Apparatus and method for the determination of the relative proportions of gases

• US 6,520,001 B2
• Filed: 03/02/2001
• Issued: 02/18/2003
• Est. Priority Date: 01/14/1998
• Status: Expired due to Term

• Current Assignee: Life Measurement, Inc. (COSMED S.r.l.)
• Original Assignee: Life Measurement, Inc. (COSMED S.r.l.)
• Inventors: Payne, John, Dempster, Philip
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): Politzer, Jay L
• Application Number:
  US09/798,882
• Publication Number:
  US 20020017124A1
• Time in Patent Office: 718 Days
• Field of Search: 73/250.2, 73/310.4, 73/232, 324/204
• US Class Current: 73/25.02
• CPC Class Codes: G01N 2291/0215 Mixtures of three or more gases, e.g. air
• CPC Class Codes: G01N 2291/0217 Smoke, combustion gases
• CPC Class Codes: G01N 2291/02809 Concentration of a compound, e.g. measured by a surface mass change
• CPC Class Codes: G01N 29/024 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 33/497 of gaseous biological material, e.g. breath

Abstract

The present invention relates to a method of determining the relative proportions of gases in a mixture, such that the relative proportions of N gases are determined using N-1 sensors, not all of which are specific to a particular gas. In the preferred embodiment, oxygen and carbon dioxide are measured in the presence of nitrogen by measuring magnetic susceptibility and speed of sound. The described method of gas analysis leads to very fast response times and exceptional stability, making the technology suitable for breath-by-breath analysis of respired air. Notably, the method does not require high-temperature components, electrochemical cells, or consumable components.

Claims

1. An apparatus for measuring the concentration of oxygen in an unknown or sample gas by comparison with a reference gas, comprising:

a sample chamber with an inlet and an outlet,

a valving system which connects either the sample or the reference gases to the inlet,

a source of low pressure connected to the outlet which tends to draw one or the other into the chamber,

a magnetic circuit for generating a flux path,

one or more coils of wire surrounding the flux path in the magnetic circuit,

control means for the valves, amplification and detection of induced voltages in the coil(s), which detected signal is a representation of the difference of magnetic susceptibility of the reference and test gases,

in which the chamber is in the form of an annulus, with inner and outer pole pieces made of soft iron enclosed on the sides by a non magnetic material and in which the control and detection system includes an integrator for the signal from the coil, and in which the integrated signal is synchronously multiplied by a periodic signal of the same frequency as valve operation to produce a detected signal to reject noise and spurious signals.

2. The apparatus as in claim 1 in which a pneumatic compliance is arranged between the two valve inlets for mitigating acoustic pulsation effects.

3. The apparatus as in claim 1 in which a pneumatic compliance is arranged at each valve inlet for mitigating acoustic pulsation effects.

4. The apparatus as in claim 1 in which the magnetic circuit and the coil is split into two halves in a symmetrical mirror image arrangement for cancelling external magnetic fields.

5. The apparatus as in claim 1 in which there are two sensing coils of similar electrical impedance driving a preamplifier system including an operational amplifier, the non-inverting input of which is driven by one coil whose other end is connected to the common conductor of the circuit, the inverting input of which is driven by the other coil whose opposite end is driven by a small proportion of the output voltage from a source whose impedance is significantly lower than that of the coil.

6. The apparatus as in claim 1 in which the low pressure source includes a biased vacuum regulator, whose reference input is connected to any of the gas inlets to correct for pneumatic resistance in the sample and reference tubes.

7. The apparatus as in claim 1 in which the temperature of the sensor head is monitored and the monitored temperature is used in the calculation of the detected signal to correct for changes in gas density.

8. The apparatus as in claim 1 in which the absolute pressure in the sensor head is monitored and monitored pressure is used in the calculation of the detected signal to correct for changes in gas density.

9. The apparatus as in claim 1 in which the control means includes means for gathering data over a time period, calculating the detected signal over a multiplicity of phase relationships between the valves and the periodic signal used in detection, and choosing and storing that phase relationship which gives a maximum output for future use in the detection system.