Calibrating a nondispersive infrared gas analyzer

Compensation and calibration method for a nondispersive infrared gas analyzer which includes an infrared source of radiation, a measuring path, a reference path, modulation devices, a detector for differential pressure measuring, comprises the following steps: first and prior to measurement, the reference gas having a variable basic concentration in the measuring component is forced through the reference branch and through the measuring branch and the detector is set to a zero position; next a dual calibration chamber is placed into the two reference paths while both of them are still passed through by the reference gas, one of the calibration chambers including a particular concentration in the measuring gas the other one lacking that measuring gas; now the sensitivity and amplification of the detector that obtains as a result of changes in the radiation on account of placing the calibration chamber into the reference and measuring path is adjusted, whereupon the calibration chamber is removed, reference gas still is passed through the reference chamber, while measuring gas now flows through the measuring chamber.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for range and offset adjustment 
and calibration of nondispersive infrared gas analyzer provided for the 
determination of concentration of a gas component having a basic 
concentration that is relatively high while the variations in 
concentration that are of interest cover but a relatively small level and 
range. 
Gas measuring engineering and technology is frequently faced with the task 
of measuring a gas component and constituent, as well as variations 
thereof which are basically very small as compared to a "background" of a 
relatively large basic concentration. This measuring problem at hand is 
complicated particularly when the basic concentration of the measuring 
component changes continuously or frequently, thereby obliterating the 
range and variance of interest. An example is the measurement of 
assimilation on the basis of the heretofore used optical methods. Here one 
uses the so called Lambert-Beer law which yields a nonlinear relation 
between the absorption of light radiation on the one hand, and 
concentration of a gas in the medium passed through by the light on the 
other hand. If the sensitivity changes in time depending on the basic 
concentration, one has to provide a continuous range tracking. 
In the example mentioned above the assimilation of interest involves the 
CO2 content in air. This concentration, considered as a background, may 
depend on conditions that vary between about 320 ppm and 1000 ppm. Since 
the assimilation covers only small variations such as 50 ppm in the CO2 
value measuring range, background variations can easily obliterate 
measurement errors up to 50% and more may readily arise. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
method of operating a nondispersive infrared gas analyzer for measuring 
the gas concentration over and above or on the basis of and with reference 
to a basic concentration which is variable, to obtain a variable 
concentration measurement wherein the measuring sensitivity of the 
component remains independent from the basic concentration and its 
variations. 
It is therefore a specific object of the present invention to provide a 
sensitivity adjustment, offset controlling and range setting method for 
nondispersive infrared gas analyzer, for the determination of the 
concentration of a gas component by means of a device that includes an 
infrared radiation source, a sample and reference chamber arrangement 
which normally provides for measuring gas as well as reference gas, and 
these two chambers are situated in two separate beam paths originating at 
the source and are thus traversed by separate rays. The rays are usually 
continously interrupted for modulation purposes. Furthermore the device 
used in the method includes a receiving or detection chamber either filled 
with an absorbing gas that corresponds to the measuring gas or it is 
itself the measuring gas. The radiation beams will be absorbed by 
separated chambers in the detection device. A pneumatic pressure sensitive 
device is connected to the two detection chambers providing an electrical 
signal that is responsive to the pressure difference in the two detection 
chambers as an indication of differentiation in the absorption of the 
radiation that has traversed the reference chamber and measuring gas 
chamber respectively. 
In accordance with the preferred embodiment of the present invention the 
objects are attained specifically, in that in a first method step, 
preceding measurement proper, the reference gas with an actual basic 
"background" concentration flows simultaneously through the measuring 
chamber and through the reference chamber to obtain and set a zero 
reference point for gas analysis. In a second and subsequent step also 
preceding the measurement proper, a calibration chamber is shifted in 
alignment with the measuring and the reference chambers, which calibration 
chamber has a portion filled with the measuring component of a very 
accurately known concentration which corresponds to the desired 
measurement range; subsequently the sensitivity of the detector generating 
the electrical signal (i.e. the detector that responds to the pressure 
difference resulting from this manipulation) will be adjusted so that its 
sensitivity corresponds to that measuring range. Finally and following the 
preceding steps the calibration chamber is removed for regular operation 
wherein the reference chamber receives the same background gas as 
reference gas as per the first step, but the measuring chamber receives 
real measuring gas to obtain measurements. 
The method that is proposed here precedes measurement proper. The reference 
gas is for example branched off the measuring gas, i.e. the same gas is 
used as reference gas, as host gas to which the source of measuring gas is 
exposed. The problem results from the fact that the reference gas has 
already, as a background, the same gas that is the measuring gas 
corresponding to a basic concentration; variations in the background may 
falsify the measurement. This reference gas which includes interfering 
measuring gas is used to set the basic response level of the equipment to 
serve as point of departure for determining the sensitivity of the 
measurement with reference to the basic concentration of measuring gas as 
background. This procedure involves these following steps. 
First the reference gas with the actual background content of measuring gas 
is fed to both the measuring chambers and to the reference chambers of the 
gas analyzer. This step permits a setting of a and set a zero point for 
that instrument. In the second step a calibration chamber is shifted into 
the radiation path of both measuring and reference beam paths. One of the 
calibration chambers is filled with some of the measuring component 
variety at a predetermined concentration. For example, a measuring range 
is thereby defined being about 50 ppm by way of example. The sensitivity 
of the instrument is now adjusted so that the measuring range covered by 
the instrument is equal to that particular range, from the previously 
adjusted zero level to, say 50 ppm, measurement within which the measuring 
component can vary. Following this the instrument is actually correctly 
adjusted to the desired measuring range as far as sensitivity is concerned 
and normal measurement may not proceed. 
In case the basic concentration of the host and reference gas will later on 
change, then these two steps have to be repeated. One can for example 
provide for an automated basis which repeatedly provides a readjustment 
and recalibration procedure conceivably any variation in the basic 
concentration of the measuring gas may be tracked separately to insitute 
and control the performing of recalibration as desired. This way one 
obtains a quasicontinuous correction of sensitivity.

The gas analyzer is basically of known construction; it includes an IR 
source 1 sending radiation into a measuring branch 1 and 2 and into a 
reference branch 1-3. In front of the source a modulating diaphragm 15 
rotates in a conventional manner changing the two beams to provide for a 
basic modulation. The reference numeral 2 refers to a measuring chamber 
that is normally passed through the gas, and parallel thereto is provided 
a reference chamber 3 that includes or is being passed through by a 
reference gas. A basic, physical set up of this kind is shown for example 
in U.S. Pat. Ser. No. 359,510 (filed 06/01/1989, now U.S. Pat. No. 
5,003,175). These two chambers are of course physically separated from 
each other but they are individually traversed by the two beams of 
radiation 1-2 and 1-3. The radiation in each instance having passed 
through the chamber 2 or 3, as the case may be, is received in detection 
chambers 16, 17 respectively. These two chambers pertain to a detector 
having a pressure sensitive branch by means of which the pressure 
differential in the two chambers 16, 17 is ascertained on a running basis. 
The pressure differential is ascertained by the somewhat schematically 
indicated detector 18. These aspects are known by and in themselves and do 
not constitute the part of invention. See also the following U.S. patents 
as representative examples: U.S. Pat. Nos. 4,682,031; 4,373,137; 
4,180,732; 4,156,812. 
In the foregoing, the basic set up for infrared nondispersive gas analyzing 
measurement was described; we now proceed to the description of the 
inventive calibration procedure. It may now be assumed that the reference 
gas is air which by means of pump 9 is fed from a conduit 10 through the 
measuring system. A first branch includes a comparing or reference vessel 
6 having a volume V. The gas is passed on through a conduit 11 and into 
the reference chamber 3. This particular procedure is consistently carried 
out as a part of the method. The gas, as it is removed from the chamber 3, 
flows through a branch 12 leading to the pump 9. The pump operates in the 
suction mode. 
A part of the air that is sucked through the conduit 10 will flow into 
another branch that may include a measuring vessel 7 with the same volume 
V, and as schematically indicated that measuring chamber 7 may include a 
leaf as a sink for CO2. Depending upon the position of a valve-switch 5 
(alternative) the air that has passed through the chamber 7 will be fed 
into the conduit 13 and passes into the measuring chamber 2. Conduit 14 
refers to the removal path for the measuring gas; everything is pumped out 
of the system by pump 9. 
The switch valve 5 connects the chamber 7 in the normal mode of measurement 
to the measuring chamber 2. The figure illustrates an alternative 
connection for the calibration mode. It can be seen that the pump 9 sucks 
air that enters the system through the conduit 10 into a first branch that 
includes the vessel 6, conduit 11 reference chamber 3, conduit 12 and out 
into the pump, while a parallel branch runs from conduit 10 to the 
measuring vessel 7, the valve 5, the feeder line 13, the measuring chamber 
2 and the output branch, conduit 14 thereof, to be united with the air 
that is arriving through the conduit 12. 
If the pump is in continuous operation, of course, no back feeding is 
expected. The measuring vessel 7 is of course the chamber in which the 
desired assimilation process obtains. The resulting gases emerging from 
the leaf mix with the reference air but prior thereto the relative 
component here is measured by the instrument. 
A vessel 8 is connected in parallel to measuring source 7. Vessel 8 has the 
same volume as the measuring source vessel 7. This vessel 8 has an input 
duct that is also connected to the conduit 10. In the illustrated position 
of valve 5, air is in fact sucked through vessel 8 rather than through 
chamber 7. In this calibration case the measuring chamber 2 receives also 
reference gas. 
As was mentioned earlier the "normal" CO2 content in air may vary from 320 
ppm to about 1000 ppm. On the other hand the assimilation of CO varies 
within a smaller range such as the range from zero to about 50 ppm. it can 
readily be seen that the background CO2 of the reference/host gas air may 
vary over a much larger range. That may not happen directly but unforseen 
changes in CO2 content, even sudden ones, must be expected. 
The variation in basic concentration is eliminated by the inventive method 
in order to obtain a removal of the concomitant error. This procedure is 
carried out as follows. First, air having whatever basic concentration in 
CO2 happens to obtain, is also fed into the measuring chamber 2 through 
the valve 5 having the illustrated position and bypassing leaf chamber 7. 
In other words chambers 2 and 3 receive the same kind of gas having a 
basic concentration of CO2 whatever the situation happens to be. As a 
consequence, specific radiation absorption processes occur in chambers 2 
and 3. Aside from certain basic errors, this absorption should result in 
similar detection processes for both branches. Any inequality reflects 
instrument inaccuracies which are also eliminated by this procedure. 
Whatever happens which is to be indicated by 18 under these references and 
conditions, the output of the detector 18 is now set to zero. 
The figure shows also a device 4, which is a calibration. The figure also 
shows a device 4, which is a calibration U.S. Pat. (Ser. No. 359,510, 
filed 06/01/1989). Device 4 includes four chambers 4a, 4b, 4c and 4d. 
During this first step, when chambers 2 and 3 receive reference gas, 
chambers 4a and 4b are in the illustrative position, just downstream from 
the chopper wheel and modulator 15. The chambers 4a and 4b are filled with 
a neutral gas, that is, a gas which has no absorption bands overlapping 
with absorption bands of the measuring gas (e.g. CO2). 
Following the zero setting as described, the calibration chamber 4 is 
shifted to the alternative position in which chambers 4c and 4d are 
aligned with chambers 3 and 2. Calibration chamber 4c is again filled with 
that neutral gas, and so is chamber 4d, but either the latter or the frame 
is in addition filled with measuring gas, i.e. it is either filled with 
the measuring component that is CO2 or with a gas having the same kind of 
IR absorbing properties. The concentration of this gas in chamber 4d (or 
4c) is selected to cover a measuring range that is equal to the measuring 
range of the assimilation process. It may include a 50 ppm concentration 
in CO2. Please note that this measuring gas may be put into chamber 4c 
(for placement in the reference path) if the measuring process actually 
involves absorption (assimilation) of measuring gas. The object in 
measuring chamber 7 may actually be a sink rather than a source, but owing 
to the overall symmetry involved it makes no difference whether the 
measuring gas component is determined on an additive or subtractive basis; 
one just has to adjust the range accordingly- 
The calibration chamber 4 when having chamber 4c and 4d placed into the 
reference and measuring paths, permits the sensitivity of the detector 18 
to be adjusted. In other words what is adjusted is the amplification 
factor because there should be a particular output that should be equal to 
(i.e. represent) the concentration of 50 ppm. Following these two 
calibration processes the calibration chamber, i.e. the two neutral gas 
containing chambers 4a and 4b, are respectively placed both into the 
reference and measuring paths. Valve 5 changes position to reconnect 
measuring chamber 7 to the system. Now normal measurement commences or is 
resumed. 
The gas analyzer is now adjusted correctly to cover the desired measuring 
range as far as sensitivity is concerned. If for any reason the basic 
concentration in CO2 varies in the air then the compensation offset and 
calibration procedure as described has to be repeated. This repetition can 
well be carried out automatically, it can even be carried out on a 
temporal basis and/or it may be instigated through separate independently 
carried out CO2 measurements. In either case a quasicontinuous correction 
obtains in the sensitivity of the measuring procedure. 
The invention is not limited to the embodiments described above but all 
changes and modifications thereof, not constituting departures from the 
spirit and scope of the invention, are intended to be included.