Abstract:
The disclosure relates to a portable apparatus for decontaminating an enclosed room or other space which includes a passageway having an air inlet at one end and an outlet at the other end. A pump causes a flow of air through the passageway from the inlet to the outlet. A heater heats the air flowing through the passageway to a predetermined temperature, a flash evaporator being in communication with the passageway. Liquid decontaminant is pumped from a supply of decontaminant to the evaporator to be evaporated and for the evaporant to be delivered to the air flow in the passage to flow in the air flow from the outlet to the rooms to be decontaminated. A universally rotating nozzle is provided at the outlet to distribute the decontaminant containing air throughout the enclosure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and apparatus for decontaminating enclosed spaces such as hospital wards and clean rooms in which a manufacturing or other processes take place in sterile conditions. 
     2. Present State of the Art 
     Vaporised aqueous solution of hydrogen peroxide has been used to decontaminate the internal surfaces of enclosures used for aseptic processing in the pharmaceutical industry since about 1990, but it has always been difficult to use the same technology to decontaminate larger enclosed volumes such as rooms. 
     The conventional apparatus for decontaminating enclosures comprises a gas generator in a closed circuit including the enclosure such as described in U.S. Pat. No. 5,173,258. In this design the hydrogen peroxide and water vapours are produced by flash evaporation of an aqueous solution into a heated air stream, which then carried the gas to the space to be decontaminated. The air and mixture of gases then mixes with the air inside the chamber before being returned to the gas generator, where the gas is decomposed, dried, heated and more liquid is flash evaporated and the air mixture is returned to the chamber. 
     The processes performed on the returned gas are complex, and include the steps of decomposing the gas, drying and re-heating. This complete process was considered necessary because it was understood that the hydrogen peroxide gas decomposed according to a half-life rule and hence to maintain an adequate concentration inside the chamber a circulating system that decomposed the gas was thought to be necessary. Recent work by Watling, ISPE Conference Zurich, September 1999 has shown that the gas does not decompose but is stable. It is therefore not necessary to remove the returning gas from the chamber. 
     S.S. Block reports in the 5th Edition of Disinfection, Sterilisation and Preservation page 189 that a 3% hydrogen peroxide aqueous solution gives a log 8 reduction of  Staphylococcus aureus  in under 20 minutes. A slower rate of deactivation has been found in experimental work when exposing  Staphylococcus aureus  to gas generated from 35% solution, when the process was operated at a temperature below the dew point thus causing condensation. Under these gassing conditions the first droplets of dew form on the organism at a much higher concentration than that of the original liquid, typically about 65% w/w, the exact value depending on the moisture content of the carrier gas. 
     As stated above, in the conventional system the air in the chamber to be decontaminated is dried prior to injecting the decontaminating gas. This is done either to allow a high level of gas concentration to be achieved before the onset of condensation, or to operate the process avoiding condensation maintaining the gas in a dry state. The vapour pressure equations for hydrogen peroxide and water may be used to calculate the concentration of the hydrogen peroxide and water vapour that will cause condensation and hence may be used either to avoid the conditions that will cause the onset of condensation or to calculate the concentration of any condensate that may be formed as a result of passing the flash evaporated vapours into the sealed enclosure. If the RH in the chamber is high the condensation will form quickly but as a relatively weak solution. Evaporating 35% w/w hydrogen peroxide into a chamber at 20° C. and 85% RH will cause the condensate to form at in excess of 6% w/w, although the concentration of the vapour will be about 120 ppm. It is well known that 6% hydrogen peroxide is active against microorganisms and will cause bio-deactivation of surfaces. If it is intended to operate a process where condensation is formed it is therefore not necessary to reduce the humidity in the chamber under normal operating conditions as the RH will be less than 85% and hence the condensation will form at a concentration greater than 6%. The same is not true when operating a process that is intended to avoid condensation, in such a process it is essential to ensure that the moisture content of the air inside the enclosed space at the start of the process is low. 
     It is believed that the difference between the liquid process as reported by Block and a gaseous dew process is the rate of delivery of the hydrogen peroxide condensation. It follows that using a standard recirculating gas generator placed outside the space to be bio-decontaminated; there may not be an adequate evaporation capacity to achieve a sufficiently high condensation rate to deactivate the organism inside the chamber. The deactivation process may be enhanced by the use of mixtures of chemicals but the principal of the rate of delivery still remains. Whilst for a dry gas process the rate of delivery of hydrogen peroxide and water vapour are not so critical it is still important to evaporate the liquid as fast as is practical as this will shorten the time required to raise the gas concentration and achieve a satisfactory bio-decontamination. 
     An analysis of the equations governing the vapour pressure of water and hydrogen peroxide by Watling et al and published in the PDA Journal of Science and Technology November/December 2002 vol 56, No 6 291-299, shows that the gas concentration inside a chamber may be raised to the dew point by passing flash evaporated vapour into the sealed enclosure, but as soon as the dew point is reached condensation will form at a higher concentration than the evaporated liquid thus reducing the gas concentration. The gas concentration will continue to fall as more liquid is evaporated until the equilibrium vapour pressure for the evaporated liquid is reached at the temperature of the chamber. 
     There are two views about the mechanisms involved in the bio-decontamination using hydrogen peroxide and water vapour. The first is that it is important to ensure that the gas remains in the dry state and the second that condensation is essential. It has been well established that dry hydrogen peroxide gas at elevated temperatures will bio-deactivate micro-organisms, and the same dry process has been shown to work at room temperatures. The condensation process in which the gas concentration is raised to the dew point and condensation is allowed to form appears to be faster at room temperatures. 
     SUMMARY OF THE INVENTION 
     The apparatus and method described in the present invention will work equally well with both the dry and condensation processes. When operating a dry process it is essential to monitor the water and hydrogen peroxide concentration in the gaseous phase to ensure that they remain below the saturated vapour concentrations. When operating a condensation process it is helpful to have an indication of the point during the cycle when condensation starts to form and the subsequent rate of formation. A technique and apparatus to make such a measurement of condensation is described patent application UK 0291983.1 
     An ideal bio-decontamination cycle is in three phases. The first phase is to bring all of the equipment to thermal stability but may also be used to adjust the relative humidity in the chamber to a pre-set level, the second is used to raise the gas concentration to the required level and maintain that concentration for a sufficient length of time to achieve the required level of bio-decontamination, and the third and last phase to reduce the concentration of the sterilant in the enclosed space to a predetermined value. 
     U.S. Pat. No. 4,863,688 discloses a method of selectively destroying organisms within a chamber such as an incubator comprising the steps of introducing vapour phase hydrogen peroxide into the chamber at a rate sufficient to cause a predetermined concentration of hydrogen peroxide to be reached while preventing a substantial change in pressure or condensation of the hydrogen peroxide in the chamber. When the predetermined period of time has elapsed, the vapour phase hydrogen peroxide is removed from the chamber. In a preferred embodiment disclosed an incubator is provided with a separate apparatus for producing a flow or air containing hydrogen peroxide vapour which is delivered to the incubator. Alternatively the apparatus for producing the air flow containing hydrogen peroxide vapour may be built into the incubator. 
     RU-C-2054295 discloses a device for sanitary treatment of air for use in livestock and poultry facilities and in various branches of industry including biological, food, light industry, chemical, coal, construction and other applications. The device includes a housing with an inlet and an outlet, a heating element, disinfected evaporator in the form of a perforated header closed at one end and enclosed in a porous sheath, the header is installed along the housing axis. The device has a reservoir containing disinfectant solution secured to the housing and connected to the open end of the evaporator. The tubular evaporator is arranged in the porous sheath along a spiral line and the heating element is mounted within the centre of the spiral. 
     This invention provides a method of decontaminating an enclosed space comprising the steps of providing an aqueous solution of hydrogen peroxide in the enclosed space, producing hydrogen peroxide/water vapour from said aqueous solution, creating an air stream in the enclosed space, introducing hydrogen peroxide/water vapour into the air stream, distributing the hydrogen peroxide/water vapour containing air stream throughout the space to be decontaminated and then removing the hydrogen peroxide/water vapour from the space; characterised in that the air stream is heated before hydrogen peroxide/water vapour is introduced to it, the hydrogen peroxide/water vapour is flash evaporated from an aqueous solution of hydrogen peroxide/water vapour from said supply into the air stream, and the air stream carrying the flash evaporated hydrogen peroxide/water vapour is distributed throughout the enclosed space to achieve bio-decontamination of the enclosed space. 
     By placing the gas generator inside the room and simply heating the carrier gas and then evaporating this sterilant into the air stream it is possible to use the available energy much more efficiently. The increase in efficiency is derived from the removal of the system for decomposing and drying the carrier gas, and also because there is no need for any pipe work to transport the carrier gas and decontaminant from an external generator. 
     This increased efficiency provides more energy for the primary function of heating the carrier gas and flash evaporating the liquid. The efficiency increase is so great as it allows a trebling of the rate of flash evaporation from the same energy source and hence the rate of increase in the gas concentration or the achievable rate of formation of condensation once the dew point has been reached is also trebled. 
     The simplified design is also much smaller and lighter than a conventional gas generator and hence considerably less expensive to manufacture. It is therefore realistic to place a number of such devices inside a sealed enclosure to be decontaminated. This reduction in size and weight makes the apparatus portable and hence makes it practical to use the same apparatus to bio-decontaminate a number of facilities either on the one site or at different locations. As stated above it is important to make measurements of the hydrogen peroxide and water vapour concentrations. To satisfy this requirement an instrument module that is placed inside the enclosed space has been devised that will also link back to the control system that is external to the enclosed space. Provision has been made within the control systems both at the gas generator(s) and the instrument module to connect a number of condensation sensors so that the process may be operated either as a dry gas or as a saturated vapour process 
     Each simplified generator will have its own control system, which is linked to a control box external to the room and connected by a single control cable. By using a central control system, such as a laptop computer, it is possible to control a number of generators that are linked together from outside the enclosed space. With the present arrangement it is possible to control eight generators from a single laptop, should a larger number be required a second computer would be needed. It is also possible to control multiple aeration units and dehumidifiers from the same laptop computer. 
     Because the apparatus is portable and may therefore be used at different sites in order to ensure that the apparatus does not carry contamination from one location to another it is essential that all of the external and internal surfaces are bio-decontaminated during the gassing cycle. To achieve this objective components have been mounted in such a way to ensure that they are exposed to the sterilising gas. The tubular steel frame has been sealed and the control box is purged with the sterilising gas drawn from the room. Tests have been performed to check that following a bio-decontamination cycle all of the surfaces of the apparatus have been rendered safe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings, in which: 
         FIG. 1  is a wholly diagrammatic view of an apparatus for generating and delivering an air flow containing an evaporated decontaminant to an enclosed space; 
         FIG. 2  is a similar view to  FIG. 1  showing the components of the apparatus including the evaporator, liquid sterilant supply and outlet nozzle in greater detail; 
         FIG. 3  is a perspective view of a portable unit embodying the apparatus of  FIGS. 1 and 2 ; 
         FIG. 4  in an exploded view of the unit of  FIG. 3 ; 
         FIG. 5  is a plan view of the evaporator; 
         FIG. 6  is a cross-sectional view on the line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a cross-sectional view of an alternative form of evaporator; 
         FIG. 8  is a perspective view of a control box for the apparatus of  FIGS. 3 and 4  with a lid of the box shown open; 
         FIG. 9  is an exploded view of a monitoring unit for use in conjunction with the apparatus of  FIGS. 3 and 4 ; and 
         FIGS. 10 and 11  show further embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The gas generator apparatus will be described firstly with reference to  FIGS. 1 and 2 . Room air, which may or may not already contain previously supplied hydrogen peroxide and water vapour, is drawn into an inlet conduit  10  through a HEPA filter  11  by a variable speed motor driven fan  12 . The HEPA filter  11  removes any particles from the air stream to ensure that the delivered air is of the correct quality when the generator is used in a clean room. The conduit delivers the air to a heater  13  where the temperature is raised to a predetermined level as described below. The heated air then passes into an evaporator  14  where a liquid sterilant comprising aqueous hydrogen peroxide is flash evaporated. By way of example, the sterilant may comprise an aqueous solution containing 30 to 35% hydrogen peroxide. If the sterilent includes peracetic acid, the proportion of hydrogen peroxide can be reduced to 15% with 0.5% peracetic acid and a balance of water. In practice the heater  13  and the evaporator  14  are combined in a single unit as shown in  FIGS. 2 to 7  to which reference will be made later. The physical shape and dimensions of the combined heater/evaporator are designed to control the energy balance between that used to heat the carrier gas and that used for flash evaporation. 
     A supply of aqueous hydrogen peroxide liquid is stored in a container  15  and is pumped to the evaporator  14  by a liquid pump  16 . The carrier gas and vapours are delivered from the evaporator through a conduit  17  to a distribution nozzle  18  for delivery of the sterilant vapour to the space to be decontaminated. The liquid container is demountable from the frame  19  to reduce the weight of the unit and make it more easily hand carried. 
       FIGS. 3 and 4  show a practical embodiment in which the gas generator apparatus is supported in a tubular steel framework or structure  19  for ease of movement. Structure  19  has opposing lateral sides  60  that each extend between a front side  62  and an opposing back side  64 . The structure  19  at least partially bounds a compartment  66  and has a plurality of spaced apart openings  68  that communicates between the exterior atmosphere and compartment  66 . As shown in the depicted embodiment, each side may incorporate a separate one of the openings  68 . One or more of the gas generator apparatus components may be disposed within compartment  66 . For example, as shown in the depicted embodiment, HEPA filter  11 , fan  12 , heater  13 , flash evaporator  14 , container  15 , liquid pump  16 , and/or conduit  17  may be wholly or partially disposed within compartment  66  so that respective exterior surfaces thereof are freely exposed to the exterior atmosphere by way of the plurality of openings  68 . The apparatus is light enough to be carried by the user and as can be seen in  FIG. 4  can have caster wheels  20  to enable it to be easily manoeuvred into position. The tubular framework is sealed to prevent any contamination being introduced to the enclosure by the frame. Ideally, the apparatus should not be placed inside a housing unit. Any covering of the apparatus would restrict the sterilant gas movements around and through the apparatus, which is essential to ensure that the apparatus itself is also surface decontaminated because otherwise it may contaminate the area in which it is placed.  FIGS. 3 and 4  also show the enclosed control box  70  for the apparatus disposed at least partially within compartment  66  so that the exterior surface of control box  70  is also freely exposed to the exterior atmosphere by way of the plurality of openings  68 . Control box  70  will be described in greater detail below. 
       FIG. 3  shows the outlet nozzle in greater detail. The nozzle has a motorised power unit  18   a  which rotates the nozzle assembly about a vertical axis. The nozzle assembly includes a laterally extending arm  18   b  having an enclosed drive for rotating the nozzle tip  18   c  about a horizontal axis to provide a universal discharge of heated air/hydrogen peroxide sterilant vapour around the room or other enclosure. The motor and nozzle assembly are formed as a unit and may be detached at the coupling  18   d  shown in  FIG. 4  from the outlet of the evaporator and dismounted from the frame to be transported independently of the gas generator unit. Multiple units may be provided as necessary and separate fan units may also be provided to circulate the sterilant atmosphere throughout the room or enclosure. 
     An ideal decontamination cycle may have three distinct phases. In the first optional phase, the relative humidity in the room or other enclosure is adjusted to a pre-set level. In the second phase the gas concentration of sterilant gas is raised to form a required layer of condensation over all surfaces in the enclosure for a sufficient length of time to achieve the required level of decontamination. In the third and last phase the sterilant is removed from the enclosure. This is achieved using the room aerator system described and illustrated in International Patent Publication No. WO 02/11864. 
     If a HVAC system is available for the room or enclosure then this may be used to achieve the required level of relative humidity at the start of the process, and if the HVAC exhausts to a safe area to remove the sterilant at the end. Alternatively a portable dehumidifier may be used to adjust the initial relative humidity and a catalytic scrubber used to circulate the gas to remove the sterilant. 
     In the decontamination cycle referred to above the initial phase of treatment in the adjustment of the relative humidity in the room or chamber may be omitted and the process commenced at the current prevailing conditions in the enclosure since the relative humidity in the enclosure would normally be well below dew point and so a considerable amount of sterilant/water vapour would need to be generated in the enclosure before condensation would occur. 
     Reference is now made to  FIGS. 5 and 6  which illustrate the combined heater/evaporator  13 / 14  in greater detail. The heater/evaporator comprises a cast cylindrical aluminium block  30  which is mounted in framework  19  with the axis of the block extending vertically. The lower end of the block has a shallow cylindrical recess  31  and a circular base plate  32  is attached to the periphery of the block extending across the recess by screws (not shown). The base plate  32  has a central aperture  33  in which the end of the inlet conduit  10  is mounted to deliver a supply of air to the recess in the block. 
     The upper end of the block also has a cylindrical recess  34  and a central top plate  35  is mounted on the periphery of the block over the recess by set screws  36 . The top plate  35  has a central aperture  39  in which an outlet conduit  40  from the block is mounted. 
     The block is formed with a central cylindrical cavity  37  extending into the block from the upper end thereof in which the outlet conduit  40  extends stopping short of the bottom of the cavity. The block  30  has a multiplicity of axially extending passageways  38  adjacent the outer surface of the block and spaced around the block leading from the lower recess  31  and the block upper recess  34  for flow of air from the bottom recess to the top recess from where the air can flow into the cavity  37  and thence into the outlet conduit  40 . The liquid sterilant from the storage container  15  is delivered via one or more inlet conduits  41  providing injection points which extend through the top plate  35  adjacent to the outlet conduit  40 . The conduits  41  lead into the cavity  37  in the block but stop short of the bottom of the cavity. A second inlet conduit  41  is shown and preferably three such conduits are provided at spaced locations around the outlet conduit. 
     The body  30  is encircled by a cylindrical jacket in which an electrical resistance heater  42  is mounted for heating the body  30  to a requisite temperature to pre-heat the airflow through the block and also to ensure that sterilant delivered by the conduit  41  to the bottom of the cavity  37  of the block is flash evaporated from the bottom of the cavity to produce a vapour which is entrained in the flow of air through the flow of heated air through the outlet conduit  40  for delivery into the room to be sterilised. 
     The heating unit of the heater-evaporator is coupled to the control unit to the apparatus and a temperature probe  44  is mounted in a radial drilling  45  in the body  30  below the cavity  37  to measure the temperature of the body for adjusting, through the thereof in which the outlet conduit  40  extends stopping short of the bottom of the cavity. The block  30  has a multiplicity of axially extending passageways  38  adjacent the outer surface of the block and spaced around the block leading from the lower recess  31  and the block upper recess  34  for flow of air from the bottom recess to the top recess from where the air can flow into the cavity  37  and thence into the outlet conduit  40 . The liquid sterilant from the storage container  15  is delivered via one or more inlet conduits  41  which extend through the top plate  35  adjacent to the outlet conduit  40  and also lead into the cavity  37  in the block and again stop short of the bottom of the cavity. A second such inlet conduit  41  is shown in dotted outline and preferably three such conduits are provided at spaced locations around the inlet conduit. 
     The body  30  is encircled by a cylindrical jacket in which an electrical resistance heater  42  is mounted for heating the body  30  to a requisite temperature to pre-heat the airflow through the block and also to ensure that sterilant delivered by the conduit  14  to the bottom of the cavity  37  of the block is flash evaporated from the bottom of the cavity to produce a vapour which is entrained in the flow of air through the flow of heated air through the outlet conduit  40  for delivery into the room to be sterilised. 
     The heating unit of the heater-evaporator is coupled to the control unit to the apparatus and a temperature probe  44  is mounted in a radial drilling  45  in the body  30  below the cavity  37  to measure the temperature of the body for adjusting, through the control unit, the power supply to the resistance heating element to enable the body to be maintained at a requisite temperature for pre-heating the air flowing through the body and flash evaporating the sterilant delivered to the body. 
       FIG. 7  of the drawings shows an alternative form of heater  13  in which the outlet from the fan  12  is coupled to an inlet  50  to a lower chamber  51  containing an electrically powered air heater  52 . At the upper end of the chamber  51  there is an annular evaporator block  53  having a central port  54  for gas flow and an evaporator plate  55  is located on top of the block. The block has a spirally wound heating element  56  embedded adjacent the surface of the block. Thus the heater  52  can be used to raise the temperature of the air flowing through the device to one level and the second heater  56  can be used to maintain the surface of the evaporator plate at the requisite temperature for flash evaporation of an aqueous solution of hydrogen peroxide. 
     The heater has an upper chamber  57  in which an outlet conduit  58  is mounted having ports  59  spaced around the conduit through which air can enter the conduit from the upper chamber as indicated by the arrows. The lower end of the conduit is closed by an air deflector  61  which partially overlies the evaporation plate and causes the air flow emerging from the port  54  in the evaporator heater to disperse outwardly over the evaporator plate before flowing upwardly and hence through the port  59  into the inlet conduit. Delivery tubes for aqueous hydrogen peroxide extend downwardly through the upper chamber  57  to stop just short of the surface of the evaporation plate to drip aqueous hydrogen peroxide onto the plate which flash evaporates and is entrained in the air flow over the plate which passes upwardly into the outlet conduit  58 . The arrangement is otherwise similar to that of  FIGS. 3 and 4 . 
     Reference is now made to  FIG. 8  of the drawings which shows the control box of the gas generator of  FIGS. 3 and 4  in greater detail. The control box comprises a casing  70  having a lid  71  shown in the open position in  FIG. 8 . The fan  11  which is of the centrifugal type is mounted in the upper end of the box and has an upwardly facing mounting plate  72  formed with an outlet port  73  to receive the evaporator  13 ,  14  with the inlet to the evaporator in communication with the port  73 . 
     A liquid pump  74  is mounted on one side of the box powered by an electric motor for delivering aqueous hydrogen peroxide to the evaporator. A mains cable connection for the unit for the various motors and other devices requiring power supply is indicated at  75 . The cable also provides couplings to the controllers  76  for the unit which are mounted on the inside of the lid  71 . 
     To ensure that contamination does not reach the enclosure from the interior of the control box for the gas generator, a fan  77  is mounted on one side of the control box to deliver air carrying sterilant from the surrounding atmosphere in the enclosure through the control box to sterilise the interior surfaces of the control box. 
     Reference is now made to  FIG. 9  of the drawings which shows in exploded form a monitoring unit for monitoring air temperature, gas concentration and humidity in the enclosure. The monitoring unit comprises a box  80  to receive the monitoring equipment and mounted on wheels  81  to enable the box to be readily manoeuvred around the enclosure and also moved from side to side where it is to be used. The box has a lid  82  formed with inlet and outlet ports  83 ,  84  respectively. The inlet port has a motor driven fan  85  disposed below the port to draw in air from the enclosure containing the dispersed sterilant to cause an air flow through the elements in the box to sterilise the interior surfaces of the box and thereby to ensure that the room or other enclosure is not contaminated by anything within the interior of the box. 
     The apparatus described particularly with reference to  FIGS. 3 to 9  is intended to be readily portable or transportable from room to room where it is to be used. It provides a source of heated air carrying hydrogen peroxide vapour sterilant directly into the room and distributes the air flow throughout the room until condensation occurs on all surfaces within the room. This includes the exposed exterior surfaces of the components disposed within compartment  66  of the apparatus by virtue of the hydrogen peroxide vapour sterilant passing through openings  68 . No external pipework connections are required to pass through walls of the room just power supply and control cables for the apparatus. No special installation requirements arise as in conventional gas generator circuit systems as referred to earlier. 
     Thus each of the components of the equipment required to sterilise a room, that is the gas generating apparatus, the gas distribution system, the instrument-module, the dehumidifier and the aeration unit are all manufactured such that they can readily be carried by a single person. 
     Reference is now made to a further form of apparatus in accordance with the invention shown in  FIG. 10 . The apparatus is mounted on a mobile trolley and comprises a gas generator  100 . Air is drawn in through a HEPA filter  101  by a fan  102  and passed into a vaporiser  103 . Inside the vaporiser the air is first heated by a heater (not shown) and then passes over an evaporation plate (also not shown) A pump  105  delivers liquid sanitant from a sanitant bottle  106  in the form of droplets onto the evaporation plate from which it is flash evaporated. The heated air carrying the sanitant vapour is passed to a distribution plenum  108  and exits to the room at high velocity through one or more nozzles  109 . 
     Provision is made either to connect a number of optical type condensation monitors  120  directly to the gas generator and hence to a control module  121  (see  FIG. 11 ), or the monitors may be connected directly to the control module. The optical condensation monitors measure the layer of condensation as it builds up on a surface or surfaces of the monitor. Connecting condensation monitors to the gas generator has the advantage of reducing the number of connections to the control module, especially when a number of gas generators are used. 
     The condensation monitors are placed around the room at the locations where the rate of condensation is the lowest. 
     A complete multiple installation is shown in  FIG. 11 , with three gas generators  100  each with eight condensation monitors  120 . Also connected to the control system is an aeration unit  122  used to remove the gas at the end of the cycle and the dehumidifier  123 . A separate instrument module  124  is also shown which has additional instrumentation to measure the gas concentration and the RH within the room. A single communications cable connects  24  all of the components to the control module. 
     The normal technique to establish if a decontamination process has been successful is to place Biological Indicators (BIs), in those parts of the chamber where it is the most difficult to achieve a kill. It is often undesirable or not permitted to place BIs in a room, but it is necessary to know that deactivation to the required level has been achieved. To overcome this difficulty condensation monitors may be used to establish that the mass and the rate of formation of condensate are sufficient to achieve deactivation of the microorganisms on the surfaces. It has been well established that once the required conditions have been achieved that the “D” value for the most resistant organisms is about two minutes. Therefore an exposure of the organisms under the correct conditions for twelve minutes will achieve a log 6 reduction in the count of viable organisms. 
     Satisfactory decontamination will only be achieved in a room if a sufficiently high rate of liquid sanitant vapour is delivered into the room to provide an adequate rate of formation of condensation. But to be assured that decontamination has been achieved it is necessary to measure the condensation levels with time in multiple locations in the room. The data from the condensation monitors together with the information from the other instruments in the room may then be used to establish that a satisfactory deactivation cycle has been completed. 
     The condensation sensors may be used in one of two ways. The first is to measure and then control the level of condensation by adjusting the liquid evaporation rate and the second is simply to use the monitor as a switch. When used as a switch it simply gives a signal when an adequate amount of condensation has formed and the process is then considered to be complete or allowed to dwell in that state giving a sufficient period during which the organisms are killed. There is a further variation to the “switch” method in which two sensors are used at each location set at different levels of condensation. The first indicates when condensation has started and the second when the level of condensation is sufficient to have caused a satisfactory level. It may then be necessary to have a “dwell” period during which the kill occurs. 
     The condensation monitors of the above apparatus are optical devices which measure the layer of condensation. An electronic device may be used instead that gives a switch signal when a known level of condensation has arrived. The switch level depends on the construction of the sensor plate. Sensor plates are single use disposable items and hence are inexpensive. The plates plug into a box which may be placed at a remote location within the room.