Infrared Radiation Gas Analyzer

1. Field of the Invention

The present invention relates to a gas analyzer for determining the concentration of specific ingredients in a sample gas.

2. Description of the Prior Art

The conventional method of determining concentration of ingredients in gas has been by a nondispersive infrared absorption analyzer using Lambert-Beer's law. However, such an analyzer requires an infrared light source and a power source for stabilizing said light source. In addition, it is expensive due to the complicated circuitry. Furthermore, the optical adjustment is required for adjusting the quantity of light incident upon a reference cell and the quantity of light incident upon a sample cell. The reference cell must be provided in order to prevent drift in the light source directing light on said sample cell from occurring.

It is an object of the present invention to provide an operationally useful infrared radiation gas analyzer which can measure the concentration of ingredients in a gas with high accuracy, and which eliminates the main cause of errors in such measurement.

It is a further object to provide an infrared radiation gas analyzer for determining the concentration of specific ingredients in a sample gas which has simple construction and which is inexpensive.

To this end the present invention provides an infrared light source comprising means to measure an infrared radiation dose of a specified wave length constituted by radiation from gas molecules other than monoatomic molecules and produced by the gas when the gas molecules are heated to high temperatures, whereby an infrared light source and a power source for stabilizing said light source, which have heretofore been required in a conventional infrared gas analyzer, can be dispensed with.

The invention will now be described in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an infrared radiation gas analyzer according to the invention;

FIGS. 2(a) and 2(b) are graphs showing relative radiation intensity; and

FIG. 3 is a schematic diagram of another embodiment of the present invention.

Referring now to FIG. 1, a sample cell 1 has an inlet 2 for a sample gas under a particular pressure and an outlet 3 for the sample gas, the internal surface of said sample cell 1 being a mirror surface, and said sample cell 1 having cell windows 1a and 1b made of infrared ray-transmitting materials at the opposite ends thereof to transmit infrared radiation from within the sample cell. A heater 4 is provided in the wall of cell 1 for heating the sample gas in said sample cell 1, the heater having a capacity to heat the sample gas to temperatures of at least 100.degree. C. to cause it to radiate infrared rays, in order to increase the radiation from a sample gas and decrease the background radiation relative to the radiation from the gas. Insulating material 5 surrounds the elements of heater 4.

A chopper 6 is provided in front of the window 1a, and a dual type infrared detector 7 is provided on the opposite side of the chopper from the window. The detector 7 has two detecting elements a and b which subtracts the two signals from said elements a and b and puts out a differential signal.

A first filter 8 is mounted in front of detecting element a and is for transmitting infrared rays which will be radiated from the ingredient contained in the sample and the concentration of which is to be determined, at the temperature and pressure to which the sample gas is raised, the filter 8 transmitting wave lengths in a specified range W characteristic of the ingredient (for example a 4.3 .mu.m band of high radiation coefficient when determining the concentration of CO.sub.2). The radiation intensity will be in proportion to the concentration of the ingredient.

A second filter 9 is mounted in front of detecting element b and is for transmitting infrared rays having wave lengths in the range W' which is different from the above described specified range W but near the said specified range W.

Said filters 8 and 9 have almost the same transmission factor and half value width.

Accordingly, background infrared rays which are radiated from said cell 1 itself and incident upon said elements a and b are cancelled. Infrared detector 7 detects only a radiation dose of infrared rays radiated from the ingredient the concentration of which is to be determined and which are transmitted only through said first filter 8. Said infrared detector 7 puts out a pulsed electric signal equivalent to the concentration of the ingredient the concentration of which is to be determined due to the operation of chopper 6.

The electric signals from said infrared detector 7 are amplified in amplifier 10. In addition, although not shown in the drawing, the signals, amplified by said amplifier 10, can be outputted in the form of signals equivalent to the concentration of the ingredient the concentration of which is to be determined.

In the above described construction, infrared rays are radiated from a sample gas introduced into said sample cell 1 at a particular pressure and heated to the desired temperature, and simultaneously background infrared rays are radiated from said cell 1 itself when said sample gas is heated to the appointed temperatures by means of said heater 4. The detector 7 with the filters 8 and 9 functions to remove the background radiation so as to leave only the radiation emitted by the ingredient the concentration of which is to be determined.

For example, when determining CO.sub.2 concentration, said first filter 8 transmits infrared rays A+B, which is the total of infrared rays A radiated from only CO.sub.2 and having wave lengths in the specified range W as shown in FIG. 2(a) and infrared rays B having wave lengths in said specified range W forming part of the background infrared rays as shown in FIG. 2(b), while said second filter 9 transmits the background infrared rays B' having wave lengths in the range W' near said specified range W. These background infrared rays are almost equal in intensity as shown in FIG. 2(b), so that B is nearly equal to B'.

When infrared rays A+B and infrared rays B' are radiated onto said detecting elements a and b, respectively, the background infrared rays cancel each other leaving only the dose of radiation from the ingredient, i.e. A+B-B'.div.A. Thus a radiation dose A of infrared rays radiated from the ingredient the concentration of which is to be determined can be detected by means of said infrared detector 7, and thus the concentration of the ingredient can be accurately determined on the basis of the radiation dose A of infrared rays.

The concentration can also be accurately determined in the same manner as described above even though the amount of background infrared rays is changed because of contamination of said cell 1 and the cell-window 1a and the like, since said infrared detector 7 detects only the concentration of the ingredient to be determined on the basis of absolute values.

In addition, although in the above described embodiment the sample gas is heated to the appointed temperatures by means of said heater 4, the sample gas may be heated to a suitable temperature range and temperature compensation may be carried out by using a thermometer for measuring temperatures of gas. Alternatively, gas which has been preliminarily heated to high temperatures can be introduced into said cell 1. Furthermore, exhaust gas pipes P for exhausting exhaust from internal combustion engines and for exhausting gas from factories can constitute said cell, as shown in FIG. 3, and the window 1a can be placed over an aperture in the pipe. The detector 7 and amplifier 10 are the same as in the embodiment shown in FIG. 1.

Furthermore, the concentration of various kinds of ingredients can be detected by using as said first filter 8 and said second filter 9 filters which transmit infrared rays corresponding to those emitted by the ingredients heated to the desired temperatures, and wavelengths near thereto.

In the analyzer according to the present invention, neither an infrared light source nor a power source for stabilizing the light source, which are necessary in the conventional nondispersive infrared absorption analyzer, are required. Furthermore, neither a reference cell nor an optical adjusting mechanism for preventing drift of the light source are required. Thus the concentration of the specified ingredient contained in a sample gas can be determined by an analyzer having a simple and inexpensive construction.

The analyzer of the present invention takes into account that the background infrared rays radiated from said cell and cellwindow is several times or several tens of times as great as the infrared rays radiated from the ingredient of the sample gas. A slight change in temperature can influence the zero point, or the level of infrared rays incident upon the infrared detector may be changed due to a change in the radiation coefficient of the cell, cell-window and the like, and as a result the zero point is changed, which is an important factor causing errors of measurement in the detection of concentration. As a result, the concentration of the specified gaseous ingredient can only be accurately determined by removing the influence of background infrared rays. In the analyzer of the present invention, only the infrared rays radiated from the ingredient to be determined are detected, and this is done by means of the dual type infrared detector which cancels from the total radiation A of infrared rays radiated from the ingredient and having wave lengths in a specified range W and radiation B of background infrared rays having the same wave lengths as the infrared rays radiated from the ingredient, the radiation B by means of the radiation B' of background infrared rays nearly equal to the above described radiation B of background infrared rays.