PRESSURE-MODULATED MULTIPLE GAS ANALYZER

• US 3,728,540 A
• Filed: 08/27/1971
• Issued: 04/17/1973
• Est. Priority Date: 08/27/1971
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Marion N. Todd, Milder D. Michael
• Inventors: Todd, Marion N., Milder, D. Michael
• Primary Examiner(s): Wibert, Ronald L.
• Assistant Examiner(s): McGraw, V. P.
• Application Number:
  US05/175,657
• Time in Patent Office: 599 Days
• Field of Search: 356/51,97 250/43.5R
• US Class Current: 250/343
• CPC Class Codes: G01N 21/3518 Devices using gas filter correlation techniques; Devices using gas pressure modulation techniques

Abstract

Small, inexpensive apparatus is disclosed for precisely analyzing gas samples through radiation-absorption measurements, employing pressure modulation, a sealed calibration gas cell optically in series with the sample cell, synchronous demodulation, and preferably time-resolved spectral analysis and a captive computer for matrix inversion to read out gas constituent concentrations directly.

Claims

1. Apparatus for analysis of gaseous specimens, comprising:

a radiation source and detector, and means defining a radiation path therebetween;

means defining a sample enclosure, positioned on the radiation path and at least partially radiation-transparent, for containing such gaseous specimen;

means for supplying a flow of such gaseous specimen to the sample enclosure;

means defining a sealed calibration enclosure positioned on the radiation path and at least partially radiation-transparent, for containing a reference substance;

whereby radiation from the source sequentially traverses both enclosures in reaching the detector;

modulation means for cyclically varying gas pressure in the sample and calibration enclosures, without displacing macroscopic objects in the radiation path;

control means for selecting only one-at-a-time of the two enclosures to be acted upon by the modulation means;

wheReby the detector receives a cyclically varying radiant flux and generates a correspondingly varying output signal, whose amplitude depends on the constitution of the specimen when the control means select the sample enclosure, and whose amplitude depends on the constitution of the reference substance when the control means select the sealed calibration enclosure; and

utilization means responsive to the detector, for registering a calibration parameter when the control means select the sealed calibration enclosure and for registering a specimen parameter when the control means select the sample enclosure.

2. The apparatus of claim 1, also comprising:

means narrowly limiting the spectral interval of radiation reaching the detector, whereby each parameter registered is correlated with a respective single narrow spectral interval of predetermined nominal wavenumber.

3. The apparatus of claim 2, wherein:

the limiting means function to select a plurality of narrow spectral intervals, whereby a plurality of calibration parameters is registered, each correlated respectively with a single narrow spectral interval of predetermined nominal wavenumber, when the control means selects the sealed calibration enclosure; and

a corresponding plurality of likewise spectrally correlated specimen parameters is registered when the control means select the sample enclosure; and

the utilization means comprise means for analyzing and comparing the two corresponding sets of pluralities of parameters to derive indices of the specimen constitution.

4. The apparatus of claim 3, wherein:

the analyzing and comparing means take account of the ratio of modulation amplitudes in the two enclosures, and of the known constituency and quantity of reference substance in the calibration enclosure.

5. The apparatus of claim 3, wherein:

the limiting means operates to select the plurality of narrow intervals serially; and

the utilization means comprise storage means to retain the pluralities of parameters in their respective correlations during such serial operation.

6. The apparatus of claim 1, wherein:

the control means select each one of the two enclosures for periods of time corresponding to many cycles of the modulation means;

whereby the detector output signal performs many cyclical variations corresponding to one of the two enclosures before the control means operates to select the other enclosure; and

the utilization means comprise means for averaging the detector output signal over many cyclical variations, thereby minimizing the noise contribution to the registered parameters.

7. The apparatus of claim 1, also comprising:

phase-sensitive means, synchronous with the modulating means, for causing the utilization means to receive signals corresponding substantially only to variations in radiant flux reaching the detector in appropriate phase relationship with the gas pressure variations;

thereby minimizing the effects of spurious signals at modulation frequency, and at other frequencies, due to radiant flux variations not caused by varying gas pressure in one of the enclosures; and

also minimizing the effects of modulation-frequency and other-frequency spurious signals arising stochastically and arising systematically in the operation of the detector.

8. The device of claim 1, wherein:

the modulation means comprise electrically excited acoustic drivers in the respective enclosures; and

the enclosures are formed to be acoustically resonant at the frequency of cyclical variation of the gas pressure.

9. The device of claim 8, wherein:

the enclosures comprise tuned ports and are thereby dimensionally substantially smaller than one acoustic wavelength at atmospheric pressure and at the said frequency.

10. A small, inexpensive, hand-portable device for sensitive monitoring of air pollution and other gas constituents, comprising in combination:

a radiation source and means for projecting radiation therefrom along a radiation path to a radiation detector;

a calibration cell interposed on the radiation path and adapted for sealed containment of a reference substance;

a sample cell interposed on the radiation path and adapted for flow-through containment of sample gas for monitoring;

a pump connected to impel sample gas into the sample cell;

a cyclically driven valve for periodically varying sample-gas pressure in the sample cell, whereby the sample cell is pressure-modulated;

a chamber communicating with the calibration cell via a compliant seal;

a two-way valve for redirecting the pump'"'"''"'"'s sample gas flow output to pass into the chamber instead of the sample cell;

a cyclically driven valve for varying periodically the gas pressure in the chamber when the pump output is passing into the chamber, thereby to vary the reference-substance pressure in the calibration cell via the compliant seal, whereby the calibration cell is pressure-modulated;

a spectral analyzer interposed on the radiation path for permitting only radiation in one narrow spectral band at a time to reach the detector;

an amplifier receiving and processing electrical signals from the detector and also responsive to the time phase at which the cyclically driven valves operate, to process substantially only signals which correspond to maximum radiation transmission when the pressure in the pressure-modulated cell is minimum, and minimum radiation transmission when the pressure in the pressure-modulated cell is maximum;

a computer responsive to processed signals from the amplifier and also to the instantaneous condition of the spectral analyzer, for generating and storing parameters related to signals processed while the pump output is passing into the chamber, using these stored parameters as calibration data in computing sample-gas-constituent data while the pump output is passing into the sample cell, and storing the gas-constituent data for subsequent readout.

11. The device of claim 10, wherein:

the sample cell is a multi-pass type;

the spectral analyzer is a variable interference filter consisting of an annular interference filter, whose thickness is different at different locations along the annulus, and which is driven in rotation about the axis of the annulus by a motor; and

a shaft encoder functionally connected to the motor, to provide information to the computer as to the instantaneous position of the filter, this position being the said instantaneous condition.