Gas analyzer using thermal detectors

Details Gas analyzer using thermal detectors Gas analyzer using thermal detectors

2003-03-20

2004-02-24

Williams, Hezron (Department: 2856)

Measuring and testing

Gas analysis

By thermal property

C250S345000

Reexamination Certificate

active

06694800

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATION
The present application claims priority from European Patent Application No. 02396036.2, filed Mar. 22, 2002.
BACKGROUND OF THE INVENTION
The invention relates to a gas analyzer comprising: a measuring volume having connections for input and output flow of a sample gas mixture, at least one gas component of which is to be analyzed for determining its concentration in said mixture, and having first and second ends transparent to radiation; a radiation source for providing a beam of electromagnetic radiation having a wavelength range, said beam approaching collimation and being directed to pass said measuring volume through the first and second ends thereof; a heat sink for said radiation source; at least one thermal detector having an active radiation detecting sensor element within at least one detector housing and receiving the radiation exiting said measuring volume, said thermal detector having a reference sensor element within the same detector housing and protected from said radiation, said thermal detector generating an output signal indicative of a property of said at least one gas component of said mixture in the measuring volume; at least one optical bandpass filter between said active radiation detecting sensor element and said radiation source; electrical contact pins in said at least one detector housing for the output of said signal(s); a thermal mass formed of a material having high thermal conductance, said thermal mass: having a cavity and an outer surface, surrounding at least said detector housing in the cavity, being in contact with said detector housing, and extending towards the radiation source; and a thermal barrier between the heat sink and the thermal mass.
Thermal detectors, typically thermopile detectors are used in gas analyzers among other things owing to their capability of DC (Direct Current) measurement, which facilitates a cost-effective construction of the measurement system. In these gas analyzers the thermopile detector measures the infrared absorption of a gas introduced to a sampling chamber or a sampling volume, after which the concentration of the gas component or the gas components of interest is/are determined from the measured absorption. The useful wavelength range of thermopile detectors is suitable for infrared measurements, since their absorption bands in the wavelength region 3 &mgr;m-10 &mgr;m fall within the required spectral sensitivity wavelength range for the detector. Moreover, thermopiles have a high sensitivity and good linearity and they are cost effective components.
A characteristic of a thermopile detector is that a thermal gradient in its external housing, noticeable especially in small analyzers with small thermal mass, will cause an offset error in the detector signal, which degrades measurement accuracy. The thermopile is a very sensitive detector containing a plurality of thermocouple junctions. In a typical analyzer it has been measured that the signal change corresponding to the absorption caused by 0.1% by volume of CO
2
in a sample gas is about 2 &mgr;V. The temperature difference in the thermopile detector would then be only about 0.13 mK. It is therefore easy to understand that even small temperature gradients in the thermopile housing may cause considerable measurement errors. Similar errors also occur with a change of the external housing temperature after, e.g., a cold start-up of the analyzer or due to a change in the ambient temperature.
With reference to the patent U.S. Pat. No. 4,772,790 a gas analyzer is described where a number of thermocouples connected to form groups of thermopiles are used as the detector. The first set of thermopile groups is arranged so as to receive the radiation at their inner junctions to form hot junctions, whereas the outer junctions shielded accordingly form cold junctions of this first set. The thermocouples of the second set of thermopile groups, having the same number of shielded thermocouples as the first set, are electrically connected in series with the first set, whereupon the electrical current caused by the EMFs from the first set because of the radiation creates inverted cold as well as hot junctions in the second set of thermopile groups with opposite EMFs to those of the thermocouples of the first set. All of the thermocouples with their hot and cold junctions are positioned on a single substrate of a heat-conductive insulator material. The analyzer is further provided with a highly heat conductive section, which is in contact with the ambience and has a thermal mass substantially greater than that of the housing so that it acts as a large area heat sink. The heat-conductive substrate of the thermopiles is connected with the conductive section using a heat conductive material. Further the radiation source is thermally insulated from the heat conductive section by the wall of the sample cell made of plastic or the like. This seems to be the conventional compensation method. In this construction there is a need for additional dark junctions, which reduces the space for the sensitive area of the detector. The leads from the detector housing are directly connected to the conductive pads of a printed circuit board in contact with ambience, too. As a consequence, even an extremely small change of the temperature from the ambience would cause a considerable thermal gradient on the substrate and so within the thermopile array. Especially in a small analyzer such a gradient could induce an offset in the signal, which would not necessarily be completely compensated by the shielded junctions. For modern semiconductor thermopiles bonded to the base plate of the housing this is especially true. Under a steady state condition, in which the non-shielded hot junctions are receiving a constant radiation, there may exist even a temporal thermal drift, whereupon the DC-signal from the detector varies with time, leading to measurement inaccuracies.
The patent U.S. Pat. No. 5,081,998 discloses a gas analyzer where a group of thermocouples is connected in series and paired so that the first thermopile and the second thermopile is in opposed relationship to each other on a common ceramic substrate. The first thermopile is electrically connected to the second thermopile in series opposition to subtract the output signals from each other, and further at least a first neutral density, i.e. attenuation filter with a different transmission coefficient as compared to a second or lacking neutral density filter are positioned in front of the thermopile detectors so that the first neutral density filter affects the hot junctions of the first thermopile and the second neutral density filter or its absence affects the hot junctions of the second thermopile, whereupon these two thermopile detectors are “optically stabilized”. The thermopile detectors are further preceded by one or several analytical bandpass filters and a reference bandpass filter for passing desired wavelengths in the optical path. The difference between the outputs is used to eliminate the effects of a variation in the background signals and variations due to the thermal drift. This kind of construction makes the detector large and expensive and also difficult to construct for several different gases.
The patent U.S. Pat. No. 5,296,706 refers e.g. to those two patent publications mentioned above, and also describes, as a prior art, a further developed version of the latter patent provided a multiple of paired thermopiles and with an aperture sheet placed over the optical filters for analysis of several gas components in the gas mixture. This now discussed US-patent discloses a topography, which allows several channels to be used as independent analytical channels for detecting the absorption of a plurality of predetermined wavelengths. For this purpose the patent suggests separate reference thermopiles, which are identical with the active analytical thermopiles and are located behind these active thermopiles receiving the thermal radiation. Each reference thermopile and its correspond

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