Gas analyzer

The present invention concerns a gas analyzer, for instance a CO.sub.2 analyzer, comprising a measuring chamber for the gas to be examined, a reference chamber from which the gas to be measured has been drawn off, a light source and a chopper disk, which chops the light beam to make it pass alternatingly through the measuring and reference chambers and so that in between there is a period during which no light is passed through at all, and an automatic gain control which maintains a constant difference between the "dark" signal and the signal delivered by the reference chamber. An attenuation member is positioned adjacent the reference chamber so the light beam passing through the reference chamber is attenuated to cause a shift of the operating point of the gain control circuit into the gently ascending part of the absorption versus concentration curve.

BACKGROUND OF THE INVENTION 
The problem in gas analyzers of this type known in the art is that as the 
light has to travel a certain distance through ambient gas an error is 
introduced by variations of the content of the gas under measurement in 
the ambient gas. The magnitude of this error is dependent on the ambient 
content, on the distance which the light beam travels in the ambient space 
and the content of the gas under measurement. In a CO.sub.2 analyzer, for 
instance, where the measuring light beam travels part of its path in the 
ambient CO.sub.2 content, the varying ambient CO.sub.2 content introduces 
an error. This problem is accentuated particularly in CO.sub.2 analyzers 
for the reason that the absorption of infra-red radiation as a function of 
CO.sub.2 content is strongly non-linear. The ambient carbon dioxide causes 
equal attenuation both of the measuring and the reference beam. Since in 
the case of increasing CO.sub.2 content the absorption caused by it 
increases at a relatively slower rate, that is, the absorption curve 
plotted as a function of content becomes less steep at the upper end, even 
a minor additional absorption causes a major error. This is only minimally 
corrected by the change in gain which is caused by the attenuation of the 
reference beam due to the ambient environment. 
SUMMARY OF THE INVENTION 
The object of the invention is to improve a gas analyzer of the type 
mentioned so that the said error effect can be substantially reduced. 
This aim is achieved, as taught by the invention, in that in the path of 
the light beam passing through the reference chamber an attenuation member 
has been placed which provides a high enough attenuation in the wavelength 
range employed, to cause a shift of the operating point of the gain 
control circuit into the gently ascending part of the absorption vs. 
content curve. Hereby the additional attenuation caused by the ambient 
environment in the reference beam results in a change of gain which 
compensates for the ambient attenuation of the measuring beam. 
It is advantageous in the measuring ranges normally concerned to CO 
analyzers if the attenuation of the attenuation disk is on the order of 8% 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A motor 2, attached to the frame plate 1, rotates the chopper disk 3 
provided with a sequence of holes 5 for chopping the measuring beam 7 and 
a sequence of holes 6 for chopping the reference beam 8 in such manner 
that the beams 7 and 8 will alternate. The beams 7 and 8 are transmitted 
by the infra-red light source 9, and the measuring beam 7 passes through 
the measuring chamber 10 and the reference beam 8 through the reference 
chamber 11. In the present exemplary case the analyzer shall be described 
as a CO.sub.2 analyzer. Through the measuring chamber 10 that gas is 
conducted of which the CO.sub.2 content is measured. In the reference 
chamber 11 a gas has been enclosed from which the CO.sub.2 has been drawn 
off. The filter 12 is employed to leave of the radiation only that 
wavelength range in which CO.sub.2 displays the strongest absorption. 
After the measuring and reference beams 7 and 8 have in succession arrived 
at the detector 14, there follows a dark period, which is used towards the 
automatic gain control to be described later on, by keeping constant the 
difference between the dark signal and the signal 8 produced by the 
reference chamber 11. The tripartite signal (measuring signal plus 
reference signal plus dark signal) thus formed at the detector is 
conducted through a pre-amplifier 15 to an AC amplifier 16, the operating 
point of the latter being governed by an automatic gain control circuit 
22. 
In order to render the said three signals (measuring, reference and dark 
signal) identifiable and distinguishable from each other, the chopper disk 
3 has been provided with three sequences of synchronizing holes 4, through 
which synchronizing light pulses are transmitted from a LED unit 17, these 
pulses being received in the light receiver and synchronizing signal 
transmitter unit 18. The positions of the holes 4 in the chopper disk 3 
have been so chosen that the signal A coincides with the measuring signal 
received by the detector 14, signal B with the reference signal and signal 
C with the dark signal. With the aid of these synchronizing signals A, B 
and C the synchronizing and measuring difference signal forming unit 20 is 
controlled by the timing control unit 19. The unit 20 has the task to form 
the analog and difference signals between the three different phases of 
the signal coming from the unit 16. First, therein is generated a 
reference voltage by measuring the difference between the reference signal 
and the dark signal. This difference voltage is monitored by an automatic 
gain control circuit 22 to keep it at a predetermined constant value. The 
control circuit 22 governs the AC amplifier 16 for maintaining the said, 
predetermined reference voltage. 
Secondly, in the unit 20 the measuring signal proper is formed. This signal 
is obtained as the difference between the signal received through the 
measuring chamber 10 and that received through the reference chamber 11. 
This analog difference signal is conducted to the DC amplifier 23 and 
thence further over the linearizing and measuring circuits 24 to the 
display instrument 25, on which the CO.sub.2 content can be read. The 
linearizing process taking place in the circuit 24 serves to compensate 
for the non-linear relationship between the absorption and the CO.sub.2 
content in the measuring chamber 10, displayed in FIG. 2. 
The above-described principle of design and operation of the analyzer is 
known in the art. It is unavoidable that the measuring beam 7 travels part 
of its path through the ambient gas. Hereby the non-linear relationship 
between absorption and CO.sub.2 content, displayed in FIG. 2, introduces 
the effect that an additional absorption is caused by the ambient CO.sub.2 
content variations, at comparatively low concentrations already, and which 
in the gently sloping portion of the curve is large enough to cause an 
appreciable error in the content that is being measured. 
It has been understood in this invention to compensate to a remarkable 
extent for this error, by placing in front of the reference chamber 11 an 
attenuation plate 13, which within the wavelength range selected by the 
filter 12 has a large enough attenuating capacity to cause the result that 
the automatic gain control circuit 22 shifts the operating point of the AC 
amplifier 16 to the shallow part of the curve reproduced in FIG. 2. As can 
be seen in FIG. 2, a suitable absorption for the attenuation disk, in a 
CO.sub.2 analyzer, is about 8%. In that case the additional attenuation 
due to the ambient environment is sufficient at the reference level, too, 
to cause such a change of the gain that this change will compensate for a 
substantial part of that interference attenuation which the measuring beam 
suffers due to the environment. 
This compensating effect is illustrated by the aid of the test examples 
following below. In both experiments which were carried out, the ambient 
CO.sub.2 content was first 0.0 and, next, 0.3% by volume. The length of 
the light beam path through ambient air was 4 mm and the path within the 
measuring chamber, 4 mm. The gas which was measured had in both instances 
a content of 8% CO.sub.2 by volume. 
__________________________________________________________________________ 
1. Without attenuation disk 
Measuring Total at- 
Total at- 
Ambient 
chamber 
Ambient 
tenuation 
tenuation Total 
content, 
attenu- 
attenu- 
in ref. 
in meas. 
Equivalent 
error, 
% CO.sub.2 
ation ation chamber 
chamber 
to CO.sub.2 -% 
CO.sub.2 -% 
__________________________________________________________________________ 
0 9.2 0 0 9.2 8 0 
0.3 9.2 0.5 0.5 9.65 9.2 1.2 
__________________________________________________________________________ 
2. With attenuation disk 
Absorption of the attenuation disk is 8% 
Measuring Total at- 
Total at- 
Ambient 
chamber 
Ambient 
Attenua- 
tenuation 
tenuation 
Equiva- 
content, 
attenua- 
attenua- 
tion disk, 
in ref. 
in meas. 
lent to 
Error, 
% CO.sub.2 
tion tion, % 
% chamber 
chamber 
CO.sub.2 -% 
CO.sub.2 -% 
__________________________________________________________________________ 
0 9.2 0 8 8.0 9.2 8 0 
0.3 9.2 0.5 8 8.46 9.65 8.2 0.2 
__________________________________________________________________________ 
From the foregoing, the conclusion can be drawn that, by using an 
attenuation disk 13 according to the invention, it has been possible in a 
simple and inexpensive way to substantially improve the accuracy of 
measurement of the analyzer by causing such a shift of the reference level 
which moves the operation of the analyzer into the gently sloping part of 
the absorption curve, whereby the effect of the incremental attenuation 
due to the ambient environment will produce an adequate change in the 
amplifier gain for substantial compensation of the incremental attenuation 
of the measuring beam due to ambience. 
Although in the foregoing the invention has been described in association 
with a CO.sub.2 analyzer, it is fully obvious that it is applicable in all 
and any types of gas analyzers. Among other things the invention has been 
tried out in an oxygen analyzer, and there too it has been found to 
eliminate the ambient interference effects to a remarkable extent.