Apparatus and method for determining the proportion of a substance in a gas sample using an infrared measuring instrument

• US 20050274893A1
• Filed: 06/01/2005
• Published: 12/15/2005
• Est. Priority Date: 06/11/2004
• Status: Active Grant

• Current Assignee: Dragerwerk AG
• Original Assignee: Dragerwerk AG
• Inventors: Dicks, Bernd-Michael, Dreyer, Peter, Matthiessen, Hans, Behring, Rasmus, Hattendorff, Horst-Dieter
• Granted Patent: US 7,268,352 B2
• Time in Patent Office: Days
• US Class Current: 250/343
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis

Abstract

The invention relates to an apparatus and a method for improved correction of drift in an infrared measuring instrument. The measurement signal furnished by a thermal detector is split into a direct voltage component and an alternating voltage component. By means of calibration curves (24, 27), a calculated comparison variable $T_{\mathrm{DC},\mathrm{korr}}^{900}$
is formed from a measured, averaged concentration value $c_{\mathrm{AC1}}^{900}.$
The correction value ΔT for the drift correction is obtained from the difference between the corresponding measured size of the direct voltage component $T_{\mathrm{DC}}^{900}$
and the comparison variable $T_{\mathrm{DC},\mathrm{korr}}^{900}.$

Claims

1. An apparatus for infrared-optical determination of the concentration of a substance in a gas sample, having the following characteristics:

a modulated radiation source;

a thermal detector;

a measurement chamber between the radiation source and the detector, as a measurement distance for recording the gas sample;

means for splitting the measurement signal, furnished by the detector, into a direct voltage component and an alternating voltage component;

means for forming averaged transmissions

$T_{\mathrm{DC}}^{900},T_{\mathrm{AC}}^{900}$

of the direct voltage component and of the alternating voltage component;

calibration curves, for the functional relationship between transmission and concentration, associated individually with the transmissions of the direct voltage component and the transmissions of the alternating voltage component; and

means for forming a correction value for the direct voltage component, by forming the correction value Δ

T from the deviation of a comparison variable

$T_{\mathrm{DC},\mathrm{korr}}^{900},$

formed from the averaged alternating voltage component

$T_{\mathrm{AC1}}^{900}$

with the calibration curves, from the associated, averaged direct voltage component

$T_{\mathrm{DC1}}^{900}.$

2. The apparatus according to claim 1, characterized in that the correction value Δ

T serves to correct an unaveraged direct voltage component T_DC2.

3. The apparatus according to claim 1, characterized in that as an alternative to the transmission difference Δ

T for correction, a concentration difference is used.

4. A method for determining the concentration of a substance in a gas sample with an infrared measuring instrument, which includes a thermal detector, a modulated radiation source, and a measurement distance between the detector and the radiation source, characterized by the following steps:

splitting the measurement signal, furnished by the detector, into a direct voltage component and an alternating voltage component;

from the alternating voltage component, using a first calibration curve, ascertaining a first, averaged concentration value

$c_{\mathrm{AC1}}^{900};<br><br>$

from the first concentration value, using a calibration curve, determining a calculated comparison variable

$T_{\mathrm{DC},\mathrm{korr}}^{900}$

of the direct voltage component;

from the deviation of the calculated comparison variable

$T_{\mathrm{DC},\mathrm{korr}}^{900}$

from the corresponding size of the measured direct voltage component

$T_{\mathrm{DC1}}^{900},$

forming a drift signal Δ

T, U_Drift for correcting the direct voltage component.

5. The method according to claim 4, characterized in that the drift signal is used for correcting an unaveraged direct voltage component T_DC2.

6. The apparatus according to claim 2, characterized in that as an alternative to the transmission difference Δ

T for correction, a concentration difference is used.