Airborne contaminant indicator

An indicator system provides a visual indication of the cumulative level of an airborne contaminant. The indicator is a transparent sheath with support medium having an acidic surface and treated with a color pH indicator. As a basic contaminant is adsorbed by the medium, the medium color changes. As additional contaminant is adsorbed, the color front progresses the length of the indicator. The indicator system may be used with an adsorptive filter system to predict the life of adsorption bed assemblies. A sample flow is taken upstream of the adsorption beds and is passed through the indicator. Preferably, the flow rate is calibrated so that the rate of the color change in the indicator is proportional to the rate of depletion of the adsorption bed. By monitoring the indicator, an accurate prediction of adsorption bed life may be made.

BACKGROUND
 1. Field of the Invention
 The present invention generally relates to a method and apparatus for
 indicating the presence of airborne contaminants and accumulation of
 airborne contaminants, such as organic and inorganic bases, and more
 particularly to predicting the life of filter systems for removing the
 airborne contaminants.
 2. Prior Art
 Gas adsorption beds are used in many industries to remove airborne
 contaminants, such as organic bases, to protect people, the environment
 and often, a critical manufacturing process for the products which are
 manufactured. A specific example of an application for gas adsorption beds
 is the semiconductor industry where products are manufactured in an
 ultra-clean environment, commonly known in the industry as a "clean room".
 The manufacturing process typically requires the use of substances such as
 solvents to be used in the clean room environment. The use of these
 substances presents a problem when vapors are formed during the process
 which may contaminate the air and other processes in the room if they are
 not properly removed. In addition, many environments have several gases
 that may naturally occur in the ambient air that may contaminate the
 products and/or processes and are not removed by normal particulate
 filters. Typical contaminants that are produced by such processes are
 airborne bases, such as ammonia, organic amines and
 N-Methyl-2-pyrrolidone.
 To eliminate the problem, contaminated air is often drawn through a
 granular adsorption bed assembly having a frame and adsorption medium,
 such as activated carbon retained within the frame. The adsorption medium
 adsorbs the gaseous contaminants from the air flow and allows clean air to
 be returned to the clean room and/or process. It can be appreciated that
 the removal efficiency of such beds is critical in order to protect the
 processes and the products that are involved.
 It can further be appreciated that since the removal process involves
 passage of air through an activated carbon bed that adsorbs or chemically
 reacts with the airborne contaminants, there is no measurable pressure
 change as occurs when particulate filters are loaded. Therefore, it is
 difficult to directly monitor the status and deterioration of the
 activated carbon bed. Monitors placed downstream may detect performance,
 efficiency, or when a failure has occurred and that the adsorption beds
 are spent. However, presently available sensors may not be sensitive
 enough to work at the contaminant threshold levels which are critical in
 the semiconductor industry and are often quite costly. A problem with
 sensors having acceptable sensitivity is that they are often specific to a
 single contaminant. Although such sensors may detect a low level of one
 contaminant, others may accumulate to high levels and remain undetected.
 However, once there is an indication that an adsorption bed is spent, it
 is often too late and the process or products have often been ruined or
 damaged.
 Other systems have been devised which can monitor adsorption bed life such
 as placing the beds in series. When adsorption beds are placed in series,
 a sensor may be placed in series intermediate the two adsorption beds.
 Therefore, as one adsorption bed becomes spent and the sensor indicates
 the presence of a contaminant, the second adsorption bed is still
 effective and failures are prevented. However, such detection systems have
 several drawbacks. When two adsorption beds are used, the pressure drop is
 doubled. This may be critical in some applications. In addition, once the
 first bed has been indicated as being spent, the adsorption beds are
 normally rotated in a sometimes complicated manner. Such rotation
 increases the maintenance and down time of such a system. At other times,
 both adsorption beds may be changed out, thereby decreasing labor, but
 also shortening the useful life of the downstream adsorption beds as they
 are removed prior to being fully spent.
 Other systems utilize a sensor placed directly in the adsorption bed.
 However, in very thin adsorption beds, such a sensor may take up valuable
 space. In addition, an interface for detecting the presence of
 contaminants within an adsorption bed requires seals and can be
 complicated and expensive.
 It can be appreciated that if the filtered air can be distributed in a
 balanced, even manner over the adsorption beds, a reliable prediction of
 the expected useful duration of each bed would enable a longer change out
 interval period without failure. It can be appreciated that achieving the
 greatest possible change out interval without failure would decrease
 filter materials cost and labor costs utilized in changing the adsorption
 bed filters.
 It is desirable to have an indication of the actual amount of contaminant
 that the filter beds have been exposed to based on a known filter capacity
 and being able to accurately predict an optimal change out period for the
 adsorption beds. Such a process is more precise if the actual flow passing
 through the filters is known and the prediction based on a flow which is
 proportional to the actual flow through the adsorption beds. By sampling
 upstream of the adsorption beds, an accurate prediction of the amount of
 contaminants flowing to the adsorption beds can be made.
 It can be seen then that an indicator system is needed that detects the
 cumulative levels of airborne contaminants. Such a system should be able
 to sample a proportional amount of airborne contaminants that are flowing
 past an adsorption bed device. Such an indication system should provide a
 clear visual indication of the bed usage and indicate when the adsorption
 beds should be changed. In addition, as contaminant concentrations may
 vary, the system should provide a real time indication of cumulative
 contamination levels for predicting the change out interval based on the
 actual contaminant flow past the adsorption bed. It can also be
 appreciated that such a system should provide for a variable safety factor
 to ensure that adsorption bed failures do not occur. Such a system should
 also be able to measure the presence and cumulative level of such
 contaminants in an environment and provide a visual indication. The
 present invention addresses these as well as other problems associated
 with indicating the presence of airborne contaminants.
 SUMMARY OF THE INVENTION
 The present invention is directed to a system and method for indicating the
 presence of airborne bases. Such a system may be used in clean rooms and
 other applications wherein air quality is critical.
 The indicator system includes a valving arrangement to control flow into
 and out of the system. A probe is utilized to obtain a sample of air. In
 one embodiment, a sample taken is proportional to the actual flow rate of
 the air being sampled. Therefore, the system can be used as a predictor
 with greater accuracy as contamination levels vary.
 The system uses a flow meter to monitor and calibrate the system. An
 indicator device includes a sheath, such as a tube, having an indicating
 medium therein. As contaminants enter the tube from a first end, the
 specially treated medium will change color to indicate the presence of
 contaminant. Because the tube is preferably substantially transparent, the
 color changing medium within the sheath is visually detectable. As
 additional contaminant passes through the medium, additional treated
 medium is affected and changes color to indicate the increasing levels.
 Thus, an advancing front of color is observed. A calibrated flow meter
 maintains the flow rate through the indicator proportional to the actual
 flow. A pump, for example an ejector type pump actuated by compressed air,
 maintains flow through the system.
 As typical clean room processes emit ammonia, amines and other bases, it is
 desirable to provide a medium capable of detecting bases. Preferably, the
 medium is a silica gel or zirconium oxide having an acidic surface. This
 acidic surface is achieved by treating the medium with an acid, such as
 sulfuric acid. The acidic medium is then treated with a pH sensitive
 indicator, such as bromophenol blue, which changes color from yellow to
 blue upon an increase in pH. Therefore as airborne bases pass through the
 medium, a front of color, indicating spent medium, advances along the
 tube.
 The adsorption type beds commonly used in clean room settings are designed
 to not clog in the same manner as normal filters. Unfortunately, because
 of this, the beds are difficult to monitor to determine their change
 interval. Although sensors may be utilized downstream of the filters to
 detect when the adsorption beds are spent, such sensors are expensive and
 often do not have sufficient sensitivity to monitor the low levels of
 airborne contaminants. In addition, control of the clean room processes
 may be so critical that by the time the expiration of the adsorption beds
 is detected, damage may have already occurred to the process. Therefore,
 it would be advantageous to be able to predict the interval when the
 adsorption beds will need to be changed. This will allow scheduling of the
 downtime required for the maintenance and will maximize the chance of
 changing of the filters prior to any failure.
 To predict an interval, an indicator probe is placed upstream of the
 absorption beds to sample the air flow. Since the air passing through the
 indictor has the same composition as the air passing through the
 absorption beds, the advancement of the color change in the indicator
 probe can be an accurate indicator for the deterioration rate of the bed.
 As proportional flow is maintained, the color front on the indicator
 medium advances at a rate proportional to the deterioration of the
 adsorption bed. Thus, when the rate of the front advancement in the tube,
 the flow rate over the adsorption beds, and the bed deterioration rate,
 are known, the monitoring indicator can predict when the adsorption bed
 interval expires.
 These features of novelty and various other advantages which characterize
 the invention described herein are pointed out with particularity in the
 claims. However, for a better understanding of the invention, its
 advantages, and the objects obtained by its use, reference should be made
 to the drawings which form a further part hereof, and to the accompanying
 descriptive matter, in which there is illustrated and described a
 preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring now to the drawings, and in particular FIG. 1, there is shown a
 system, generally designated 10, for indicating the presence and level of
 an airborne contaminant. The system includes a probe 12 for sampling air
 from a duct or ambient air volume 100. The indicator system 10 also
 includes a flow meter 16 with a control valve 14 between the probe 12 and
 the flow meter 16. An indicator 18 is downstream of the flow meter 16. A
 pump 40 maintains flow through the indicator system 10. In a preferred
 embodiment, the pump is an ejector type pump such as Model No. LX-5,
 available from Piab Company, which is actuated by a compressed air supply
 44. A valve 42 acts as a safety valve between the compressed air supply 44
 and the pump 40. The valve 14 controls flow to the system 10 and is
 calibrated by flow meter 16. In a preferred embodiment, the flow is
 proportional to the sampled flow. The pump 44 returns the sample air
 through a vent 46 to a duct and back to the sampled or ambient air 100. It
 can be appreciated that for some monitoring applications, such as when
 sampling ambient air or flows, the pump 40, flow meter 16 and valve 14 may
 not be used. Similarly, various other monitoring applications may not
 required the use of all the equipment described herein.
 Referring now to FIG. 2, the indicator system 10 may be used with an
 adsorption filtering system 60. Such a filtering system 60 typically
 includes a housing 62 with an access door 66. The interior of the housing
 has an inlet plenum 64 extending vertically past a stack of adsorption
 beds 70. Flow passes from the inlet plenum 64 in parallel through the
 adsorption beds 70 to an outlet plenum 72. The treated air is circulated
 back to the ambient air. Another system is also shown in U.S. Pat. No.
 5,290,345 to Osendorf and assigned to Donaldson Company, assignee of the
 present invention.
 In the configuration shown in FIG. 2, the probe 12 is placed in the inlet
 plenum to intercept air flow prior to it being treated through the
 adsorption beds 70. However, such systems may be used to measure
 contaminant levels in the ambient air or at different locations in a
 filter system, such as downstream of the adsorption beds 70. In the
 configuration shown, the indicator system 10 can be used as a predictor of
 adsorption bed life. It can be appreciated that the adsorption beds 70 are
 positioned to receive a balanced air supply through the beds so that the
 deterioration rate of each adsorption bed 70 is equal. The indicator
 system 10 is used as a predictor of adsorption bed life and changeover
 interval, rather than as an indicator of when failure has already
 occurred, as in the prior art sensors. As explained hereinafter, the
 indicator 18 also gives an indication of the deterioration level of the
 adsorption beds 70.
 Referring now to FIGS. 3-5, there is shown greater detail of the indicator
 18. The indicator 18 includes a tube 20 which preferably has a
 transparency level sufficient to see a colormetric indicating medium 22
 within the tube. In the preferred embodiment, the tube 20 is quite narrow,
 on the order of an 1/8 inch outside diameter, an inside diameter of 0.08
 inches, and made from a suitable material such as Teflon.TM.. Glass,
 plastic and sufficiently transparent ceramic materials may also be used.
 At each end of the tube 20 are porous plug elements 32 such as glass wool,
 which maintain the indicating medium 22 tightly packed within the tube 20.
 The tube 20 also includes fittings 30 such as Swagelok.TM. elements or
 other suitable fittings for connecting to mating fittings. The fittings 30
 provide for easy removal and replacement of the indicator 18 when the
 indicating medium 22 is spent, or when the associated adsorption beds are
 changed and it is desired to have a fresh indicator 18 associated with the
 fresh adsorption bed assemblies 70. The overall length of the indicator 18
 is about 7.5 inches, but may range from about 2 to 12 inches.
 The medium 20 is typically a material having an acidic surface which is
 designed with advancement rate which coincides with the life of the filter
 bed system 70. Preferably, the medium comprises silica gel or zirconium
 oxide (zirconia) that has been treated to provide an acidic surface.
 Examples of other suitable medium include glass beads or bubbles, porous
 polymers, alumina, and ceramic materials. It can be appreciated that by
 calibrating the flow rate, tube size, medium type, medium mesh size and
 surface area, the amount of surface acid, and/or the flow rate through the
 indicator 18, it is possible to predict an optimum interval for changing a
 filter bed assembly 70 by monitoring the amount of spent indicator medium
 24.
 In one preferred embodiment, the indicating medium 22 is a coated or
 impregnated silica gel. A specific mesh and surface area of the indication
 medium is chosen for the specific needs of each indicator system 10. It
 can be appreciated that a smaller particle size will provide for a sharper
 divide in the color change of the advancing front of affected medium 24,
 but will result in a higher pressure drop for the sampled air. An example
 of a typical medium for airborne bases is a 100/200 mesh silica gel or
 beads which has a specific surface area of approximately 500 square meters
 per gram. To prepare the gel, it is first immersed in a sulfuric acid
 solution for approximately two hours after which the excess acid is poured
 off and the silica gel is washed with distilled water several times. It
 can also be appreciated that other types of acid such as phosphoric acid,
 nitric acid, acetic acid, hydrochloric acid, trifluoromethane sulfonic
 acid, and trifluoroacetic acid may be used depending on the pH range which
 is desired, the medium being used and the indicator being used. The final
 solution of the silica gel is filtered and dried. Dried samples are wetted
 with an aqueous solution of isopropanol, to which is added a known amount
 of an appropriate indicator. An example of an indicator that has a color
 change at an appropriate pH is bromophenol blue.
 In another preferred embodiment, the indication medium 22 is a coated or
 impregnated zirconia particle. Similar to the silica gel, a smaller
 particle size will provide a sharper divide in the color front. To acidify
 the surface of the zirconia particle, the zirconia is boiled in acid, for
 example, sulfuric acid, to provide a sulphate-zirconia (ZrO.sub.2
 /SO.sub.4.sup.-2) which is commonly referred to as a "superacid". To
 prepare the zirconia, the particles are boiled in a sulfuric acid solution
 for approximately three hours after which the particles are washed with
 distilled water several times and dried. In general, zirconia is capable
 of being modified to be strongly acidic, more so than silica gel. A color
 change indicator is applied to the surface of the acidic particle from an
 aqueous isopropanol solution.
 It can be appreciated that depending on the needs of the system and the
 type of contaminants that are being removed, that indicator mediums such
 as m-cresol purple, thymol blue, xylenol blue, cresol red, bromothymol
 blue, resolic red, phenolphthalein, thymolphtalein phenol red, and other
 colormetric indicators changing at a different pH may also be utilized. In
 addition, it can be appreciated that the concentration of the indicator
 that is used depends on the intensity of the color that is desired. It has
 been found that a 0.5% by weight bromophenol blue concentration works
 well, however concentrations of 0.1% to 5% by weight are useable.
 To apply the color changing indicator to the particle, a solution of the
 acidified medium (for example silica gel or zirconia), a water/isopropanol
 mixture and the color indicating substance (for example bromophenol blue)
 is stirred for several minutes. After the mixture is allowed to stand, the
 medium is decanted and washed with isopropanol. The resulting medium is
 dried, for example in an oven at approximately sixty degrees (60.degree.)
 Celsius. If silica gel medium is treated with bromophenol blue, the
 resulting silica gel is bright yellow, but upon exposure to a base such as
 ammonia, the color will change from yellow to blue. Higher concentrations
 of the bromophenol blue can yield an orange silica gel which changes to a
 blue/purple color upon exposure to a base. The indicating medium 22 is
 vacuum packed into the tube 20 and retained by the plugs of glass wool 32.
 Once a tube is prepared, it can be calibrated by exposing it to a
 controlled air flow that contains a known amount of the contaminant or
 airborne base. With the flow meter 16, the amount of sample air passing
 through the system may be measured. The rate of the advancing color front
 28 between spent medium 24 and fresh medium 26, as shown in FIG. 4, is
 measured as a function of the amount of contaminant. A graphical curve can
 be derived to estimate the relation between the color change in the
 indicating medium 22 and the deterioration rate of the absorption bed.
 This curve can be compared to the known capacity of adsorption bed 70 and
 the expected breakthrough time or failure point of adsorption bed 70 can
 be predicted. The flow rate through the indicating system 10 (including
 indicator 18) can be increased or decreased as desired depending on the
 safety factor required by manipulating flow meter 16 and valve 14.
 Referring to FIGS. 3-5, it can be appreciated that when the indicator 18 is
 fresh and unexposed to any base, the indicating medium 22 shows an
 unchanged medium 26 of the initial color, typically yellow if the
 indicator used is bromophenol blue. As shown in FIG. 4, as more
 contaminant passes through the indicator 18, the medium exposed to the
 contaminant base changes color and the spent medium 24 can be visually
 detected through the tube 20. A front 28 provides a clearly visually
 perceptible line advancing along the tube 20. As shown in FIG. 5, when the
 medium 22 is substantially entirely affected, all of the medium 22 has
 changed color to show the spent medium 24, indicating that the adsorption
 bed filter 70 should be changed.
 It can be appreciated that the above example is for a system for detecting
 airborne bases such as ammonia. However, it can be appreciated that other
 substances may be utilized for measuring the presence of other types of
 airborne compounds and used just as effectively. In addition, although the
 above described example provides for predicting the life of an adsorption
 bed assembly, the indicator system may be used to measure the presence of
 a contaminant and the cumulative concentration of such a contaminant over
 time and in ambient conditions that do not have a flow or an adsorption
 device.
 Referring now to FIG. 6, there is shown a method for predicting the life of
 an adsorption filter using the indicator system 10 shown in FIG. 1. When
 new adsorption medium, such as for example, activated carbon, is installed
 in an adsorption bed 70, a fresh indicator tube 18 having fresh medium 24
 is also installed. Using flow meter 16 and control valve 14, the flow
 through the indicator 18 is calibrated to provide a relation between the
 rate of color change in the indicator 10 with the rate of deterioration of
 the adsorption bed 70. Preferably, a time safety factor is included in the
 calibration. After the initial installation and calibration, further
 calibration is not necessary unless flow through the adsorption bed 70
 changes. However, monitoring of the flow to ensure that the system is
 working properly can be done at any time with flow meter 16.
 After calibration, the flow meter 16 and valve 14 ensure that the proper
 flow is maintained so that the flow through the indicator 18 is
 proportional to and provides a relation to the flow through the absorption
 bed 70. Over time, as the indicator tube 18 is exposed to more
 contaminants, the front 28 of spent medium 24 moves along the length of
 the tube 18 as shown in FIGS. 3-5. This moving color front provides a
 visually perceptible indication which can be monitored. Because the rate
 of advancement of the front 28 is comparable to the deterioration rate of
 the adsorption bed 70, it can be appreciated that the usage and life of
 the adsorption beds 70 can be predicted by viewing the indicator 18. When
 the indicator medium 22 is substantially spent by a total change in color,
 as shown in FIG. 5, system operators know that it is time to change the
 adsorption beds 70.
 Replacement of the indicator 18 is preferably done at the same time as the
 media for the adsorption bed 70 are changed. If the flow within the
 indicator system 10 has already been calibrated to the flow of the
 adsorption filtering system, it is not necessary to recalibrate, so that
 the indicator tube 18 is the only element replaced at the same time as the
 adsorption beds 70. A new indicator tube 18 serves to predict the rate of
 usage of the new adsorption bed 70.
 Although the indicator 18 has been described in detail as a predictor for
 the life of adsorption bed 70, it should be understood that it is also
 possible to place indicator 18 downstream of bed 70 to function as a
 detector or contaminants, rather than a predictor of bed life. In such a
 configuration, indicator 18 would change color if any contaminants are
 found to remain in the air flow stream after passing through the
 adsorption filtering system. As a downstream indicator, indicator 18 would
 typically be shorter (approximately one to 2 inches), because the goal is
 not to observe the color front moving the length of the indicator tube,
 but to see any color change. In some systems, for example, if the
 contaminant is on the order of about 100 parts per billion (ppb), the bed
 has already failed and the overall system may be adversely affected by the
 time indicator 18 shows any color change. For such an embodiment,
 indicator 18 may not be desirable in the downstream position. However, for
 some systems where the contaminant is about 5-10 ppb, color change in
 indicator 18 would warm that the bed is close to its failing point. The
 examples given above are examples used for the purpose of description only
 and should not be read as limiting values.
 It is to be understood, however, that even though numerous characteristics
 and advantages of the present invention have been set forth in the
 foregoing description, together with details of the structure and function
 of the invention, the disclosure is illustrative only, and changes may be
 made in detail, especially in matters of shape, size and arrangement of
 parts within the principles of the invention to the full extent indicated
 by the broad general meaning of the terms in which the appended claims are
 expressed.