Method and apparatus for trace gas detection

• US 7,324,204 B2
• Filed: 02/21/2006
• Issued: 01/29/2008
• Est. Priority Date: 02/22/2005
• Status: Active Grant

• Current Assignee: Siemens AG
• Original Assignee: Siemens AG
• Inventors: Kluczynski, Pawel
• Primary Examiner(s): Pham; Hoa Q.
• Application Number:
  US11/358,854
• Publication Number:
  US 20060192967A1
• Time in Patent Office: 707 Days
• Field of Search: 356432-440, 250/345, 250/343, 250/339.13
• US Class Current: 356/437
• CPC Class Codes: G01N 2021/151 Gas blown
• CPC Class Codes: G01N 2021/354 Hygrometry of gases
• CPC Class Codes: G01N 2021/3545 Disposition for compensating effect of interfering gases
• CPC Class Codes: G01N 2021/399 Diode laser
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/39 using tunable lasers
• CPC Class Codes: G01N 21/61 Non-dispersive gas analysers G01N21/3504 takes precedence
• CPC Class Codes: G01N 21/85 Investigating moving fluids or granular solids

Abstract

In spectroscopic devices the sections of an optical measuring path from a light source to a measuring volume containing a measuring gas and from there to a measuring detector are often sealed off from the ambient atmosphere and purged with a purge gas such as dry nitrogen to prevent penetration of atmospheric gas components, such as water vapor, which may interfere with the trace gas measurement. The moisture content in the nitrogen supply is usually in the range of a few ppm at the gas source and can increase dramatically at the measuring site depending on the length of the nitrogen pipe net and due to porosity of the pipe walls, leakage of seals and residual moisture trapped in so-called dead legs. In order to compensate interfering absorption of atmospheric gas components and other impurities in the purge gas the purge gas is collected after flushing the optical path sections, a portion of the light of the light source is transmitted along a second optical path to a compensation detector, the second optical path is flushed with the collected purge gas and the trace gas component in the measuring gas is determined based on a difference between the output of the measuring detector and the output of the compensation detector.

Claims

1. A method for quantitatively determining a trace gas component in a measuring gas, comprising:

introducing the measuring gas into a measuring volume;

emitting light from a light source;

passing a first portion of the light along a first optical path from the light source through the measuring volume to a measuring detector;

generating a measuring detector output dependent on the light absorption in the first optical path;

flushing sections of the first optical path between the light source and the measuring volume and the measuring volume and the measuring detector with a purge gas;

collecting the purge gas after flushing the first optical path sections;

transmitting a second portion of the light along a second optical path from the light source to a compensation detector;

flushing the second optical path with the collected purge gas;

generating a compensation detector output dependent on the light absorption in the second optical path; and

determining the trace gas component in the measuring gas based on a difference between the measuring detector output and the compensation detector output.

2. The method of claim 1, further comprising the step of varying the frequency of the light of the light source over a frequency spectrum including an absorption frequency of the trace gas component.

3. The method of claim 1, further comprising the step of prior to determining the trace gas component adjusting the ratio of the measuring and compensation detector outputs by a scaling factor, wherein said scaling factor depends on the ratio of the purged optical path length in the measuring volume and the purged length of the second optical path.

4. An apparatus for quantitatively determining a trace gas component in a measuring gas, comprising:

a measuring volume configured to contain the measuring gas;

a light source;

a device for obtaining a first portion and a second portion of the light from the light source;

a measuring detector for generating a measuring detector output;

a device for passing the first light portion along a first optical path through the measuring volume to the measuring detector;

a compensation detector for generating a compensation detector output;

a device for transmitting the second light portion along a second optical path to the compensation detector;

a device for providing a purge gas;

a device for flushing sections of the first optical path between the light source and the measuring volume and the measuring volume and the measuring detector with the purge gas;

a device for collecting the purge gas after flushing the first optical path sections;

a device for flushing the second optical path with the collected purge gas; and

evaluation device for determining the trace gas component in the measuring gas based on a difference between the measuring detector output and the compensation detector output.