Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Patent
1993-04-12
1994-06-14
Fields, Carolyn E.
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
250345, G01N 2161
Patent
active
053212665
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a pneumatic, two-layer detector for use in NDIR (Non-dispersive Infrared) gas analyzers having two, coaxially arranged, gas-filled chambers with radiation permeable frontal areas, a line which couples the chambers, and a pressure or flow-rate sensor of a pneumo-electrical transducer arranged in the line.
These types of detectors are employed in gas analyzers, in which modulated infrared (IR) radiation strikes the detector through a cuvette filled with the gas being analyzed. As a rule, the gas being analyzed is a gas mixture having one or more components to be evaluated. The components to be evaluated or a gas having the same kind of absorptive capacity for IR-radiation constitute the gas filling in the detector chambers (see, e.g., German Patent 1 017 385).
Distortions in the measuring-signal occur because of so-called "cross sensitivity". That is, there are instances when the absorption bands of other components contained in the gas being analyzed overlap with those of the component to be measured. The known two-layer detector configuration, in which the two chambers are pneumatically connected back to back, improves the selectivity ratio. In this manner, the sensitivity due to the absorption bands of all disturbing components may be reduced but can not be totally eliminated. Thus there is a need for a detector for use in a gas analyzer which eliminates cross-sensitivity due to the absorption bands of other components contained in the gas being analyzed.
SUMMARY OF THE INVENTION
The present invention meets the aforementioned need by providing a pneumatic two-layer detector, which comprises two chambers, two lines, two pneumo-electric assemblies, a buffer volume, and a differential element. The first and second chambers are adapted to be filled with a gas and have a radiation permeable frontal window. Further, the two chambers are coaxial. The first line couples the first chamber with the second chamber. The first pneumo-electrical transducer includes a pressure or flow rate sensor arranged in the first line and provides an output. The second line couples at least one of the first and second chambers with the buffer volume. The second pneumo-electrical transducer includes a pressure or flow rate sensor arranged in the second line, is adapted to provide an output, and includes means for adjusting the amplification of its output. The differential element has inputs electrically coupled with the outputs of the first and said second pneumo-electrical transducers. Not only does the present invention further reduce the sensitivity to disturbing components, but it also selectively compensates one of these disturbing components to zero. The present invention does so electrically, by attenuating the signal coming from the second pneumo-electrical transducer before forming the differential. In this manner, the detector becomes insensitive to this disturbing gas.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically depicts an exemplified embodiment of the present invention.
FIG. 2 schematically depicts an alternative embodiment in which a two-beam variant of an NDIR gas analyzer includes detectors according to the present invention.
DETAILED DESCRIPTION
As shown in FIG. 1, a two-layer detector has two chambers K1 and K2, which are arranged coaxially, one behind the other. The frontal area of each chamber is provided with a radiation permeable window F. The chambers K1 and K2 are filled with the gas to be analyzed or with another gas having the same kind of absorptivity. Modulated radiation from the measuring path of rays MS enters through the window F into the chambers K1 and K2. The radiant energy is absorbed by the gas contained therein. This absorbed radiant energy leads to a warming of the gas and, thus, to an increase in pressure of the gas in the chambers. A flow-rate sensor SF1, which operates based on the anemometer principle, is arranged in a line L12 which couples the chambers K1 and K2. Together with the amplifier V1, this flow-rate
REFERENCES:
patent: 4288693 (1981-09-01), Fabinski et al.
patent: 4468561 (1984-08-01), Speeter
patent: 4742229 (1988-05-01), Weinel
Fields Carolyn E.
Siemens Aktiengesellschaft
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