Measuring and testing – Instrument proving or calibrating – Gas or liquid analyzer
Reexamination Certificate
1999-08-03
2001-05-22
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Gas or liquid analyzer
Reexamination Certificate
active
06234001
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns improved apparatus and methods for generating calibration gas.
BACKGROUND OF THE INVENTION
Most gas analyzers require periodic calibration to ensure proper performance. A typical calibration process consists of feeding a calibration gas into the inlet of a gas analyzer. The concentration of calibration gas can range from parts per billion (ppb) to parts per million (ppm) and higher.
Calibration gas can be purchased compressed in large, heavy cylinders or tanks. Because most calibration gases are volatile, tanks of compressed calibration gas are inherently dangerous. For field use, smaller and often disposable cylinders are available, but these cylinders are expensive and the quantity of contained gas comparatively small. Cylinders of calibration gas also have a limited shelf life depending on the gas composition and concentration.
As an alternative to purchasing cylinders of compressed calibration gas, permeation devices can be used. A typical permeation device comprises a small container of a highly concentrated calibration gas, usually in liquid form. Part of the container consists of a permeable material that permits molecules of the gas to pass through at a controlled rate. This rate is extremely temperature dependent. Therefore, for a constant emission, the permeation device is usually maintained in a temperature controlled oven. A carrier gas can be moved past the permeation device to produce a blended concentration of the calibration gas. Such systems are not available for all calibration gases and the temperature control issue is problematic for field use. Even in the laboratory, stabilization of a permeation system can take a considerable amount of time and effort. Furthermore, permeation devices cannot be turned off, although refrigeration can slow the emission to a level low enough to assure a reasonable shelf life.
Electrochemical cells can also be used to produce a controlled quantity of calibration gas. Although electrochemical cells stabilize quickly and are available in compact packages, they cannot be used with ammonia and other important calibration gases, and replacement cells are costly.
In view of these and other deficiencies in the art, it would be highly desirable to provide new and improved apparatus and methods for generating calibration gas.
Accordingly, it is a purpose of the invention to provide new and improved apparatus for generating calibration gas that is easy to construct.
It is another purpose of the invention to provide new and improved apparatus for generating calibration gas that is easy to use.
It is still another purpose of the invention to provide new and improved apparatus for generating calibration gas that is inexpensive.
It is a further provision of the invention to provide new and improved apparatus for generating calibration gas that uses low-cost consumables.
It is still a further provision of the invention to provide new and improved apparatus for generating calibration gas that is not temperature dependent.
It is yet still a further purpose of the invention to provide new and improved apparatus for generating calibration gas that is lightweight and portable.
It is yet a further purpose of the invention to provide new and improved apparatus for generating calibration gas that is neither chemically nor physically hazardous.
It is another purpose of the invention to provide new and improved apparatus for generating calibration gas that provides a wide range of calibration gas concentrations.
SUMMARY OF THE INVENTION
The above problems and others are at least partially solved and the above purposes and others realized in new and improved apparatus for generating calibration gas comprising a structure for containing a stream of carrier gas, a chamber coupled to the structure for containing at least part of the stream, and a wick structure contained in the chamber for receiving and holding a volatile reference liquid that, when exposed to a stream of carrier gas, evaporates from the wick structure into a gas that blends into the carrier gas to form a calibration gas. A source of volatile reference liquid is preferably coupled with the wick structure in liquid communication. A pump transfers the volatile reference liquid from the source to the wick structure at a desired flow rate and is adjustable for varying the flow rate of the volatile reference liquid. Another pump is also coupled to the structure for providing a stream of carrier gas through the structure at a desired flow rate. The other pump is also adjustable for varying the flow rate of the stream. By varying the flow rate of the stream and/or the volatile reference liquid, the concentration of volatile reference gas in the calibration gas can be specifically and efficiently controlled. A baffle chamber may be coupled to the structure for receiving and mixing the calibration gas to rid it substantially of volatile reference gas concentration irregularities.
The wick structure is preferably constructed of a porous high surface area media and comprises a stage and a wick element. The stage is preferably engaged directly to the wick element and functions to receive the volatile reference liquid from the source and conduct it evenly to the wick element. The wick element is preferably elongate and tubular for providing a large evaporative surface area.
Consistent with the foregoing, the invention also provides associated methods of generating a calibration gas.
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Lamb Edward A.
Sorensen Ian W.
Goltry Michael W.
Parsons Robert A.
Parsons & Goltry
Raevis Robert
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