Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-05-19
2002-05-21
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339010, C250S344000, C250S343000
Reexamination Certificate
active
06392234
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to an infrared optical gas sensor with at least one infrared radiation source and with at least one infrared radiation detector.
BACKGROUND OF THE INVENTION
Such an infrared optical gas sensor is disclosed, e.g., in DE 197 13 928 C1 and it contains especially two radiation sources and two radiation detectors as well as associated optical concentrators and a beam splitter.
An essential drawback of the infrared optical gas sensors used hitherto is due to the relatively complex optical design and the cost of the optical components that is associated with it, because two wavelengths, namely, a measuring wavelength and a reference wavelength, are usually used to compensate the effect of the reduction of the radiation intensity of the infrared radiation source due to its aging or due to the contamination of optical surfaces in the beam path. The measuring wavelength (measuring radiation) is selected to be such that the corresponding measured signal of an infrared radiation detector displays a characteristic dependence on the concentration of the gas to be measured. On the other hand, the reference wavelength (reference radiation) is selected to be such that the reference signal measured is affected by the gas to be measured as little as possible. The geometric design of the measuring and reference channels is selected to be such that the radiation will possibly take the same optical path in the measuring and absorption sections for both channels.
The radiation is divided between two separate optical paths for the measuring radiation and the reference radiation in a usually encapsulated area not affected by the gas to be measured, especially by means of a beam splitter. The effect of the above-mentioned changes in the radiation intensity, which affect both channels, is to be eliminated by dividing the signal values of the measuring channel and the reference channel, while the change in the measured signal of the narrow spectral band absorption by the measured gas is preserved.
These hitherto usual measuring arrangements and the corresponding measurement methods have the fundamental drawback that a change in the geometry of the beam distribution between the measuring channel and the reference channel leads to a drift or deterioration of the measured signal. In addition, the measuring effect, i.e., the change in the signal quotient due to absorption, is frequently very small, so that it corresponds to a deviation of the signal quotient by 0.5%, e.g., in the case of a methane sensor based on the required accuracy of 1% of the lower explosion limit. However, such deviations of the signal quotient are likewise easily possible if, e.g., the beam splitter in the optical arrangement does not guarantee ideal splitting of the radiation between the active surfaces of the measuring radiation detector and the reference radiation detector, or if a shift in the image of the radiation source in the plane of the detectors is brought about by aging effects in the radiation source. If the beam spot moves over the edge of the active detector surface, the detector signal will change. A slight asymmetry in illumination thus produces an undesired deviation in the signal quotient. Similar deviations may also be caused by thermal deformations of the sensor structure or by dirt or liquid drops in the beam path, by mechanical shocks or other causes. As an end result, radiation components that are weighted unequally in the measuring and reference channels are thus blanked out.
Complicated technical measures are taken to avoid or reduce the above-described effects. For example, the housing and the carrying structure are made of high-quality metallic materials, and fits prepared with precision and true-to-angle mounts as well as adjustment steps are provided in the manufacturing process. Furthermore, structured intensity distributions in the effective detector surface are smoothened by blurred imaging or by a specific scattering of the radiation. The use of only one detector element with one movable filter wheel arranged in the beam path with different interference filters has been known as well.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the present invention is to propose a simple design for an improved infrared optical gas sensor, which delivers more stable measured signals.
According to the invention an infrared optical gas sensor is provided with at least one infrared radiation source and with at least one infrared radiation detector. The infrared radiation detectors used as reference radiation and measuring radiation detectors are formed of thin layers of a partially transparent material, which delivers an electric measured signal that depends on the radiation intensity received. The infrared radiation detectors are stacked one over the other and are arranged with an intermediate narrow-band filter each, which transmits at the measuring wavelength. The infrared radiation detectors have an electrically conductive coating and are contacted on their top sides and undersides. The measuring radiation detector follows the reference radiation detector in the direction of the beam. The reference radiation detector is transparent for at least part of the measuring radiation at the measuring wavelength.
According to an alternative embodiment of the invention an infrared optical gas sensor is provided with at least one infrared radiation source and with at least one infrared radiation detector. The infrared radiation detectors used as reference radiation and measuring radiation detectors are formed of thin layers of a partially transparent material, which delivers an electric measured signal that depends on the radiation intensity received. The infrared radiation detectors are stacked one over the other and are arranged with an intermediate narrow-band filter each, which stop or block at the measuring wavelength. The infrared radiation detectors on the top side and the underside each have an electrically conductive coating and are contacted. The reference radiation detector follows the measuring radiation detector in the direction of the beam. The measuring radiation detector is transparent for at least part of the reference radiation at the reference wavelength.
One essential advantage of the present invention arises from the compact, layered design of a multiple detector arrangement for the measurement of at least two different wavelengths, namely, a reference wavelength and a measuring wavelength. In the simplest case, the gas sensor according to the present invention may be designed such that the measuring gas holder filled with the gas to be measured is an internally reflecting tube, which has a broad-band infrared radiation source at one end face and a layered multiple detector arrangement according to the present invention at the other, opposite end face. An infrared optical gas sensor with stable output signal is thus provided without an additional imaging optical system. The cylindrical reflector, i.e., the measuring gas holder, ensures increased radiation intensity in the central longitudinal axis of the measuring gas holder and thus also in the entry window of the multiple detector arrangement, which is mounted centrally in one end face of the measuring gas holder.
Contrary to the present invention, a double detector usually used hitherto with entry windows arranged next to one another has drawbacks in this regard due to reduced stability over time and geometric stability of the irradiated effective detector surfaces as well as due to the reduced radiation intensity received because of the impossibility of the geometrically identical arrangement of the two detectors in the central longitudinal axis of the sample holder. The multiple reflections on the cylindrical surface of the measuring gas holder lead to a radiation intensity distribution with a maximum in the central longitudinal axis and a steep drop in intensity in the radial direction. The radiation-sensitive detector surfaces are thus located on the flanks of this intensity distribution,
Drager Sicherheitstechnik GmbH
Gagliardi Albert
Hannaher Constantine
McGlew & Tuttle, P.C
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