1. Field of the Invention
The present invention generally relates to an infrared gas analyzer for detecting gases contained in air, exhaust gas, and the like. More specifically, it relates to an improved infrared gas analyzer that prevents influenced results by interfering gases.
2. Description of Related Art
As to an infrared gas analyzer for detecting gases contained in air, exhaust gas, and the like, in the case where interfering component gases having an absorption wavelength range partially overlapping a wavelength range of a gas to be measured are contained in a sample gas, an infrared gas analyzer capable of preventing an influence by the interfering component gases comprising an interference filter as shown in FIG. 2 has been known.
This gas analyzer comprises a reference cell 1, in which a reference gas is enclosed. A sample cell 2, in which a sample gas is to be supplied, is disposed in parallel. The sample cell 2 is provided with a supply port 3a for supplying the sample gas and an exhaust port 3b for exhausting the sample gas. Reference numerals 4a, 4b designates a light source disposed on one side of the reference cell 1 and the sample cell 2, respectively. Reference numeral 5 designates a detector, such as condenser microphone, disposed on the other side of the reference cell 1 and the sample cell 2.
Reference numeral 6 designates a chopper disposed between the reference cell 1 and the sample cell 2 and the light sources 4a, 4b. Reference numerals 7a, 7b designates interference filters disposed between the reference cell 1 and the sample cell 2 and the detector 5. The filters 7a, 7b pass radiation including a wavelength range in any absorption bands of the gas to be measured but reflect radiation that is in a non-absorbing wavelength range. Reference numeral 9 designates a preamplifier.
With this infrared gas analyzer, a radiation emitted from the light sources 4a, 4b is intermittently incident upon the reference cell 1 and the sample cell 2 by rotating the chopper 6. Thereupon, a part of the radiation is absorbed by a gas enclosed in the reference cell 1 and the sample cell 2, respectively, so that the energy of radiation incident upon the detector 5 from the reference cell 1 is different from that from the sample cell 2. The sample gas is analyzed on the basis of this difference in energy of radiation.
And, radiation including any wavelengths within the absorption bands of the interfering gases contained in the sample gas supplied in the sample cell 2 and incident upon the reference cell 1 and the sample cell 2 are reflected by the interference filter 7 to prevent them from being incident upon the detector 5, whereby the influences of the interfering component gases are reduced.
Also an infrared gas analyzer shown in FIG. 3 has been known. In this infrared gas analyzer, the interference filter 7 in the gas analyzer shown in FIG. 2 is replaced with gas filter cells 8a, 8b for absorbing the interfering component gases. Other constructions of FIG. 3 are the same as in the gas analyzer shown in FIG. 2, so that they are marked with the same reference numerals and marks.
In the gas analysis by this gas analyzer (FIG. 3), radiation having wavelengths of the absorption bands of the interfering components contained in the sample gas and incident upon the reference cell 1 and the sample cell 2 are absorbed by the gas filter cell 8 to reduce the influences of the interfering components.
Also, an infrared gas analyzer of an interference compensation type as shown in FIG. 4 has been known. In this gas analyzer, a detector 5a is adapted to be able to pass radiation therethrough. An interference-compensating detector 5b, upon which the radiation passing through the detector 5a is incident, is provided. A subtracter 10 for subtracting an output of the interference-compensating detector 5b from an output of the detector 5a is provided in the gas analyzer as shown in FIG. 4.
Other constructions are the same as in the gas analyzer shown in FIG. 2, so that they are marked with the same reference numerals and marks.
With the conventional infrared gas analyzer as shown in FIG. 2, a reflection factor of the interfering component of radiation by the interference filter 7 is high, so that the influences of the interfering component radiation can be reduced.
Here, the transmission and reflection of the radiation by the inteference filter in the case where the window on the side of the detector of the gas cell S is replaced with the interference filter f, as shown in FIG. 5, is investigated.
Radiation which shall be illustrated by a light beam I.sub.o emitted from a light source L passes through the window W of the gas cell S to enter the gas cell S. After a part of the light I.sub.o is absorbed by the component to be measured contained in the gas cell S, the remaining light I.sub.o passes through the interference filter F to enter the detector (not shown).
The total transmission quantity T.sub.1 of radiation having a wavelength range capable of passing through the interference filter F at this time is expressed by the following equation (1): ##EQU1## Even though tf is replaced with tw and rf is replaced with rw, it is one and the same thing, so that the transmission quantity of radiation capable of passing through the interference filter has nothing to do with their incident direction. That is to say, it is one and the same thing even though the radiation is are incident from the side of the interference filter F.
On the other side, the total reflection quantity R of radiation having a reflection wavelength range of the interference filter is expressed by the following equation (2): ##EQU2## The quantity of the reflected radiation is changed by replacing tf with tw and rf with rw. That is to say, the case where the radiation is incident from the side of the interference filter F is different from the case where the radiation is incident from the side of the window of the gas cell S.
It is found from the above described matters that since both the window W of the gas cell S and the interference filter F have a high transmittance and a low reflectance for the radiation of a component to be measured, the quantity of the reflected radiation can be deemed as constant regardless of the direction of the surface upon which the radiation is incident. However, the filter has a low transmittance and a high reflectance for the interfering component radiation, so that the quantity of the radiation reflected toward the side of the radiation source in the case where the radiation is incident from the side of the filter is larger than that in the case where the radiation is incident from the side of the window W of the gas cell S.
If the radiation reflected toward the side of the radiation source is not return by reflecting again by means of a radiation source mirror and the like, there is no difference in quantity of the transmitted radiation regardless of the direction of the surface upon the light is incident.
However, in order to increase the quantity of radiation in fact, a mirror is frequently used with the light source, so that the difference in incident direction always leads to a difference in quantity of transmitted radiation.
I.sub.o designates a quantity of an incident radiation; tw designates a quantity of a transmitted radiation; rw designates a quantity of a reflected radiation by the window W; tf designates a quantity of lights passing through the interference filter F; rf designates a quantity of a reflected radiation by the interference filter F; c designates a concentration of a gas contained in the cell S; l designates a length of the cell S; and .epsilon. designates a constant determined by the gas.
Accordingly, referring to FIG. 2 and FIG. 4, a comparatively large quantity of interfering component radiation reflected by the interference filters 7a, 7b is reflected by the inside surface of the reference cell 1 or the sample cell 2, the windows of the reference cell 1 and the sample cell 2, reflecting mirrors of the light sources 4a, 4b and the like to arrive at the interference filters 7a, 7b again. This is repeated.
Moreover, since the interference filters 7a, 7b are disposed between the reference cell 1 and the sample cell 2 and the detector 5, a large quantity of the interfering component radiation is reflected by the inside surface of the reference cell 1 and the sample cell 2 to increase an oblique component incident upon the interference filters 7a, 7b. In general, a transmission spectrum of the interference filter has physical characteristics of shifting toward shorter wavelengths in the case of an oblique incidence. Accordingly, as a result, a problem occurs in that the quantity of the interfering component radiation passing through the interference filters 7a, 7b to enter the detector 5 is increased which lowers the accuracy of analysis.
Next, an infrared gas analyzer shown in FIG. 3 has no problem in the reflection of the interfering component radiation in the interference filter since the radiation having wavelength within the absorption bands of the interfering component gases are absorbed by the gas filter 8.
However, in the case where the degree of absorption of the interfering component radiation is slightly lower and a plurality of kinds of interfering component gases are contained in the sample gas, it is difficult to absorb all interfering component radiation, so that the quantity of the interfering component radiation incident upon the detector 5 is comparatively increased which lowers the accuracy of analysis.
An interference-compensation type gas analyzer shown in FIG. 4 can improve an accuracy of analysis since the influences of the interfering component radiation are compensated by the subtractor 10.
However, an interference-compensation detector 5b and a compensation-signal treatment circuit are excessively required, so that a problem occurs in an increase of cost.