Respiratory gas analyzer

• US 5,464,982 A
• Filed: 03/21/1994
• Issued: 11/07/1995
• Est. Priority Date: 03/21/1994
• Status: Expired due to Term

• Current Assignee: LumaSense Technologies Holdings, Inc. (Advanced Energy Industries, Inc. (China)), AC Limited (Emirate of Dubai (United Arab Emirates))
• Original Assignee: Andros, Inc. (Advanced Energy Industries, Inc. (China))
• Inventors: Drucker, Steven, Williams, Kevin, Goder, Alexey, Major, Emery, Christensen, Ken, Khalili, Davood
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): Glick, Edward J.
• Application Number:
  US08/215,481
• Time in Patent Office: 596 Days
• Field of Search: 250/339.12, 250/339.13, 250/343, 250/345, 250/346, 250/349, 250/353, 250/252.1 A, 250/339.01, 250/339.03, 250/339.04
• US Class Current: 250/343
• CPC Class Codes: A61B 5/0836 Measuring rate of CO2 production
• CPC Class Codes: G01N 2021/158 Eliminating condensation
• CPC Class Codes: G01N 21/15 Preventing contamination of the components of the optical system or obstruction of the light path
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence

Abstract

An improved apparatus and method for measuring the concentration of carbon dioxide in respiratory gases. The apparatus is a non-dispersive gas analyzer attached in series with the respiratory airway which is universally compatible with any host system. The analyzer includes a housing which supports a sample cell containing the respiratory gases, and which contains an infrared radiation source encased in shock absorbing material, means for directing the radiation into a collimated beam path through the sample cell, means for splitting the beam path and directing it towards a pair of infrared detectors for measuring the amount of absorption of carbon dioxide in the sample cell. The analyzer is further improved by the use of two heating servos, one for regulating the temperature of the sample cell windows in order to inhibit condensation build-up and one for regulating the ambient temperature surrounding the detectors so as to ensure more accurate measurements. Calibration means is included within the analyzer housing for storing and providing information characterizing performance parameters of the analyzer.

Claims

1. An apparatus for determining the concentration of a selected gas in a mixture of gases containing the selected gas and providing an electrical signal representative thereof, comprising:

a housing;

a sample cell supported in said housing for containing the gas mixture;

source means mounted in said housing for emitting infrared radiation;

measuring detector means for detecting infrared radiation passing through said sample cell at the characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

reference detector means for detecting infrared radiation passing through said sample cell at a wavelength other than that of said characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

lenseless optical means mounted in said housing for directing said radiation into a beam traveling down a beam path through said sample cell, said optical means comprising a reflector for forming a substantially collimated beam directed through said sample cell and further comprising dichroic beam splitting means for directing a portion of said collimated beam having a bandwidth including said characteristic absorption wavelength to said measuring detector means and for directing a portion of said collimated beam having a bandwidth not including said characteristic absorption wavelength to said reference detector means, said optical means further comprising first and second collector means positioned, respectively, adjacent to each of said measuring detector means and said reference detector means for reflectively focusing said respective beam portions thereon;

filter means associated with said measuring detector means for further limiting energy passing therethrough to said bandwidth including said characteristic absorption wavelength; and

filter means associated with said reference detector means for further limiting energy passing therethrough to said bandwidth not including said characteristic absorption wavelengths.

2. The apparatus of claim 1 further comprising holder means formed in said housing for manually releaseably holding said sample cell in said beam path between said source means and said beam splitting means.

3. The apparatus of claim 1 wherein said source means comprises a tungsten filament lamp encased in a shock absorbing material.

4. The apparatus of claim 1 wherein said reflector is a parabolic mirror positioned so that said source means is located substantially at the focal point thereof.

5. The apparatus of claim 1 further comprising thermal regulation means for inhibiting condensate build-up within said sample cell and for minimizing variations in the ambient temperature surrounding said measuring detector means and said reference detector means.

6. An apparatus for determining the concentration of a selected gas in a mixture of gases containing the selected gas and providing an electrical signal representative thereof, comprising:

a housing;

a sample cell supported in said housing for containing the gas mixture, said sample cell having first and second windows positioned transversely to said beam;

source means mounted in said housing for emitting infrared radiation;

optical means mounted in said housing for directing said radiation into a beam traveling down a beam path through said sample cell;

measuring detector means for detecting infrared radiation passing through said sample cell at the characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

reference detector means for detecting infrared radiation passing through said sample cell at a wavelength other than that of said characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

holder means formed in said housing for manually releasably holding said sample cell in said beam path, said holder means having first and second windows corresponding to said first and second windows of said sample cell and positioned transversely to said beam, said first holder means window being positioned proximate and upstream of said first sample cell window, said second holder means window being positioned proximate and downstream of said second sample cell window;

window temperature regulation means, for inhibiting condensate build-up on said first and second sample cell windows, comprising window heating means for applying heat to said first and second sample cell windows and window temperature sensor means for monitoring the temperature of said sample cell, said window temperature sensor means providing an output signal indicative of said temperature to a temperature control means capable of increasing or decreasing the amount of energy provided to said window heating means so as to inhibit condensate build-up on said sample cell windows, wherein said window heating means comprises resistive elements attached to said first and second windows of said holder means and means for supplying a current to said resistive elements, and further wherein said window temperature sensor means comprises a thermistor mounted on said second window of said holder means; and

detector temperature regulation means, for minimizing variations in the ambient temperature surrounding said measuring detector means and said reference detector means, comprising a thermally-conductive block in which said measuring detector means and said reference detector means are seated, block heating means for applying heat to said thermally-conductive block, comprising a heater strip of resistive material attached to the outside of a wall of said thermally-conductive detector block, and block temperature sensor means for monitoring the temperature of said block and for providing an output signal indicative of said block temperature to said temperature control means, said temperature control means being capable of increasing or decreasing the amount of energy provided to said block.

7. The apparatus of claim 6 further comprising shutoff means for turning off said window heater means when the value of said output signal indicative of said window temperature is outside a selected band of temperature values for the energy provided to said window heater means.

8. The apparatus of claim 6 wherein said block temperature sensor comprises a thermistor embedded in said thermally-conductive block.

9. The apparatus of claim 6 wherein said thermally-conductive block is composed of aluminum.

10. The apparatus of claim 6 further comprising shutoff means for turning off said block heater means when the value of said output signal indicative of said block temperature is outside a selected band of temperature values for the energy provided to said block heater means.

11. For use with a signal processing system, an apparatus for determining the concentration of a selected gas in a mixture of gases containing the selected gas and providing an electrical signal representative thereof, comprising:

a housing;

a sample cell supported in said housing for containing the gas mixture, said sample cell having first and second windows positioned transversely to said beam;

source means mounted in said housing for emitting infrared radiation;

optical means mounted in said housing for directing said radiation into a beam travelling down a beam path through said sample cell;

measuring detector means for detecting infrared radiation passing through said sample cell at the characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

reference detector means for detecting infrared radiation passing through said sample cell at a wavelength other than that of said characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;

holder means formed in said housing for holding said sample cell in said beam path between said source means and said receiver means;

calibration means mounted in said housing for storing characterization information characterizing the performance of said source means, said measuring detector means, and said reference detector means, wherein said calibration means provides electrical signals representative of said information to the processing system; and

span check filter means, for determining whether said characterization information accurately represents the performance of said apparatus, comprising hardware means for simulating the radiation absorption characteristics of a selected gas concentration and software means for running a span check algorithm, wherein said hardware means comprises a holder with a first window, a filter, and a second window, and means for securing said first window, said filter and said second window within said holder, whereby said hardware means is supported in said housing in lieu of said sample cell, and further wherein said software means verifies whether the value of the detector output signals representative of said simulated selected gas concentration lies within a range of prescribed values representing said selected gas concentration.

12. The apparatus of claim 11 wherein said calibration means comprises memory means for storing said characterization information so as to enable said apparatus to interface with one of a plurality of processing apparatus.

13. The apparatus of claim 12 wherein said memory means comprises a non-volatile random accessible memory.

14. The apparatus of claim 11 further comprising thermal regulation means for inhibiting condensate build-up within said sample cell and for minimizing variations in the ambient temperature surrounding said measuring detector means and said reference detector means.

15. A method for measuring the concentration of carbon dioxide in a stream of respiratory gases and processing the measurement so as to provide the user with information indicative of an individual'"'"'s respiratory status, comprising the steps of:

emitting radiation;

directing said radiation into a beam;

exposing said stream of gases to said beam of radiation;

splitting said beam into first and second component beams having preselected wavelengths;

filtering said first and second component beams;

collecting said first and second component beams;

detecting said first and second component beams;

measuring the intensity of radiation in said first and second component beams;

ratioing the measured values of said radiation in said first and second component beams;

providing an electrical signal indicative of the ratioed value to a processing apparatus, said ratioed value indicative of the concentration of carbon dioxide in said stream of respiratory gases; and

processing said signal for providing the individual'"'"'s respiratory rate and respiratory carbon dioxide concentration continuously at end-tidal peak and during the inspired phase to a host unit.

16. The method of claim 15 wherein the steps of emitting radiation, directing said radiation into a beam, exposing said stream of gases to said beam of radiation, splitting said beam into first and second component beams having preselected wavelengths, filtering said first and second component beams, collecting said first and second component beams, and detecting said first and second component beams are performed at a predetermined temperature.