Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-05-30
2003-01-21
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S343000, C250S575000
Reexamination Certificate
active
06509567
ABSTRACT:
The present invention relates to a method and to portable apparatus for detecting gas by selective absorption of electromagnetic radiation to detect the presence of a particular gas within a mixture of gases.
BACKGROUND OF THE INVENTION
Gas leak detectors are characterized by three properties, namely: selectivity, i.e. the ability to detect a particular gas amongst a mixture of gases present in an atmosphere; sensitivity, i.e. the minimum quantity of gas that can be detected; and stability, i.e. insensitivity to variations in climatic conditions so as to ensure that performance remains constant regardless of climatic conditions.
Various types of detector are already known for detecting a particular gas, such as methane, in an atmosphere made up of a mixture of gases.
Thus, semiconductor detectors exist in which a combustible gas reacts on coming into contact with the semiconductor, thereby reversibly altering the electrical resistance of the semiconductor, and this resistance is very easy to measure. That type of sensor is of low cost and is used above all for detecting leaks in the home. It has medium performance and stability, but no selectivity.
Catalytic detectors can be used for detecting the presence, e.g. of methane, and they are fitted to detector appliances for detecting methane at concentrations in a measurement range of several hundred parts per million (ppm) to several percent, by volume. Although such detectors have acceptable stability, their selectivity is very poor.
Thermal conductivity detectors use the ability of a gas to evacuate heat. The presence of a gas such as methane gives rise to a variation in thermal conductivity, and this variation is measured. Such appliances are not selective and they are not adapted to measuring low concentrations of gas, e.g. less than 1% by volume of methane.
Flame ionization appliances make use of the fact that hydrocarbon flames conduct electricity. The presence of a hydrocarbon such as methane modifies the conductivity of a hydrogen flame between two electrodes. Sensitivity is good and response times are short. Thus, appliances of that type can be used to measure in the range 1 ppm to several hundred ppm with good stability. However selectivity is zero.
Known infrared optical detectors present medium performance in terms of sensitivity and selectivity.
Various types of non-dispersive infrared (NDIR) type gas detectors are known. Nevertheless, most gas analyzers using standard NDIR type techniques lose their effectiveness when the gases to be detected and measured present absorption bands that are non-specific and overlap in the infrared range.
FIGS. 3 and 3A
are diagrams showing an example of an optical type gas analyzer as described in U.S. Pat. No. 4,914,719.
In such an embodiment, a source
10
of infrared radiation powered with alternating current (AC) produces a beam of infrared radiation which passes through a chamber
14
containing a sample of a gas mixture, and it impinges on a beam splitter
12
. The beam splitter
12
directs a fraction of the incident radiation to a wheel
16
carrying filters, and the fraction of the infrared radiation which passes through the wheel
16
carrying filters
22
,
24
, and
26
is picked up by a photodetector
20
.
A stepper motor
18
rotates the wheel
16
so as to position the various filters
22
,
24
, and
26
in turn between the beam splitter
12
and the detector
20
.
The fraction of the infrared radiation that passes through the beam splitter
12
passes initially through an interference filter
30
and is then picked up by a photodetector
28
.
FIG. 4
shows typical curves representing the transmission spectrum as a function of wavelength for three gases A, B, and C having absorption bands that overlap. It can be observed that a standard NDIR type gas detection technique using a bandpass filter centered on wavelength &lgr; and presenting a half-maximum bandwidth &Dgr;&lgr;, as shaded in
FIG. 4
is incapable of distinguishing between the three gases A, B, and C insofar as all three gases A, B, and C present various absorption bands in this zone of the spectrum. Insofar as a standard technique makes it possible to perform a transmission measurement only, it is possible to obtain only one equation having three unknowns (three gas concentrations).
The apparatus shown in
FIGS. 3 and 3A
enables this problem to be remedied by placing three bandpass filters
22
,
24
, and
26
on the wheel
16
, the filters having narrow bands centered on wavelengths &lgr;
1
, &lgr;
2
, and &lgr;
3
, with respective bandwidths &Dgr;&lgr;
1
, &Dgr;&lgr;
2
, and &Dgr;&lgr;
3
thus enabling a set of three equations to be obtained. Under such circumstances, the filter
30
is itself connected in such a manner as to correspond to a reference beam centered on the wavelength &lgr;
0
which is close to the characteristic absorption wavelength of the gases present in the chamber
14
, but which does not overlap these characteristic absorption wavelengths.
The prior art apparatus of
FIGS. 3 and 3A
thus provides a set of four measurement signals that can be used to detect the concentration of three different gases. The apparatus can be adapted to detecting the concentrations of N different gases providing N filters
22
,
24
, and
26
are selected that are centered on different wavelengths.
Although such an apparatus as known from U.S. Pat. No. 4,917,719 enables a plurality of gases to be detected simultaneously, it is not adapted to detecting a particular gas simply and quickly using a portable appliance. The apparatus described above with reference to
FIGS. 3 and 3A
has moving parts, in particular the rotary disk
16
, which increases the weight and the size of the apparatus and also the amount of energy it consumes, while also making the apparatus relatively fragile, particularly in the presence of vibration. Furthermore, such apparatus can be used only on condition that the composition of the gas mixture for analysis is known in advance, and it needs to be calibrated for each gas whose concentration is to be determined within the mixture of gases.
Other types of NDIR gas analyzer are known, that implement a gas filter correlation (GFC) technique. By way of example, such gas analyzers are described in the work entitled “Techniques and mechanisms in gas sensing” by P. T. Moseley, J. O. W. Norris, and D. E. Williams, published by Adam Hilger, Bristol, Philadelphia, and New York.
In that technique, the gas to be measured is used in high concentration as a filter for the infrared radiation passing through the chamber of the gas analyzer that is filled with the gas mixture to be analyzed. The basic components of a gas analyzer using the GFC technique and designed to measure ambient carbon monoxide CO are shown in FIG.
5
.
The infrared radiation emitted by a source
40
is chopped and then passes through a gas filter which comprises in alternation a reference filter
42
containing a high concentration of a gas of the same kind as that which is to be detected in the mixture of gases (such as carbon monoxide), and a measurement gas filter
43
containing nitrogen in this example. The gas filters
42
and
43
pass in alternation in front of the source
40
, given that they are placed on a support which is rotated by a motor
41
defining the chopping of the beam from the source
40
. Once the beam of radiation has passed through the gas filter device
42
,
43
it can pass through an additional bandpass filter
44
and then penetrates into a chamber
45
containing the mixture of gases and within which absorption occurs due to the gases in the mixture of gases. Once the infrared beam has passed through the chamber
45
it reaches a photodetector
47
, after passing through a lens system
46
that performs focusing and that also contains an interference filter having a narrow passband. The detector
46
is associated with electronic processor circuits
48
and with a display device
49
.
If the chamber
45
does not contain a gas that causes infrared radiation to be absorbed and if the radiation fr
Boudet Thierry
Fantini Jérôme
Gamarts Emelian
Krylov Vladimir
Gaz de France
Hannaher Constantine
McDermott & Will & Emery
Moran Timothy J.
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