Gas sensor with open optical measurement path

Details Gas sensor with open optical measurement path Gas sensor with open optical measurement path

2000-06-15

2003-03-25

Rosenberger, Richard A. (Department: 2877)

Optics: measuring and testing

Inspection of flaws or impurities

Having predetermined light transmission regions

Reexamination Certificate

active

06538728

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a gas sensor with an open optical measurement path.
2. Description of the Related Art
One such device is known for instance from U.S. Pat. No. 5,591,975, in which a device for measuring the exhaust gases of automobiles driving past it is described. The vehicles traverse the measurement path, which is bounded on one by a light source and on the other by a field of photodiodes.
In environmental analysis and industrial monitoring, the analysis of gaseous mixtures has gained increasing significance. There is accordingly increasing interest in the development of novel gas sensors that are optimized with respect to their sensitivity, selectivity, service life, and ease of manipulation.
Along with gas sensors that monitor a spatially narrowly limited region, recently gas sensors that monitor a larger area are increasingly being employed. The so-called gas sensors with an open optical measurement path (or open path sensors) record the mean concentration of the target gas over a path with a length of ten meters to a few hundred meters.
U.S. Pat. No. 5,339,155 describes a device in which the light of a light source is modulated in its wavelength, and this frequency modulation is converted, in the presence of the target gas, into an amplitude modulation that can be measured by a detector. The path is bounded, that is, defined, by a light source unit and a detector unit that are spatially separate from one another.
As light sources, laser light sources have increasingly been used in recent years. DFB laser diodes, in particular, are distinguished in that on the one hand the wavelength of the emitted light is very much narrower in its band than the absorption lines of gases, and on the other, this wavelength can be varied both by way of the temperature of the laser diode and via the triggering laser diode current.
In many laser diode-supported systems, so-called derivative spectroscopy is employed. In it, the wavelength of the laser diode is initially set, for instance by specifying the laser diode temperature, in such a way that the very narrow-band laser line is located spectrally within the absorption of a single gas line, for instance, of the target gas. The desired monitoring of the laser diode temperature can be performed for example by locating the laser diode chip on a Peltier element, which can be brought to a desired temperature by varying the Peltier current. The laser diode is operated with a current modulation in such a way that the gas line is periodically swept at the frequency f, and the modulation is preferably sinusoidal. Not only is the laser diode varied in its wavelength, but furthermore, as a parasitic effect, an amplitude modulation of the radiation intensity at the frequency f, the so-called
1
f
component, also occurs, and this amplitude modulation can be utilized to standardize the intensity.
Once the measurement path has been traversed, the intensity of the light is detected with a detector sensitive to the light of the light source; the detector generates an electrical signal that is proportional to the incident light intensity. This detector is equipped with an optical filter, which filters out interfering components of the spectrum, such as incident daylight. In the absence of the target gas, the detector signal is likewise sinusoidal at the frequency f, because of the corresponding amplitude modulation of the laser diode current. If target gas is present within the measurement path, however, then the intensity measured by the detector after the path has been traversed includes, as a function of time, components that are modulated with n-times the frequency, which are known as n
th
harmonic components or n
th
harmonics. The generation of these harmonic components is dictated by the nonlinear curvature of the absorption line of the gas. With the aid of suitable phase-sensitive measuring amplifiers (known as lock-in amplifiers], these harmonic components of the detector signal can be determined. While the
1
f
component of the detector signal is influenced hardly at all by the gas concentration, the higher
2
f
,
3
f
and further components are approximately proportional to the gas concentration. Thus the quotient of the
2
f
component and the
1
f
component, for instance, known as the
2
f
:
1
f
quotient, represents a standardized number for determining the gas concentration that is independent of such external effects as aging of the light sources, and broad-band attenuation from dirt, fog, and so forth.
To compensate for zero drift and to increase the sensitivities, the fast
1
f
modulation of the laser diode wavelength is additionally underlayed by a slower modulation of the mean wavelength at the frequency F (f>F), by varying the laser diode current accordingly. This slow modulation can for instance be in the form of a linear tuning ramp (sawtooth); one period of this slow modulation is called “scan”. During one scan, a previously defined number of n
2
f
:
1
f
quotients at n different wavelengths is picked up. The amplitudes of both the fast f- and the slow F-modulation of the wavelength are each selected such that they correspond approximately to the width of the gas line. Thus instead of the single value of a
2
f
:
1
f
quotient described previously for a fixed wavelength, a plurality or a tuplet of n
2
f
:
1
f
quotients at n different wavelengths is now obtained. This measurement value tuplet can serve, with suitable mathematical evaluation, for instance by a PCA (Principal Components Analysis) process or the like, both to determine the target gas concentration and to identify the target gas with certainty.
To prevent the originally set temperature of the laser diode or the Peltier element from varying during operation and thus varying the wavelength of the laser light, a beam splitter is mounted on the side of the light source unit or the detector unit; it deflects part of the light emerging from the laser diode through a cell (reference cell) in which a gas of suitable absorption capacity—for instance, the target gas itself—is confined. This portion of the light is detected, after passing through the reference cell, by a light-sensitive detector. With the aid of a phase-sensitive measuring amplifier, analogously to the measurement tuplet, a set of reference measurement values, a so-called reference tuplet, can be determined that is preferably again composed of
2
f
:
1
f
quotients. By a comparison of this reference tuplet with values stored in memory, it is possible to detect any wavelength drift and to correct the temperature of the laser diode in such a way that this wavelength drift is precisely compensated for.
Open path gas sensors, when there is a large three-dimensional spacing between the light source unit and the detector unit, can be manipulated upon assembly and in operation only with difficulty and at great effort. For instance in derivative spectroscopy, problems of adaptation arise between the control of the light source and the evaluation of the detector signals. As much as possible, adaptations between the control of the light source and the evaluation for the detector signals must be done in a separate control and evaluation device, which is in communication with both the light source unit and the detector unit.
Such adaptation and control problems can be partially avoided by gas sensor systems in which the light source unit and detector unit are disposed directly adjacent one another and a long open measurement path is realized by providing that the light source aims a measurement beam at a remotely located retroreflector, by which the measurement beam is reflected and thrown back at the detector unit. One such gas sensor is known from German Patent DE 196 11 290 C2, for instance. In this known gas sensor device, the light source unit and the detector unit are not accommodated separately from one another but rather in one housing. This gas sensor has the advantage that the light source unit and the detector unit can function, adapted

Affiliated with

Also associated with

Rating

Gas sensor with open optical measurement path does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Gas sensor with open optical measurement path, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Gas sensor with open optical measurement path will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3056361