GB2360583A

GB2360583A - Optical condensation monitor

Info

Publication number: GB2360583A
Application number: GB0006822A
Authority: GB
Inventors: Anthony Martin
Original assignee: MICROFLOW Ltd; Bioquell UK Ltd
Current assignee: MICROFLOW Ltd; Bioquell UK Ltd
Priority date: 2000-03-21
Filing date: 2000-03-21
Publication date: 2001-09-26
Also published as: WO2001071321A1; GB0006822D0; CA2403745A1; JP2003528310A; AU2001239403A1; EP1266205A1

Abstract

An arrangement for monitoring condensation on one side of a window 1 comprises a pair of optical devices, e.g. prisms 4, 7, positioned on the other side of the window, where light emitted by a source 2 is transmitted through one device 4 and propagates within the window medium 1 by total internal reflection to the second device 7 and a light detector 9. The presence of condensation would facilitate the transmission of the light beam through one side of the window by refraction, decreasing the amount of light received by the detector 9. Fan means may be provided for ensuring that the window 1 is maintained at a suitable temperature. This arrangement is particularly suitable for use in controlling a chamber sterilisation process.

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 0 C is injected into a chamber at 300 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 W098112546 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.

Thus the invention provides a condensation monitor for measuring condensation on one side of a window from the other side thereof, the monitor comprising a pair of optical devices to be 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.

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 Fig 1, in which A is a glass window in the wall of the chamber to be sterilised. A light source A passes light through a lens A, which is selected and positioned 4 - so that it produces a parallel beam of light, which is projected into a prism A. The light is reflected inside the prism and then passes out of the prism through a contact pad A into the glass window A at an angle, which will cause total internal reflection in the glass window A. The contact pad A is positioned between the prism A and the glass window A to ensure that the light passes into the interior of the glass window A.

The contact pad A 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 A it is reflected internally until it reaches the contact pad A, which is similar to the contact pad A, where because of the refractive index of the pad the light escapes from the glass window A into the prism AE. The light is reflected inside the prism before escaping and passing through the lens Q which concentrates the beam onto the light sensors L The wavelength of the light emitted by the source 2 is matched to the sensitivity of the sensor 9 to minimise the effects of stray light.

At the start of the sterilisation cycle the inside surface of the glass window A is clean and all of the light is reflected inside the glass window A and is directed onto the light sensors L 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 L. 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 P.

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 A that reducing the temperature by 1 OOC 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 A. If the glass window A 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 A 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 Fig 2 and Fig 3.

In Fig 2 the condensation monitor as shown in Fig 1 is mounted inside a box A with the glass window A. The box A is then mounted inside the chamber to be sterilised and connected to the outside by a suitable short pipe A. The whole of the box A and short pipe A are constructed to be airtight and free from leaks. A small axial fan A is placed in a tube inside the short pipe A to draw air out of the box A. The act of drawing air out of the box A causes airflow of room air at room temperature into the box, thus keeping the inside surface of the box A and glass window A at a temperature similar to that of the rest of the enclosure.

A similar technique may be applied as shown in Fig 3 where the box A is mounted on the outside of the chamber to be sterlised. In this arrangement the small axial fan A 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 A.

The difference between the arrangements shown in Fig 2 and Fig 3, is that in Fig 2 the conditions inside the box replicate the conditions on the outside of the chamber, and hence the condensation monitor A is mounted inside the box, whereas in Fig 3 the arrangement is different.

In Fig 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 A.

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 M.A. Marcus et al. Pharmaceutical Technology Europe Vol 8 No 2 Feb, 99(24-32) 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 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 monitor comprising a pair of optical devices to be 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 1, 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.

3. A condensation monitor as claimed in claim 2, wherein the prisms are adhered by light transmitting contact pads to said one side of the window.

4. 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.

5. 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.

6. 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 processes is performed to measure the level of condensation in the chamber.

7. A condensation monitor as claimed in any of the preceding claims, wherein the monitor is mounted in an enclosure, mounted on the chamber wall with a port opening into the interior of the chamber and the window in the enclosure is exposed to the interior of the chamber.

8. A condensation monitor as claimed in claim 7, 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.

9. A condensation monitor as claimed in claim 8, wherein a fan means is provided for causing an air flow over the condensation monitor within the enclosure.

10. A condensation monitor as claimed in any of the claims 1 to 7, 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 airflow.

11. A condensation monitor substantially as described with reference to and is illustrated in Figure 1, Figure 2 or Figure 3 of the accompanying drawings.

:71296: GM VAT: FURNDOCS