Measuring and testing – Gas analysis
Reexamination Certificate
1999-05-06
2001-04-24
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
C073S023210
Reexamination Certificate
active
06220076
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an instrument for measuring the difference in concentration of a selected gas, such as O
2
, between two flowing gas streams, and use of the instrument to measure the rate of production or consumption of a selected gas from living or non-living materials. This invention also contemplates measurement of absolute concentrations of a selected gas in each of the flowing gas streams.
BACKGROUND OF THE INVENTION
Many industrial, medical, biological, and research situations require measurements of concentrations of selected gases in flowing gas streams. Such measurements may comprise determining the absolute concentration of a selected gas in a flowing gas stream, or the difference in concentration of a selected gas between two flowing gas streams. The latter is typically employed in research and biological applications where, for example, quantification of the O
2
consumption or production by biological material is required. Measurements involve placing biological material in a cuvette containing a known concentration of O
2
, and monitoring changes in the atmosphere within the cuvette with time. Preferably, an open gas exchange system is employed, which involves placing the biological material in a cuvette through which gas of known composition flows at a measured rate. The O
2
concentration of the effluent gas from the cuvette is monitored by an O
2
analyzer, and the difference in O
2
concentration between the input and effluent gases multiplied by the flow rate through the cuvette gives a measure of the rate of O
2
exchange. If the O
2
analyzer used in the open system is itself a flow-through instrument, the O
2
concentration in the effluent gas stream can be monitored continuously, and real-time measurements of O
2
exchange can be performed.
The most accurate method of measuring O
2
exchange in an open flow gas exchange system is to use a differential O
2
analyzer. Such instruments continuously monitor the difference in O
2
concentration between a reference gas stream and a branch of the reference gas stream which has passed through a cuvette containing the biological material under study. Where large oxygen differentials between the reference and sample gas streams occur, sensitivity of the differential analyzer is not critical and instruments such as those containing either paramagnetic O
2
sensors (e.g. the Oxygor™ 6N, Maihak AG, Hamburg, Germany) or zirconium oxide sensors (e.g. Model S-3A/II, Servomex Company, MA 02062, USA) may be used. The sensitivities of these instruments are limited; the Oxygor 6N can resolve a minimum O
2
differential of only 100 ppm O
2
when air is used as the reference gas, and under the same conditions the Servomex S-3A/II has an accuracy limit of only ±30 ppm O
2
in differential mode (note that 1 PaO
2
is approximately equivalent to 10 ppm O
2
). However, neither instrument has the sensitivity required to measure very small O
2
differentials (e.g., less than 10 ppm) that occur when the biological material under study has a low metabolic rate, or the sample is very small. Also, both types of differential O
2
analyzer are essentially laboratory-based intents which are not readily adaptable for field use, as each requires AC power and stable environmental conditions for most accurate function. They also require calibration by laboratory-based calibration systems involving compressed gases and/or gas mixing instruments. This adds to the considerable expense of the analyzers.
The differential O
2
analyzer described in our U.S. Pat. No. 5,542,284, issued Aug. 6, 1996, was developed to overcome the sensitivity limitations of the above-mentioned analyzers. Using O
2
sensors, which are O
2
cells that operate on the principle of a lead-oxygen battery, the prior device permits measurement of as little as 2 ppm differential in O
2
concentration between two flowing gas streams that contain 21% O
2
(210,000 ppm). While the prior analyzer provides for measurement of very small differentials in O
2
concentration, it has other limitations. For example, it is not possible to obtain an absolute measurement of the O
2
concentration in either of the sample and reference gas steams. Since many applications require information about the absolute O
2
concentration in the gas streams, this is a drawback of the prior design.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome the limitations of the prior art outlined above by providing an inexpensive differential gas analyzer, adaptable for use in the laboratory or in the field, that is capable of continuously measuring differential and absolute concentrations of a selected gas in reference and sample flowing gas streams containing any partial pressure of the selected gas, and capable of resolving differentials of less than 0.5 Pa in a background range from about 10 Pa to about 100 kPa of the selected gas.
SUMMARY OF THE INVENTION
According to a broad aspect, the present invention relates to an apparatus for measuring the differential in concentration of a selected gas between a first flowing gas stream and a second flowing gas stream, comprising: first and second sensors for the selected gas respectively disposed in the first and second flowing gas streams, the gas sensors being connected to each other in series with electrodes of same polarity connected; and means for amplifying a differential signal produced across both of the sensors; wherein the differential signal is indicative of the differential in concentration of the selected gas. Preferably, the electrodes of same polarity are cathodes. According to a preferred embodiment of the invention, the apparatus further comprises means for applying a voltage to the amplifier means to offset the differential signal without affecting a calibrated gain setting. Also according to a preferred embodiment of the invention, the selected gas is oxygen.
An apparatus in accordance with the invention further comprises at least one amplifier connected to a said sensor, the amplifier providing a signal proportional to absolute concentration of the selected gas in the flowing gas stream corresponding to the sensor. In accordance with a preferred embodiment, the apparatus comprises first and second amplifiers respectively connected to the first and second sensors, to obtain signals proportional to absolute concentrations of the selected gas in each of the first and second flowing gas streams.
According to another aspect of the invention, there is provided a method for measuring the differential in concentration of a selected gas between a first flowing gas stream and a second flowing gas stream, comprising: respectively disposing first and second sensors for the selected gas in the first and second flowing gas streams, the gas sensors being connected in series with electrodes of same polarity connected; and amplifying a differential signal produced across both of the sensors; wherein the differential signal is indicative of the differential concentration of the selected gas.
In accordance with yet another aspect of the invention, there is provided a method for measuring the absolute concentration of a selected gas in a first flowing gas stream and in a second flowing gas stream, comprising: respectively disposing first and second sensors for the selected gas in the first and second flowing gas streams, the gas sensors being connected in series with electrodes of same polarity connected; and amplifying a signal produced across each of the first and second sensors; wherein the signal across the first sensor is proportional to the absolute concentration of the selected gas in the first flowing gas stream, and the signal across the second sensor is proportional to the absolute concentration of the selected gas in the second flowing gas stream.
In a preferred embodiment of the invention, the method further comprises measuring the differential in concentration of the selected gas between the first flowing gas stream and the second flowing gas stream, wherein the differential in concentr
Dowling Adrian N.
Hunt Stephen
Layzell David B.
Young Roy A.
Garber Charles D
Hicks Richard J.
Miernicki Steeg Carol
Queen's University at Kingston
Scribner Stephen J.
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