Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
2003-04-28
2004-06-01
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
Reexamination Certificate
active
06742378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for calibration of instruments. More particularly, this invention relates to a method for calibration of instruments that monitor the concentration of a sterilant, e.g., hydrogen peroxide, in a system.
2. Discussion of the Art
Aseptic processing of consumable products, such as nutritional compounds and food products, is typically effected by separate sterilization of the products and the containers within which the products are packaged. Subsequent to sterilization, the sterilized products are placed in sterilized containers and sealed in a sterile environment for shipment, storage, and use.
Sterilization of such containers, which may include sterilization of separate closures as well, can be performed efficiently by use of a sterilant, such as hydrogen peroxide (H
2
O
2
) vapor, prior to the introduction of the desired sterilized products into the containers. In such a process, the containers are introduced into a sterilization apparatus in which the containers are flushed with hydrogen peroxide vapor. The containers are subsequently flushed with warm air or any other fluid suitable for achieving desirably low levels of residual hydrogen peroxide. This general procedure is highly effective in achieving sterilization of the containers and can be performed on any other articles that will come into contact with the material to be introduced into the containers.
Notwithstanding the effectiveness of sterilization by hydrogen peroxide, accurate monitoring of concentration levels of hydrogen peroxide vapor can be problematic. Problems in monitoring the concentration of hydrogen peroxide vapor result in part from changes in the physical and chemical properties of hydrogen peroxide vapor under processing conditions and the decomposition of hydrogen peroxide vapor upon contact with surfaces of various objects within the processing area. As such, undesired deviations of the concentration of hydrogen peroxide vapor from a process set point, along with excessive decomposition of hydrogen peroxide vapor, can result in loss of sterility of the containers and surrounding aseptic processing area. Moreover, hydrogen peroxide vapor is corrosive in nature, and thus excessive concentration levels of hydrogen peroxide may bring about detrimental effects to the equipment in and surrounding the processing area and surfaces of objects within the processing area. Furthermore, in accordance with government standards, subsequent use of sterilized containers requires low levels of residual sterilant.
Heretofore, detection systems for hydrogen peroxide vapor have been undesirably bulky, as exemplified by conventional near infrared (NIR) analysis apparatus. In addition, current off-line testing methods are typically too slow for monitoring levels of sterilant with sufficient accuracy. Previous arrangements have not allowed real time monitoring throughout an aseptic processing cycle, and in particular, have not been capable of monitoring concentrations of hydrogen peroxide vapor within the sterilization apparatus at select locations along the sterilant supply system during actual operations. However, U.S. Pat. No. 5,608,156 and Taizo et al., “Application of a Newly Developed Hydrogen Peroxide Vapor Phase Sensor to HPV Sterilizer”, PDA Journal Of Pharmaceutical Science & Technology, Vol. 52, No. 1/January-February 1998, pp. 13-18, describe methods of detecting the concentration of hydrogen peroxide vapor and an apparatus therefor that appear to address some of the foregoing problems.
The concentration of sterilants detected within a system is generally a function of various environmental parameters, such as, for example, temperature, relative humidity, and various measurement conditions, such as, for example, proximate location of measurement. Conventional detection systems for sterilant typically cannot or do not account for fluctuations of environmental parameters and measurement conditions. However, such fluctuations can substantially affect the results of signal generation and data collection when commercially available sensors and equipment are used. It is therefore beneficial to maintain operating parameters proximate the location of measurement as uniform as possible during data collection.
U.S. Ser. No. 09/443,768, filed Nov. 9, 1999, entitled STERILANT MONITORING ASSEMBLY AND APPARATUS AND METHOD USING SAME, incorporated herein by reference, describes an integrated system for determining the concentration of hydrogen peroxide for aseptic process validation, control, and monitoring. This system is compact and can be used for on-line determination of the concentration of hydrogen peroxide. The system requires a unique calibration procedure at regular intervals to guarantee reliable and accurate test results. This system utilizes a sensor having elements made of SnO
2
. When SnO
2
is heated to a high temperature, around 400° C., in the absence of oxygen, free electrons flow easily through the grain boundary of the SnO
2
particles. In clean air, oxygen, which traps free electrons by its electron affinity, is adsorbed onto the surface of the SnO
2
particle, forming a potential barrier in the grain boundaries that restricts the flow of electrons, thereby causing the electronic resistance to increase. When the sensor is exposed to hydrogen peroxide vapor, SnO
2
adsorbs its gas molecules and causes oxidation. This lowers the potential barrier, allowing electrons to flow more easily, thereby reducing the electrical resistance. Thus, the sensor uses an indirect method to measure the concentration of hydrogen peroxide vapor.
Voltage data from the output of the sensor must be compared to a database derived from a calibration process. The output of two different sensors cannot be compared directly without calibration. The calibration procedure uses several representative points (i.e., concentration at a given voltage) to establish a mathematical relationship that covers a specific test window. Only by means of calibration can the output voltage of a sensor be converted to a value of concentration.
Calibration procedures are important for minimizing deviations caused by such components as semiconductor chips, batteries, and signal conditioning circuits in a sensor in a portable detection system. Calibration procedures are important for minimizing deviations caused by such components as temperature and humidity compensation circuits, heating coils, data recording systems, and memory chips in a sensor in a fixed detection system.
If the calibration method is not reliable, the concentration of hydrogen peroxide vapor detected by a sensor might be misleading. In turn, an erroneous determination of the concentration of hydrogen peroxide vapor can bring about contamination in the operation system and result in spoilage. For example, a drop in voltage in the response of the sensor caused by an increase in the rate of flow of air may be interpreted as a decrease in the concentration of hydrogen peroxide vapor in the system. This apparent decrease may cause the controls in the system to increase the quantity of hydrogen peroxide delivered, thereby providing an excessive amount of hydrogen peroxide vapor. An excessive amount of hydrogen peroxide in the system may result in an excessive amount of residue. Conversely, an increase in voltage in the response of the sensor may result from a decrease in the rate of flow of air. If the delivery rate of hydrogen peroxide is correspondingly reduced, a breach in the sterility of the system may occur.
Calibration of sensors one at a time is inefficient, and, consequently, costly. It is well-known that no two sensors chosen at random are likely to be identical. Accordingly, it would be desirable to find way to calibrate individual sensors accurately and at reasonable cost. In addition, it would be desirable to find a way to calibrate individual sensors so that one or more of them could be used in portable units. The use of a greater number of portable units is desirable so that measurement of
Boruszewski Dennis V.
Guntrip David P.
Mondiek David A.
Wang Jianjun
Abbott Laboratories
Raevis Robert
Weinstein David L.
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