the required level of biological kill. There are two major disadvantages to such a technique they are, firstly that an excessive amount of chemical would be required with the associated cost implications, and secondly, and perhaps of greater importance, such gross condensation will increase the down time of the chamber that is being sterilised. The time taken to produce the gross condensation will obviously be longer than would be required to produce the optimum level, but as more sterilant must be removed at the end of the cycle then the aeration time would also be increased. The removal of the sterilant from a chamber is frequently the longest part of a sterilisation cycle, partly because aeration must continue to remove the gas concentration to very low levels, but also because of absorption of the sterilant into the surfaces. The longer the condensed sterilant is in contact with the surface the greater the degree of absorption and hence the greater the time taken for aeration. The way in which droplets of moisture condense from a saturated vapour onto a surface are discussed and explained by M. A. Marcos et al A. Droplets of dew are formed on a surface when a saturated vapour comes into contact with a surface at a lower temperature. Given a sufficient supply of the saturated vapour the droplets on the surface will generate a concentration gradient in the vapour around the droplet drawing in more vapour to increase the size of the droplet. This process will continue until the droplets grow to such a size that they touch and combine. The shape of the droplets will be defined by the wetting angle of the liquid on the surface. Where the wetting angle is large, such as with droplets of water forming on clean glass, the droplets remain almost spherical. If the wetting angle becomes zero because of some treatment of the surface then a thin film of dew would form over the whole surface. With hydrogen peroxide and water vapour the droplets of dew which form on clean glass are initially very small and separated, and as the level of condensation increases so does the percentage of the surface area that is covered by droplets. Patent W098/12546 describes a technique for measuring the proportion of the surface area that is obscured by the formation of droplets. The technique taught in this patent is to shine a light onto a specially prepared surface and measure the change in the reflected light as the droplets are deposited on the surface. Whilst the technique gives excellent results it has the disadvantage that the instrument needs to be placed inside the chamber to be sterilised, and it must be in intimate thermal contact with the chamber surface. The present invention overcomes this problem by placing the instrument on the outside of a window fitted to the wall of the isolator, and measuring the droplet formation on the inside of the window. This invention provides a condensation monitor for measuring condensation on one side of a window from the other side thereof, the window being located in an enclosure mounted on a chamber wall with a part opening with the interior of the chamber, the monitor comprising a pair of optical devices positioned on the other side of the window at spaced apart locations to provide transmission of an obliquely angled beam of light into the window and transmission of an obliquely angled beam of light out of the window, a light source adjacent one of the devices to provide a parallel beam of light to be transmitted by said device into the window to travel along the window by reflection from side to side to the other device from where the light beam emerges and a light sensor to measure the amount of light emerging from the window, the amount of light transmitted along the portion between the optical devices being dependent on the amount of condensation on said one side of the window, condensation facilitating transmission of the light beam through said one side by refraction to reduce the amount of light transmitted along the window to the light sensor. When the surfaces are dry, a light beam directed from the outside of the chamber into a glass window at an oblique angle may be trapped by total internal reflection between the two surfaces of the glass, in the same way that light is trapped in an optical fibre. However should a droplet of dew form on the inside of the window then because of the change in refractive index light will be allowed to escape through the droplet. As more of the surface becomes covered with droplets of dew so more light will escape. An optical sensor placed at the end of the light path will see a diminution of light intensity as more and more droplets of dew form on the internal surface of the glass. In one arrangement according to the invention the enclosure may project into the chamber, the monitor is mounted on a side of the window on the inside of the chamber and the other side of the window is exposed to the interior of the chamber. In that arrangement a fan means may be provided for causing an air flow over the condensation monitor within the enclosure. In a further arrangement the enclosure maybe located on the outside of the chamber, the monitor is mounted on the outside of the window externally of the chamber and the fan means are provided for drawing an air flow from the chamber into the enclosure over the window to monitor the condensation in the air flow. In any of the above arrangements the optical devices may comprise prisms mounted on said one side of the window to transmit light into and receive light from the window respectively. More specifically the prisms may be adhered by light transmitting contact pads to said one side of the window. In any of the above arrangements the light source may have a lens for producing a parallel beam of light from the source. Also in any of the above arrangements the light sensor may have a lens for focussing the parallel beam of light from the window to a point on the light sensor. According to a further feature of the invention the window is formed in a wall of a chamber within which a condensation dependent process is performed to measure the level of condensation in the chamber. The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings, in which: Figure 1 is a detailed view of a condensation measure monitor; Figures 2 and 3 illustrate different applications of the monitor; and Figure 4 illustrates to an enlarged scale, the build up of moisture in the droplets on a surface. The basic instrument is best understood by reference to Figure 1, in which 10 is a glass window in the wall of the chamber indicated at 11 to be sterilised. A light source 12 passes light through a lens 13, which is selected and positioned so that it produces a parallel beam of light, which is projected into a right angled prism 14 mounted on the window with the hypotenuse 14a of the prism extending obliquely to the window a second side 14b lying parallel to the window and the strict side 14c extending at right angles to the window. Light from the lens is incident normal to the hypotenuse and the prism is reflected by face 14c inside the prism and passes out of the prism face through 14b and a contact pad 15 between the face and glass 10 into the glass window at an angle, which causes total internal reflection in the glass window. The contact pad is positioned between face 14b of the prism and the glass window to ensure that substantially all the light passes into the interior of the glass window. The contact pad is constructed from an optically clear gel type material that has a refractive index sufficiently high as to avoid any reflection at the surface. A suitable material would be an optically clear pad of soft silicon. Once the light has entered the interior of the glass window it is reflected internally several times between the parallel faces 16, 17 of the window as indicated at 18 until it reaches a second contact pad 19, which is similar to the contact pad 15, where because of the refractive index of the pad, the light escapes from the glass window into a further prism 20 through face 20b of the prism. The light is reflected inside the prism by face 20c which lies at right angles to the window before escaping through the hypotenuse face 20a. The light is directed how the prism into a lens 21 which concentrates the beam onto light sensors 22. The wavelength of the light emitted by the source 12 is matched to the sensitivity of the sensors 22 to minimise the effects of stray light. At the start of the sterilisation cycle the inside surface of the glass window is clean and all of the light is reflected inside the glass window and is directed onto the light sensors. As the inside surface of the glass window is subjected to saturated vapour droplets of dew will form on the surface. At the point of formation of dew on the surface there will be a change in the refractive index and light will escape through the droplet thus reducing the amount of light energy arriving at the light sensor. As the process of formation or evaporation of droplets occurs there will be a change in the amount of light energy that escapes and hence the amount of light energy arriving at the light sensor. An amplifier circuit with both zero and full-scale adjustment must be connected to the output of the light sensor. The amplifier may either have a voltage or current output depending on the requirements of the monitoring or control systems. The sensors may be calibrated by first setting the zero point, i.e. with no droplets and a clean glass window, and then setting the full scale by placing a large contact pad on the interior surface which allows all of the light to escape from the window. Intermediate calibration points can be achieved by attaching the sensor to various glass windows, which have different areas that have been etched. The etching disrupts the internal reflections and hence changes the amount of light arriving at the light sensor. This method has been tried with various areas of etching and calibration points at 25%, 50% and 75% of full scale have been achieved. These sets of etched glass have thus been used to compare the calibration of a number of sensors and found to give repeatable results within about 2%. When used in a real chamber that has to be sterilised, visible condensation as a very fine bloom appears at about 20% of full scale on the condensation meter. At this level of condensation sterilisation is achieved between 5 and 20 minutes depending on the temperature of the chamber. Reducing the temperature increases the time to achieve sterilisation because the 'D' value, or time to reduce the viable count by a factor of 10, is temperature dependent. It has been reported by Swatling et al (Reference III before) that reducing the temperature by 10°C increases the 'D' value by a factor of 2. One of the most critical parameters in measuring the condensation is the temperature of the glass window. If the glass window is part of the wall of the chamber this condition will be satisfied providing no local heating or cooling is applied to the area of the glass. To overcome the difficulty of mounting the glass window into the wall of the chamber and ensuring that it is at the correct temperature, two alternative mounting methods are possible, these are shown in Figure 2 and Fig 3. In Fig 2 the condensation monitor as shown in Figure 1 is mounted inside a box 23 on the glass window 10. The box 23 is then mounted on a chamber wall 24 on the inside the chamber 11 to be sterilised and is connected to atmospheres by a suitable short pipe or conduit 25. The whole of the box 23 and short pipe 25 are constructed to be airtight and free from leaks. A small axial fan 26 is placed in a tube inside the short pipe to draw air out of the box. The act of drawing air out of the box causes airflow of room air at room temperature into the box, thus keeping the inside surface of the box and glass window at a temperature similar to that of the rest of the enclosure. A similar technique may be applied as shown in Figure 3 where the box 23 is mounted on the outside of the chamber to be sterlised. The small axial fan 26 still removes the air from the box but in this arrangement the air is replaced by air from within the chamber to be blown over the inside surface of the condensation monitor glass window 10. The difference between the arrangements shown in Figure 2 and Figure 3, is that in Figure 2 the conditions inside the box replicate the conditions on the outside of the chamber, and hence the condensation monitor is mounted inside the box, whereas in Figure 3 the arrangement is different. In Figure 3 the arrangement the conditions inside the box replicate the conditions inside the chamber and the condensation monitor is mounted on the outside of the box. The above embodiments are particularly suitable for measuring condensation in the enclosure of the apparatus described and illustrated in our UK Patent Application No. 9922324.6. References I. M.A. Marcus et al. Pharmaceutical Technology Europe Vol 8 No 2 Feb 99 (24-32) II. Watling et al. The implications of the physical properties of mixtures of hydrogen peroxide and water on the sterilisation process. ISPE conference Zurich Sept 1998 III. Swartling et al. The sterilizing effect against bacillus subtilis spores of hydrogen peroxide at different temperatures and concentrations. J Dairy Red (1968), 35, 423

Claims: CLAIMS 1. A condensation monitor for measuring condensation on one side of a window from the other side thereof, the window being located in an enclosure mounted on a chamber wall with a part opening with the interior of the chamber, the monitor comprising a pair of optical devices positioned on the other side of the window at spaced apart locations to provide transmission of an obliquely angled beam of light into the window and transmission of an obliquely angled beam of light out of the window, a light source adjacent one of the devices to provide a parallel beam of light to be transmitted by said device into the window to travel along the window by reflection from side to side to the other device from where the light beam emerges and a light sensor to measure the amount of light emerging from the window, the amount of light transmitted along the portion between the optical devices being dependent on the amount of condensation on said one side of the window, condensation facilitating transmission of the light beam through said one side by refraction to reduce the amount of light transmitted along the window to the light sensor. 2. A condensation monitor as claimed in claim 2, wherein the enclosure projects into the chamber, the monitor is mounted on a side of the window on the inside of the chamber and the other side of the window is exposed to the interior of the chamber. 3. A condensation monitor as claimed in claim 2, wherein a fan means is provided for causing an air flow over the condensation monitor within the enclosure. 4. A condensation monitor as claimed in any of the claims 1 to 3, wherein the enclosure is located on the outside of the chamber, the monitor is mounted on the outside of the window externally of the chamber and the fan means are provided for drawing an air flow from the chamber into the enclosure over the window to monitor the condensation in the air flow. 5. A condensation monitor as claimed in any of the preceding claims, wherein the optical devices comprise prisms mounted on said one side of the window to transmit light into and receive light from the window respectively. 6. A condensation monitor as claimed in claim 5, wherein the prisms are adhered by light transmitting contact pads to said one side of the window. 7. A condensation monitor as claimed in any of the preceding claims, wherein the light source has a lens for producing a parallel beam of light from the source. 8. A condensation monitor as claimed in any of the preceding claims, wherein the light sensor has a lens for focussing the parallel beam of light from the window to a point on the light sensor. 9. A condensation monitor as claimed in any of the preceding claims, wherein the window is formed in a wall of a chamber within which a condensation dependent process is performed to measure the level of condensation in the chamber.

Inventors: MARTIN ANTHONY, MARTIN, ANTHONY
Applicants: BIOQUELL UK LTD, MARTIN ANTHONY, BIOQUELL UK LIMITED, MARTIN, ANTHONY
Application year: 2001
Main classification: G01N21
IPC codes: C12M1, G01F23, G01N21, G01V8