Infrared gas analyser and humidity sensor

• US 5,468,961 A
• Filed: 03/31/1994
• Issued: 11/21/1995
• Est. Priority Date: 10/08/1991
• Status: Expired due to Term

• Current Assignee: Fisher & Paykel Healthcare Limited
• Original Assignee: Fisher & Paykel Limited
• Inventors: Seakins, Paul J., Gradon, Lewis, Clark, Andrew B.
• Primary Examiner(s): FIELDS, CAROLYN
• Application Number:
  US08/221,037
• Time in Patent Office: 600 Days
• Field of Search: 250/339.03, 250/339.1, 250/343, 250/345
• US Class Current: 250/343
• CPC Class Codes: A61M 16/1085 after being humidified or mixed with a beneficial agent
• CPC Class Codes: A61M 16/1095 in the connecting tubes
• CPC Class Codes: A61M 16/16 Devices to humidify the respiration air A61M16/1045 takes precedence
• CPC Class Codes: A61M 16/161 with means for measuring the humidity
• CPC Class Codes: A61M 2205/3368 Temperature
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

An infrared gas analyser. The analyser comprises a sample cell located in a gas flow, a source of infrared radiation of selected wavelengths which is propagated through the sample cell and an infrared detector which receives radiation which has passed through the cell, The radiation incident on the detector provides a measure of the concentration of selected substances in the gas flow due to one of the wavelengths being selected to be absorbed by the gas being measured. One preferred use is as a humidity sensor in a medical respiratory circuit where at least one selected wavelength is a wavelength strongly absorbed by water. Heating means am associated with the sample cell to vaporize any liquid water which may enter it and the cell is provided with internal baffles to reduce deposition or condensation of water on internal surfaces through which infrared radiation must pass. In one embodiment two selected wavelengths are derived from a single source with the second reference wavelength being a wavelength which is not significantly absorbed by the gas being measured and the source is switched between two power supplies to ensure the output of the detector is equal for the two wavelengths when the concentration of the measured gas is zero.

In a second embodiment two single wavelength sources are used and their combined outputs passed through the sample cell.

Claims

1. Apparatus for measuring the humidity of a gas or a mixture of gases flowing in a medical breathing circuit, said apparatus comprising a source of infrared radiation of one or more selected wavelengths including a wavelength strongly absorbed by water, a gas sample cell through which the radiation is passed and through which said gas or gases pass in use, an infrared radiation detector adapted to detect radiation in said one or more selected wavelengths, means defining a radiation path between said source and said detector with said sample cell being positioned intermediately of said source and said detector, signal processing means connected to the output of said detector which convert the intensity of the received infrared radiation on said detector to a signal proportional to the humidity of the gases passing through said sample cell and water-prevention means associated with said sample cell which prevent water entering or forming in the sample cell said sample cell incorporating at least one radiation transparent window and said water prevention means comprising baffles disposed within the cell around the periphery of said at least one window to prevent liquid water from contacting the window surface.

2. Apparatus according to claim 1 wherein a second set of baffles are disposed within said sample cell to provide a labyrinthic path through which gases must pass before traversing the surface of said window to thereby induce any liquid water contained in said gases to deposit on said second set of baffles.

3. An infrared gas analyser comprising a source of infrared radiation of at least two selected wavelengths, the first selected wavelength being strongly absorbed by a selected gas to constitute a measurement wavelength and the second selected wavelength selected so as to be minimally absorbed by said selected gas to constitute a reference wavelength, a gas sample cell through which the radiation is passed and through which said gas or gases pass in use, an infrared radiation detector adapted to detect radiation in said selected wavelengths, means defining a radiation path between said source and said detector with said sample cell being positioned intermediately of said source and said detector, a rotating dual segment filter assembly located in said radiation path prior to said sample cell which alternately allows transmission of radiation of the measurement wavelength and then the reference wavelength, a synchronous demodulator fed with the output of said detector and a synchronizing signal produced by sensing the period each segment of said rotating filter is present in the radiation path which converts the output signal from the infrared radiation detector to a signal proportional to the concentration of the selected gas present in the gases passing through said sample cell, a switched power supply for said source of infrared radiation, and means for switching the output of said power supply between two levels, said switching being in phase with said synchronizing signal, said levels being predetermined to ensure the amplitude of the output signal from the infrared radiation detector remains constant when the concentration of said selected gas in said sample cell is zero.

4. An infrared gas analyser according to claim 3 including means which detect blocking intervals when non filter components of the rotating filter assembly pass through the radiation path and gate means which insert into the signal from the output of the infrared radiation detector a zero signal during said blocking intervals.

5. An infrared gas analyser according to claim 4 including gate means which hold the signal from the output of the infrared radiation detector at the level it was immediately before the blocking intervals during the detected blocking intervals.

6. An infrared gas analyser comprising a first source of infrared radiation of a first selected wavelength, the first selected wavelength being strongly absorbed by a selected gas to constitute a measurement wavelength, a second source on infrared radiation of a second selected wavelength, the second selected wavelength, the second selected wavelength selected so as to be minimally absorbed by said selected gas to constitute a reference wavelength, a gas sample cell through which radiation from both the first and second source is passed and through which said gas or gases pass in use, an infrared radiation detector adapted to detect radiation at said selected wavelengths, means defining a radiation path between said sources and said detector with said sample cell being positioned intermediately of said sources and said detector, a synchronous demodulator fed with the output of said detector and a synchronizing signal which converts the output signal from the infrared detector to a signal proportional to the concentration of the selected gas present in the gases passing through said sample cell, a first switched power supply for said first source of infrared radiation, a second switched power supply for said second source of infrared radiation and means for alternately switching the outputs of said first and second power supplies to energize alternately the respective first and second sources of infrared radiation, said switching being in phase with said synchronizing signal, means for setting the levels of the power supply outputs, said levels being predetermined to ensure the amplitude of the output signal from the infrared radiation detector remains constant when the concentration of said selected gas in said sample cell is zero, radiation combining means in said radiation path prior to said sample cell which combines radiation from said first source of infrared radiation with radiation from said second source of radiation, and said combining means is also a radiation splitting means which derives a further radiation path of combined radiation and including a second infrared radiation detector positioned in said further radiation path, a second synchronous demodulator fed with the output of said second infrared radiation detector and said synchronizing signal which produces a signal representative of the imbalance of radiation intensities generated by the first and second infrared radiation sources respectively, and means responsive to said imbalance signal which controls the outputs of both power supplies to keep the ratio of the radiation intensities from the first and second infrared radiation sources constant.

7. Apparatus for measuring the humidity of a gas or a mixture of gases flowing in a medical breathing circuit, said apparatus comprising a source of infrared radiation of one or more selected wavelengths including a wavelength strongly absorbed by water, a gas sample cell through which the radiation is passed and through which said gas or gases pass in use, an infrared radiation detector adapted to detect radiation in said one or more selected wavelengths, means defining a radiation path between said source and said detector with said sample cell being positioned intermediately of said source and said detector, signal processing means connected to the output of said detector which convert the intensity of the received infrared radiation on said detector to a signal proportional to the humidity of the gases passing through said sample cell and water-prevention means associated with said sample cell which prevent water entering or forming in the sample cell, said water prevention means comprising a microporous envelope provided about said sample cell.