Stable infrared analyzer

A double beam infrared analyzer is modulated to separately and alternately project the sample beam and comparison beam into a condenser microphone type detector thereby generating alternate pulsed signals indicating the intensity of the sample beam and comparison beam. The peak comparison beam signal is detected and maintained at a constant value through a feedback circuit controlling the degree of amplification of both the comparison beam signal and the sample beam signal, thereby providing comparison and sample signals that are corrected every cycle for analyzer instabilities.

FIELD OF THE INVENTION 
This invention relates to double beam infrared fluid analyzers using 
pneumatic detectors of the condenser microphone type, and more 
particularly to an improved analyzer having exceptional stability against 
error resulting from analyzer instabilities. Such errors may result from 
changes in ambient temperature, changes in the intensity of the infrared 
energy source, or instabilities in the pneumatic detector or the 
associated preamplifier and amplifier. 
BACKGROUND OF THE INVENTION 
In the conventional double beam analyzer with pneumatic detectors, one beam 
passes through the fluid sample to be analyzed and its attenuated because 
of absorption of infrared energy in a given spectral range by the presence 
of the component to be determined; the second beam of substantially equal 
intensity passes through a comparison fluid, normally substantially 
nonabsorbent in the measured spectral range of absorbence by the component 
to be determined. In Waters, U.S. Pat. No. 2,648,775, the two beams are 
modulated by an interrupter to separately and alternately project the two 
beams at a rapid frequency into a condenser microphone type detector 
responsive to the intensity of the beams in the spectral region of 
interest. The concentration of the fluid to be determined is indicated by 
the magnitude of the signal difference resulting from the two beams. 
Although the purpose of the high-frequency alternation was to avoid 
temperature effects between successive cycles, the instrument had no 
provision for correcting errors resulting from slower changes in ambient 
conditions and other instrument instabilities. 
SUMMARY OF THE INVENTION 
The primary purpose of this invention is to provide a double beam infrared 
analyzer of enhanced stability. A further object is to provide such an 
analyzer in which the peak signal derived from the comparison beam is 
automatically maintained at a predetermined level and in which the 
comparison beam signal and sample beam signal are equally amplified. Other 
objects will be apparent from the following description and claims. 
The invention is an improvement to infrared analyzers of the type in which 
the infrared radiation travels along two beam paths, one a sample beam 
path traversing a gas sample to be analyzed, the other a reference beam 
traversing a reference gas sample, wherein the presence of the component 
to be determined in the gas sample, affects the relative intensity of the 
beams in a spectral region, and wherein the two beams are cyclically 
modulated by an interrupter to be separately and alternately received by a 
detector of the condenser microphone type responsive to changes in beam 
intensity in the said spectral region. The detector generates a series of 
pulsed signals and the magnitude of alternate pulsed signals indicate the 
intensity of the detected sample beam and reference beam respectively. 
The peak comparison signal is segregated and maintained at a constant value 
through a feed-back circuit, correcting the comparison signal for analyzer 
instabilities, and the feed-back circuit controls the degree of 
amplification of both the comparison signal and the sample signal. Thus 
the comparison and sample signals are corrected every cycle for 
instabilities.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The presently preferred embodiment is described with reference to FIG. 1, 
FIG. 2, FIG. 3 and FIG. 4. Infrared sources 2 and 4, for the sample beam 
and comparison beam respectively, are intersected by a 90.degree. segment, 
pie-shaped interruptor 6, opaque at least in the spectral region of 
interest, rotated at a speed of about 2 cps by synchronous motor 8. Each 
beam is provided with conventional trimmer screw 10 to adjust the 
intensity of the beam. The sample beam passes through a sample cell 12, 
which is provided with a sample inlet 14 and outlet 16 and is otherwise 
sealed by windows 18, transparent to infra-red radiation. The sample beam 
then enters the chamber 20 of pneumatic detector 22. The comparison beam 
follows a parallel path through the interruptor region and comparison cell 
24, similar to the sample cell except that it contains a fixed volume of 
reference gas, usually like the sample gas but without the component that 
is to be measured. The comparison beam then enters chamber 26 of the 
detector. 
The chambers 20 and 26 and the interconnecting passage, delineated by 
diaphragm 28 of a condenser microphone are filled with a gas which absorb 
radiation in the same spectral region as the gas being measured, usually 
the same gas. The gas warms and expands as radiation is absorbed causing 
the diaphragm to move. The movement of the diaphragm in relation to plate 
30 generates a capacitance output signal. 
In operation, the interruptor 6 modulates both beams to produce a signal 
from the pneumatic detector having the wave form showing in FIG. 3. When 
the interruptor is in the position shown in position A of FIG. 4, with 
neither beam interrupted, the signal is a null signal shown at point A of 
FIG. 3. At position B of FIG. 4 the interruptor completely interrupts the 
comparison beam, giving the peak signal from the sample beam at point B of 
FIG. 3. In position C of FIG. 4, the output signal is again at a null 
balance shown at point C of FIG. 3. With the interruptor in position D of 
FIG. 4, blocking the sample beam, the peak comparison beam signal is 
obtained at point D of FIG. 3. This type of interruption, which in effect 
permits the analyzer to function alternately as a single beam sample 
analyzer and a single beam reference analyzer, was used by Freilino U.S. 
Pat. No. 3,731,092 for the purpose of obtaining increased detector 
sensitivity using a flow responsive detector. Equivalent results may be 
obtained with other interruptors, e.g. an opaque disc having a transparent 
pie-shaped segment, or reciprocating shutters, it being required only that 
there be identifiable cycle positions in which the sample beam is entirely 
suppressed, in which the comparison beam is entirely suppressed and that 
these cycle portions are non-consecutive and are separated by an 
intervening cycle portion in which both beams are modulated the same way, 
thereby generating alternating pulse signals indicating the intensity of 
the reference beam and sample beams, providing positions in the wave form 
equivalent to A, B, C. and D of FIG. 3. 
With further reference to FIGS. 1 and 2, which schematically illustrate the 
main elements of the signal treating portion of the analyzer incorporated 
in conventional supporting circuitry, the capacitance output signal from 
the detector is amplified and converted to a voltage signal by 
preamplifier-transducer 34, which voltage signal amplitude is adjusted by 
attentuator 36, and amplified by operational amplifier 38. 
The peak detector comprises a diode 40, preventing back current flow, a 
capacitor 42 in which the peak voltage is stored and three normally open 
switches, 44, 46 and 48, synchronized by electronic timers 50, 52 and 54 
with the interruptor 6. The switches are preferrably optical couplers 
which close momentarily on receiving a timer pulse. The timers are clocked 
by the 50-60 Hz line supply that also supplies the synchronous motor 
driving the interruptor, and are reset by a pulse via lead 55 from 
phototransistor 56, generated when the phototransistor is blocked from LED 
58 by the interruptor when the interruptor is in the position A (FIG. 4). 
The reset pulse momentarily closes switch 44, discharging capacitor 42, and 
resets the timers 50, 52 and 54. Switch 44 reopens and, as the interrupter 
rotates, generating the signal waveform shown in FIG. 2, the peak sample 
signal at point B is stored on capacitor 42. At a time just following the 
peak, point E, Timer 52 produces a pulse momentarily closing switch 46, 
transferring the peak signal from high impedance amplifier 45 to 
operational amplifier 60 and capacitor 61, serving as a sample peak signal 
memory. At point C, timer 52 generates a pulse momentarily closing switch 
44 to again discharge capacitor 42. The capacitor then stores the peak 
comparison signal and, at a time following the peak (point F), the timer 
54 generates a pulse momentarily closing switch 48, transferring the peak 
comparison signal for storage by operational amplifier 62 and capacitor 63 
serving as a comparison memory. 
The output of operational amplifier 62 is connected as the input to 
feedback amplifier 64, which is referenced against a voltage 66. The 
output of the feed back amplifier is connected to control attentuator 36, 
which preferably includes an optical coupler operating in a linear mode to 
adjust the attentuator proportionally to the output of the feedback 
amplifier. 
The output of sample memory 60 and comparison memory 62 are connected to 
differential amplifier 68, the output of which is connected to a readout 
meter 70. The meter indicates the difference in signal amplitude of the 
sample and comparison memories. This difference is proportional to the 
sample concentration in the sample cell. 
The purpose of the feedback network is to correct for changes within the 
system by maintaining the comparison signal at a constant value. If the 
comparison signal does vary and the signal is less than the reference 
voltage, the feedback amplifier controls the attentuator to increase the 
amplitude of the signal. Likewise, if the comparison signal is larger than 
the reference voltage, the feedback amplifier controls the attentuator to 
decrease the amplitude of the incoming signal. The attentuator controls 
the amplification of both the comparison and sample signals, so both 
signals are equally amplified. By performing this level correction in 
signal, changes in signal level due to instabilities in the infrared 
detector, preamplifier or amplifier are corrected and the stability of the 
analyzer is enhanced.