Method and apparatus for determining the concentration of a gas

• US 5,464,983 A
• Filed: 04/05/1994
• Issued: 11/07/1995
• Est. Priority Date: 04/05/1994
• Status: Expired due to Fees

• Current Assignee: Industrial Scientific Corporation (Fortive Corp.)
• Original Assignee: Industrial Scientific Corporation (Fortive Corp.)
• Inventors: Wang, Annie Q.
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US08/222,825
• Time in Patent Office: 581 Days
• Field of Search: 250/343, 356/437
• US Class Current: 250/343
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

A method and apparatus for determining the concentration of a gas using nondispersive infrared absorption. The method includes measuring the output signal for a plurality of instantaneous time periods, calculating the change of the output signals (CS), calculating the change of the change of the output signal (CCS) which creates a pattern of CCS data and comparing the CCS data with corresponding CCS data for a known reference gas at a known concentration and at a range of known temperatures. The apparatus for determining the concentration of a known gas in a sample gas mixture is a gas chamber, a light source, a detector, a signal converter, and a data acquisition and microprocessor unit which measures the output signal and has a memory with a program to calculate the change of the signal (CS) and the change of the change of the signal (CCS), and compare the CCS data generated from the gas sample mixture to CCS data of known gases with known concentrations and at known temperature ranges.

Claims

1. A method for determining the concentration of a known type gas present in a gas sample mixture comprising:

(a) providing light through a chamber;

(b) introducing the gas sample mixture into the chamber;

(c) detecting light in the chamber and producing an electric output signal which is a function of the amount of light detected;

(d) measuring the output signal for three sequential instantaneous periods of time t₁, t₂, t₃ ;

(e) determining the relative difference between the output signals at t₁ and the output signal at t₂ which gives a change of output signal value CS₁ ;

(f) determining the relative difference between the output signal at t₂ and the output signal at t₃ which gives a change of output signal value CS₂ ;

(g) determining the relative difference between the change of output signal value CS₁ and the change of output signal value CS₂ which gives a change of the change of the output signal value CCS₁ ;

(h) repeating the method steps of (d) through (g) to develop a plurality of CCS data which generates a pattern; and

(i) comparing the CCS data of step (h) to corresponding CCS data for the known type of gas at a range of known concentrations and at a range of known temperatures to determine the concentration of the known type gas in the sample mixture contained in the chamber.

2. A method for determining the concentration of a known type gas present in a gas sample mixture comprising:

(a) providing light through a chamber;

(b) introducing the gas sample mixture into the chamber;

(c) detecting light in the chamber and producing an electric output signal which is a function of the amount of light detected;

(d) measuring the output signal for four sequential instantaneous periods of time t₁, t₂, t₃, t₄ ;

(e) determining the relative difference between the output signals at t₁ and the output signal at t₂ which gives a change of output signal value CS₁ ;

(f) determining the relative difference between the output signal at t₃ and the output signal at t₄ which gives a change of output signal value CS₂ ;

(g) determining the relative difference between the change of output signal value CS₁ and the change of output signal value CS₂ which gives a change of the change of the output signal value CCS₁ ;

(h) repeating the method steps of (d) through (g) to develop a plurality of CCS data which generates a pattern; and

(i) comparing the CCS data of step (h) to corresponding CCS data for the known type of gas at a range of known concentrations and at a range of known temperatures to determine the concentration of the known type gas in the sample mixture contained in the chamber.

3. A method for determining the concentration of a known type gas present in a gas sample mixture comprising:

(a) providing light through a chamber;

(b) introducing the gas sample mixture into the chamber;

(c) detecting light in the chamber and producing an electric output signal which is a function of the amount of light detected;

(d) measuring the output signal for a first instantaneous period of time t₁ ;

(e) measuring the output signal for a second sequential instantaneous period of time t₂ ;

(f) measuring the output signal for a third sequential instantaneous period of time t₃ ;

(g) subtracting the output signal at t₂ from the output signal at t₁ which gives a change of signal value CS₁ ;

(h) subtracting the output signal at t₃ from the output signal at t₂ which gives a change of signal value CS₂ ;

(i) subtracting the value CS₂ from the value CS₁ which gives the change of the change of the signal value CCS₁ ;

(j) repeating the method steps of (d) through (i) to develop a plurality of CCS data which generates a pattern; and

(k) comparing the CCS data of step (j) to corresponding CCS data for the known type of gas at a range of known concentrations and at a range of known temperatures to determine the concentration of the known type gas in the sample mixture contained in the chamber.

4. The method as recited in claim 3 wherein a wavelength of the light is a wavelength in the absorption spectrum of the known type of gas.

5. The method as recited in claim 3 wherein the step of providing light further comprises repeatedly switching the light on and off such that the step of detecting the light occurs both when the light is present in the chamber and when the light is absent in the chamber.

6. The method as recited in claim 3 further comprising the steps of providing a second sealed chamber containing a known reference gas having a known concentration such that the detecting step comprises alternatively detecting the amount of light in the sample chamber and detecting the amount of light in the second sealed chamber.

7. The method as recited in claim 6 wherein the step of providing light comprises positioning the light such that it is alternatively directed through the second sealed chamber containing the known reference gas and then directed through the chamber containing the gas sample mixture.

8. The method as recited in claim 3 wherein the step of providing light comprises providing a continuous stream of light from the light source through the chamber containing the gas sample mixture.

9. The method as recited in claim 3 wherein the step of comparing the CCS data comprises creating a database containing CCS data for the known type of gas at a range of known concentrations and at a range of known temperatures and alternatively accessing the CCS data from the database and the CCS data generated in step (j).

10. The method as recited in claim 3 wherein the step of comparing the CCS data comprises comparing each peak value of the CCS data generated in step (j) with a peak value of the CCS data of the known gas at known concentrations and at a known range of temperatures.

11. The method as recited in claim 3 wherein the step of comparing the CCS data comprises comparing a summation of all absolute value of the CCS data generated in step (j) with a summation of all absolute values of the CCS data of the known gas at known concentrations and at a known range of temperatures.

12. The method as recited in claim 3 wherein the step of comparing the CCS data comprises comparing a summation of each CCS data generated in step (j) multiplied by its corresponding time interval to a summation of the CCS data of the known gas of a known concentration at a known range of temperatures multiplied by its corresponding time interval.

13. The method as recited in claim 3 wherein the step of comparing the CCS data further comprises eliminating any CCS data having an absolute value less than a predetermined minimum CCS peak value.

14. An apparatus for determining the concentration of a known type of gas present in a gas sample mixture comprising:

(a) a gas chamber having an inlet and an outlet;

(b) an infrared light source connectable to a power supply and mounted within the gas chambers;

(c) a detector positioned in the gas chamber for detecting the light in the chamber and converting the light detected to an electric output signal which is a function of the light detected;

(d) an optical band pass filter also positioned within the chamber;

(e) a signal converter connectable to the detector which transforms the electrical output signal from analog output to digital output; and

(f) a data acquisition and microprocessor means connected to the signal converter, the data acquisition and microprocessor means having the following functions;

(i) measuring the output signal for four sequential instantaneous periods of time t₁, t₂, t₃, t₄ ;

(ii) determining the relative difference between the output signals at t₁ and the output signal at t₂ which gives a change of output signal value CS₁ ;

(iii) determining the relative difference between the output signal at t₃ and the output signal at t₄ which gives a change of output signal value CS₂ ;

(iv) determining the relative difference between the change of output signal value CS₁ and the change of output signal value CS₂ which gives a change of the change of the output signal value CCS₁ ;

(v) repeating the functions of (i) through (iv) to develop a plurality of CCS data which generates a pattern; and

(vi) comparing the CCS data of function (v) to corresponding CCS data for the known type of gas at a range of known concentrations and at a range of known temperatures to determine the concentration of the known type gas in the sample mixture contained in the chamber.

15. The apparatus as recited in claim 14 further comprising a second reference gas chamber.

16. The apparatus as recited in claim 14 wherein the data acquisition and microprocessor means has a database containing CCS data of known gases having known concentrations at known ranges of temperatures.